Dermpath Diagnosis

The Diagnosis: Microcystic Adnexal Carcinoma

Microcystic adnexal carcinoma (MAC) is a rare, low-grade adnexal carcinoma consisting of both ductal and pilar differentiation.¹ It typically presents in young to middle-aged adults as a flesh-colored or yellow indurated plaque on the upper lip, medial cheek, or chin. Histologically, MACs exhibit a biphasic pattern consisting of epithelial islands of cords and lumina creating tadpolelike ducts intermixed with basaloid nests (quiz image). Keratin horn cysts are common superficially. A dense red sclerotic stroma is seen interspersed between the ducts and epithelial islands creating a "paisley tie" appearance. The lesion displays an infiltrative pattern and can be deeply invasive, extending down to the fat and muscle (quiz image, inset). Perineural invasion is common. Atypia, when present, is minimal or mild and mitoses are rare. Although this tumor's histologic pattern appears aggressive in nature, it lacks immunohistochemical staining such as p53, Ki-67, bcl-2, and c-erbB-2 that correlate with malignant behavior.² A common diagnostic pitfall is examination of a superficial biopsy in which an MAC may be mistakenly identified as another entity.

Syringomas are benign adnexal neoplasms with ductal differentiation.³ They are more common in women, especially those of Asian descent, and in patients with Down syndrome. They typically present as multiple small, firm, flesh-colored papules in the periorbital area or upper trunk. Histologically, syringomas also display comma-shaped tubules and ducts with a tadpolelike appearance and a dense red stroma creating a paisley tie-like pattern. Ductal cells have an abundant pink cytoplasm. Syringomas are well-circumscribed and more superficial than MACs without an infiltrative pattern. They lack mitotic activity or perineural invasion (Figure 1).

Desmoplastic trichoepithelioma (DTE) is a benign follicular neoplasm.⁴ It presents in adulthood with a female predominance. Clinically, it appears as a solitary flesh-colored to yellow annular plaque with raised borders and a depressed central area, often on the medial cheek. Histologically, DTEs are well-circumscribed with narrow branching cords lined with polygonal cells. A dense red stroma in combination with the epithelioid aggregates also creates the paisley tie-like pattern in this lesion. Retraction between collagen bundles within the stroma can be seen, helping distinguish this lesion from a morpheaform basal cell carcinoma (BCC), which has retraction between the epithelium and stroma. Immunohistochemistry also can be a useful tool to help differentiate DTEs from morpheaform BCCs in that sparse cytokeratin 20-positive Merkel cells can be seen within the basaloid islands of DTE but not BCC.⁵ Also seen with DTEs are numerous keratin horn cysts that commonly are filled with dystrophic calcifications. Cellular atypia and mitoses are not seen (Figure 2). Compared to MACs, DTEs lack abundant ductal structures and also contain papillary mesenchymal bodies and a more fibroblast-rich stroma.

Morpheaform BCC is an aggressive subtype of BCC. It presents as a scarlike plaque that gradually expands. Thin infiltrating strands of basaloid cells are seen haphazardly throughout a pink sclerotic stroma. Tadpolelike basaloid islands and rarely horn cysts can be seen scattered superficially, creating the paisley tie-like pattern. This lesion is more infiltrating than a syringoma or a DTE, and perineural invasion is common. Retraction is uncommon, but when present, it is seen between the epithelial cords and adjacent stroma (Figure 3).

Trichoadenoma is another benign neoplasm of follicular differentiation.⁶ It typically presents as a dome-shaped papule or plaque on the head or neck. Histologically it displays numerous dilated cystic spaces that reflect its origin from isthmic and infundibular differentiation. There is no attachment to the overlying epidermis. It can be distinguished from MAC, DTE, and syringoma due to a lack of basaloid aggregates and only a small number of non-cyst-forming epithelial cells (Figure 4).

The Diagnosis: Microcystic Adnexal Carcinoma

Microcystic adnexal carcinoma (MAC) is a rare, low-grade adnexal carcinoma consisting of both ductal and pilar differentiation.¹ It typically presents in young to middle-aged adults as a flesh-colored or yellow indurated plaque on the upper lip, medial cheek, or chin. Histologically, MACs exhibit a biphasic pattern consisting of epithelial islands of cords and lumina creating tadpolelike ducts intermixed with basaloid nests (quiz image). Keratin horn cysts are common superficially. A dense red sclerotic stroma is seen interspersed between the ducts and epithelial islands creating a "paisley tie" appearance. The lesion displays an infiltrative pattern and can be deeply invasive, extending down to the fat and muscle (quiz image, inset). Perineural invasion is common. Atypia, when present, is minimal or mild and mitoses are rare. Although this tumor's histologic pattern appears aggressive in nature, it lacks immunohistochemical staining such as p53, Ki-67, bcl-2, and c-erbB-2 that correlate with malignant behavior.² A common diagnostic pitfall is examination of a superficial biopsy in which an MAC may be mistakenly identified as another entity.

Syringomas are benign adnexal neoplasms with ductal differentiation.³ They are more common in women, especially those of Asian descent, and in patients with Down syndrome. They typically present as multiple small, firm, flesh-colored papules in the periorbital area or upper trunk. Histologically, syringomas also display comma-shaped tubules and ducts with a tadpolelike appearance and a dense red stroma creating a paisley tie-like pattern. Ductal cells have an abundant pink cytoplasm. Syringomas are well-circumscribed and more superficial than MACs without an infiltrative pattern. They lack mitotic activity or perineural invasion (Figure 1).

Desmoplastic trichoepithelioma (DTE) is a benign follicular neoplasm.⁴ It presents in adulthood with a female predominance. Clinically, it appears as a solitary flesh-colored to yellow annular plaque with raised borders and a depressed central area, often on the medial cheek. Histologically, DTEs are well-circumscribed with narrow branching cords lined with polygonal cells. A dense red stroma in combination with the epithelioid aggregates also creates the paisley tie-like pattern in this lesion. Retraction between collagen bundles within the stroma can be seen, helping distinguish this lesion from a morpheaform basal cell carcinoma (BCC), which has retraction between the epithelium and stroma. Immunohistochemistry also can be a useful tool to help differentiate DTEs from morpheaform BCCs in that sparse cytokeratin 20-positive Merkel cells can be seen within the basaloid islands of DTE but not BCC.⁵ Also seen with DTEs are numerous keratin horn cysts that commonly are filled with dystrophic calcifications. Cellular atypia and mitoses are not seen (Figure 2). Compared to MACs, DTEs lack abundant ductal structures and also contain papillary mesenchymal bodies and a more fibroblast-rich stroma.

Morpheaform BCC is an aggressive subtype of BCC. It presents as a scarlike plaque that gradually expands. Thin infiltrating strands of basaloid cells are seen haphazardly throughout a pink sclerotic stroma. Tadpolelike basaloid islands and rarely horn cysts can be seen scattered superficially, creating the paisley tie-like pattern. This lesion is more infiltrating than a syringoma or a DTE, and perineural invasion is common. Retraction is uncommon, but when present, it is seen between the epithelial cords and adjacent stroma (Figure 3).

Trichoadenoma is another benign neoplasm of follicular differentiation.⁶ It typically presents as a dome-shaped papule or plaque on the head or neck. Histologically it displays numerous dilated cystic spaces that reflect its origin from isthmic and infundibular differentiation. There is no attachment to the overlying epidermis. It can be distinguished from MAC, DTE, and syringoma due to a lack of basaloid aggregates and only a small number of non-cyst-forming epithelial cells (Figure 4).

The Diagnosis: Microcystic Adnexal Carcinoma

Microcystic adnexal carcinoma (MAC) is a rare, low-grade adnexal carcinoma consisting of both ductal and pilar differentiation.¹ It typically presents in young to middle-aged adults as a flesh-colored or yellow indurated plaque on the upper lip, medial cheek, or chin. Histologically, MACs exhibit a biphasic pattern consisting of epithelial islands of cords and lumina creating tadpolelike ducts intermixed with basaloid nests (quiz image). Keratin horn cysts are common superficially. A dense red sclerotic stroma is seen interspersed between the ducts and epithelial islands creating a "paisley tie" appearance. The lesion displays an infiltrative pattern and can be deeply invasive, extending down to the fat and muscle (quiz image, inset). Perineural invasion is common. Atypia, when present, is minimal or mild and mitoses are rare. Although this tumor's histologic pattern appears aggressive in nature, it lacks immunohistochemical staining such as p53, Ki-67, bcl-2, and c-erbB-2 that correlate with malignant behavior.² A common diagnostic pitfall is examination of a superficial biopsy in which an MAC may be mistakenly identified as another entity.

Syringomas are benign adnexal neoplasms with ductal differentiation.³ They are more common in women, especially those of Asian descent, and in patients with Down syndrome. They typically present as multiple small, firm, flesh-colored papules in the periorbital area or upper trunk. Histologically, syringomas also display comma-shaped tubules and ducts with a tadpolelike appearance and a dense red stroma creating a paisley tie-like pattern. Ductal cells have an abundant pink cytoplasm. Syringomas are well-circumscribed and more superficial than MACs without an infiltrative pattern. They lack mitotic activity or perineural invasion (Figure 1).

Desmoplastic trichoepithelioma (DTE) is a benign follicular neoplasm.⁴ It presents in adulthood with a female predominance. Clinically, it appears as a solitary flesh-colored to yellow annular plaque with raised borders and a depressed central area, often on the medial cheek. Histologically, DTEs are well-circumscribed with narrow branching cords lined with polygonal cells. A dense red stroma in combination with the epithelioid aggregates also creates the paisley tie-like pattern in this lesion. Retraction between collagen bundles within the stroma can be seen, helping distinguish this lesion from a morpheaform basal cell carcinoma (BCC), which has retraction between the epithelium and stroma. Immunohistochemistry also can be a useful tool to help differentiate DTEs from morpheaform BCCs in that sparse cytokeratin 20-positive Merkel cells can be seen within the basaloid islands of DTE but not BCC.⁵ Also seen with DTEs are numerous keratin horn cysts that commonly are filled with dystrophic calcifications. Cellular atypia and mitoses are not seen (Figure 2). Compared to MACs, DTEs lack abundant ductal structures and also contain papillary mesenchymal bodies and a more fibroblast-rich stroma.

Morpheaform BCC is an aggressive subtype of BCC. It presents as a scarlike plaque that gradually expands. Thin infiltrating strands of basaloid cells are seen haphazardly throughout a pink sclerotic stroma. Tadpolelike basaloid islands and rarely horn cysts can be seen scattered superficially, creating the paisley tie-like pattern. This lesion is more infiltrating than a syringoma or a DTE, and perineural invasion is common. Retraction is uncommon, but when present, it is seen between the epithelial cords and adjacent stroma (Figure 3).

Trichoadenoma is another benign neoplasm of follicular differentiation.⁶ It typically presents as a dome-shaped papule or plaque on the head or neck. Histologically it displays numerous dilated cystic spaces that reflect its origin from isthmic and infundibular differentiation. There is no attachment to the overlying epidermis. It can be distinguished from MAC, DTE, and syringoma due to a lack of basaloid aggregates and only a small number of non-cyst-forming epithelial cells (Figure 4).

The Diagnosis: Solitary Fibrous Tumor

Solitary fibrous tumors (SFTs), as first described by Klemperer and Rabin1 in 1931, are relatively uncommon mesenchymal neoplasms that occur primarily in the pleura. This lesion is now known to affect many other extrathoracic sites, such as the liver, kidney, adrenal glands, thyroid, central nervous system, and soft tissue, with rare examples originating from the skin.² Okamura et al³ reported the first known case of cutaneous SFT in 1997, with most of the literature limited to case reports. Erdag et al² described one of the largest case series of primary cutaneous SFTs. These lesions can occur across a wide age range but tend to primarily affect middle-aged adults. Solitary fibrous tumors have been known to have no sex predilection; however, Erdag et al² found a male predominance with a male to female ratio of 4 to 1. 

Histopathologically, a cutaneous SFT is known to appear as a well-circumscribed nodular spindle cell proliferation arranged in interlacing fascicles with an abundant hyalinized collagen stroma (quiz image). Alternating hypocellular and hypercellular areas can be seen. Supporting vasculature often is relatively prominent, represented by angulated and branching staghorn blood vessels (Figure 1).² A common histopathologic finding of SFTs is a patternless pattern, which suggests that the tumor can have a variety of morphologic appearances (eg, storiform, fascicular, neural, herringbone growth patterns), making histologic diagnosis difficult (quiz image).⁴ Therefore, immunohistochemistry plays a large role in the diagnosis of this tumor. The most important positive markers include CD34, CD99, B-cell lymphoma 2 (BCL-2), and signal transducer and activator of transcription 6 (STAT6).⁵ Nuclear STAT6 staining is an immunomarker for NGFI-A binding protein 2 (NAB2)-STAT6 gene fusion, which is specific for SFT.^5,6 Vivero et al⁷ also reported glutamate receptor, inotropic, AMPA 2 (GRIA2) as a useful immunostain in SFT, though it is also expressed in dermatofibrosarcoma protuberans (DFSP). In this case, the clinical and histopathologic findings best supported a diagnosis of SFT. Some consider hemangiopericytomas to be examples of SFTs; however, true hemangiopericytomas lack the thick hyalinized collagen and hypercellular areas seen in SFT.

A cellular dermatofibroma generally presents as a single round, reddish brown papule or nodule approximately 0.5 to 1 cm in diameter that is firm to palpation with a central depression or dimple created over the lesion from the lateral pressure. Cellular dermatofibromas mostly occur in middle-aged adults, with the most common locations on the legs and on the sides of the trunk. They are thought to arise after injuries to the skin. On histopathologic examination, cellular dermatofibromas typically exhibit a proliferation of fibrohistiocytic cells with collagen trapping, often at the periphery of the tumor (Figure 2). Although cellular dermatofibromas appear clinically different than SFTs, they often mimic SFTs histopathologically. Immunostaining also can be helpful in differentiating cellular dermatofibromas in which cells stain positive for factor XIIIa. CD34 staining is negative.

Dermatofibrosarcoma protuberans usually appears as one or multiple firm, red to violaceous nodules or plaques. They most often occur on the trunk in middle-aged adults. Histopathologically, DFSP presents with a dense, hypercellular, spindle cell proliferation that demonstrates a typical storiform pattern. The tumor generally infiltrates into the deep dermis and subcutaneous adipose layer with characteristic adipocyte entrapment (Figure 3). Positive CD34 and negative factor XIIIa staining helps to differentiate DFSP from a cellular dermatofibroma. Immunohistochemically, it is more difficult to distinguish DFSP from SFT, as both are CD34⁺ spindle cell neoplasms that also stain positive for CD99 and BCL-2.² GRIA2 positivity also is seen in both SFT and DFSP.⁷ However, differentiation can be made on morphologic grounds alone, as DFSP has ill-defined tumor borders with adnexal and fat entrapment and SFT tends to be more circumscribed with prominent arborizing hyalinized vessels.⁸

Spindle cell lipoma (SCL) is an asymptomatic subcutaneous tumor commonly located on the back, neck, and shoulders in older patients, typically men. It often presents as a solitary lesion, though multiple lesions may occur. It is a well-circumscribed tumor of mature adipose tissue with areas of spindle cell proliferation and ropey collagen bundles (Figure 4). In early lesions, the spindle cell areas are myxoid with the presence of many mast cells.⁹ The spindle cells stain positive for CD34. Although spindle cell lipoma would be included in both the clinical and histopathologic differential diagnosis for SFT, its histopathologic features often are enough to differentiate SCL, which is highlighted by the aforementioned features as well as a relatively low cellularity and lack of ectatic vessels.⁸ However, discerning tumor variants, such as low-fat pseudoangiomatous SCL and lipomatous or myxoid SFT, might prove more challenging.

Nodular fasciitis typically presents as a rapidly growing subcutaneous nodule that may be tender. It is a benign reactive process usually affecting the arms and trunk of young to middle-aged adults, though it commonly involves the head and neck region in children.¹⁰ The tumor histopathologically appears as a well-circumscribed subcutaneous or fascial nodule with an angulated appearance. Spindle-shaped and stellate fibroblasts are loosely arranged in an edematous myxomatous stroma with a feathered appearance (Figure 5). Extravasated erythrocytes often are present. With time, collagen bundles become thicker and hyalinized. Immunohistochemical studies demonstrate positivity for vimentin, calponin, muscle-specific actin, and smooth muscle actin. Desmin, CD34, cytokeratin, and S-100 typically are negative.^10-12 Therefore, CD34 staining is one of the main differentiating factors between nodular fasciitis and SFTs.

The Diagnosis: Solitary Fibrous Tumor

Solitary fibrous tumors (SFTs), as first described by Klemperer and Rabin1 in 1931, are relatively uncommon mesenchymal neoplasms that occur primarily in the pleura. This lesion is now known to affect many other extrathoracic sites, such as the liver, kidney, adrenal glands, thyroid, central nervous system, and soft tissue, with rare examples originating from the skin.² Okamura et al³ reported the first known case of cutaneous SFT in 1997, with most of the literature limited to case reports. Erdag et al² described one of the largest case series of primary cutaneous SFTs. These lesions can occur across a wide age range but tend to primarily affect middle-aged adults. Solitary fibrous tumors have been known to have no sex predilection; however, Erdag et al² found a male predominance with a male to female ratio of 4 to 1. 

Histopathologically, a cutaneous SFT is known to appear as a well-circumscribed nodular spindle cell proliferation arranged in interlacing fascicles with an abundant hyalinized collagen stroma (quiz image). Alternating hypocellular and hypercellular areas can be seen. Supporting vasculature often is relatively prominent, represented by angulated and branching staghorn blood vessels (Figure 1).² A common histopathologic finding of SFTs is a patternless pattern, which suggests that the tumor can have a variety of morphologic appearances (eg, storiform, fascicular, neural, herringbone growth patterns), making histologic diagnosis difficult (quiz image).⁴ Therefore, immunohistochemistry plays a large role in the diagnosis of this tumor. The most important positive markers include CD34, CD99, B-cell lymphoma 2 (BCL-2), and signal transducer and activator of transcription 6 (STAT6).⁵ Nuclear STAT6 staining is an immunomarker for NGFI-A binding protein 2 (NAB2)-STAT6 gene fusion, which is specific for SFT.^5,6 Vivero et al⁷ also reported glutamate receptor, inotropic, AMPA 2 (GRIA2) as a useful immunostain in SFT, though it is also expressed in dermatofibrosarcoma protuberans (DFSP). In this case, the clinical and histopathologic findings best supported a diagnosis of SFT. Some consider hemangiopericytomas to be examples of SFTs; however, true hemangiopericytomas lack the thick hyalinized collagen and hypercellular areas seen in SFT.

A cellular dermatofibroma generally presents as a single round, reddish brown papule or nodule approximately 0.5 to 1 cm in diameter that is firm to palpation with a central depression or dimple created over the lesion from the lateral pressure. Cellular dermatofibromas mostly occur in middle-aged adults, with the most common locations on the legs and on the sides of the trunk. They are thought to arise after injuries to the skin. On histopathologic examination, cellular dermatofibromas typically exhibit a proliferation of fibrohistiocytic cells with collagen trapping, often at the periphery of the tumor (Figure 2). Although cellular dermatofibromas appear clinically different than SFTs, they often mimic SFTs histopathologically. Immunostaining also can be helpful in differentiating cellular dermatofibromas in which cells stain positive for factor XIIIa. CD34 staining is negative.

Dermatofibrosarcoma protuberans usually appears as one or multiple firm, red to violaceous nodules or plaques. They most often occur on the trunk in middle-aged adults. Histopathologically, DFSP presents with a dense, hypercellular, spindle cell proliferation that demonstrates a typical storiform pattern. The tumor generally infiltrates into the deep dermis and subcutaneous adipose layer with characteristic adipocyte entrapment (Figure 3). Positive CD34 and negative factor XIIIa staining helps to differentiate DFSP from a cellular dermatofibroma. Immunohistochemically, it is more difficult to distinguish DFSP from SFT, as both are CD34⁺ spindle cell neoplasms that also stain positive for CD99 and BCL-2.² GRIA2 positivity also is seen in both SFT and DFSP.⁷ However, differentiation can be made on morphologic grounds alone, as DFSP has ill-defined tumor borders with adnexal and fat entrapment and SFT tends to be more circumscribed with prominent arborizing hyalinized vessels.⁸

Spindle cell lipoma (SCL) is an asymptomatic subcutaneous tumor commonly located on the back, neck, and shoulders in older patients, typically men. It often presents as a solitary lesion, though multiple lesions may occur. It is a well-circumscribed tumor of mature adipose tissue with areas of spindle cell proliferation and ropey collagen bundles (Figure 4). In early lesions, the spindle cell areas are myxoid with the presence of many mast cells.⁹ The spindle cells stain positive for CD34. Although spindle cell lipoma would be included in both the clinical and histopathologic differential diagnosis for SFT, its histopathologic features often are enough to differentiate SCL, which is highlighted by the aforementioned features as well as a relatively low cellularity and lack of ectatic vessels.⁸ However, discerning tumor variants, such as low-fat pseudoangiomatous SCL and lipomatous or myxoid SFT, might prove more challenging.

Nodular fasciitis typically presents as a rapidly growing subcutaneous nodule that may be tender. It is a benign reactive process usually affecting the arms and trunk of young to middle-aged adults, though it commonly involves the head and neck region in children.¹⁰ The tumor histopathologically appears as a well-circumscribed subcutaneous or fascial nodule with an angulated appearance. Spindle-shaped and stellate fibroblasts are loosely arranged in an edematous myxomatous stroma with a feathered appearance (Figure 5). Extravasated erythrocytes often are present. With time, collagen bundles become thicker and hyalinized. Immunohistochemical studies demonstrate positivity for vimentin, calponin, muscle-specific actin, and smooth muscle actin. Desmin, CD34, cytokeratin, and S-100 typically are negative.^10-12 Therefore, CD34 staining is one of the main differentiating factors between nodular fasciitis and SFTs.

The Diagnosis: Solitary Fibrous Tumor

Solitary fibrous tumors (SFTs), as first described by Klemperer and Rabin1 in 1931, are relatively uncommon mesenchymal neoplasms that occur primarily in the pleura. This lesion is now known to affect many other extrathoracic sites, such as the liver, kidney, adrenal glands, thyroid, central nervous system, and soft tissue, with rare examples originating from the skin.² Okamura et al³ reported the first known case of cutaneous SFT in 1997, with most of the literature limited to case reports. Erdag et al² described one of the largest case series of primary cutaneous SFTs. These lesions can occur across a wide age range but tend to primarily affect middle-aged adults. Solitary fibrous tumors have been known to have no sex predilection; however, Erdag et al² found a male predominance with a male to female ratio of 4 to 1. 

Histopathologically, a cutaneous SFT is known to appear as a well-circumscribed nodular spindle cell proliferation arranged in interlacing fascicles with an abundant hyalinized collagen stroma (quiz image). Alternating hypocellular and hypercellular areas can be seen. Supporting vasculature often is relatively prominent, represented by angulated and branching staghorn blood vessels (Figure 1).² A common histopathologic finding of SFTs is a patternless pattern, which suggests that the tumor can have a variety of morphologic appearances (eg, storiform, fascicular, neural, herringbone growth patterns), making histologic diagnosis difficult (quiz image).⁴ Therefore, immunohistochemistry plays a large role in the diagnosis of this tumor. The most important positive markers include CD34, CD99, B-cell lymphoma 2 (BCL-2), and signal transducer and activator of transcription 6 (STAT6).⁵ Nuclear STAT6 staining is an immunomarker for NGFI-A binding protein 2 (NAB2)-STAT6 gene fusion, which is specific for SFT.^5,6 Vivero et al⁷ also reported glutamate receptor, inotropic, AMPA 2 (GRIA2) as a useful immunostain in SFT, though it is also expressed in dermatofibrosarcoma protuberans (DFSP). In this case, the clinical and histopathologic findings best supported a diagnosis of SFT. Some consider hemangiopericytomas to be examples of SFTs; however, true hemangiopericytomas lack the thick hyalinized collagen and hypercellular areas seen in SFT.

A cellular dermatofibroma generally presents as a single round, reddish brown papule or nodule approximately 0.5 to 1 cm in diameter that is firm to palpation with a central depression or dimple created over the lesion from the lateral pressure. Cellular dermatofibromas mostly occur in middle-aged adults, with the most common locations on the legs and on the sides of the trunk. They are thought to arise after injuries to the skin. On histopathologic examination, cellular dermatofibromas typically exhibit a proliferation of fibrohistiocytic cells with collagen trapping, often at the periphery of the tumor (Figure 2). Although cellular dermatofibromas appear clinically different than SFTs, they often mimic SFTs histopathologically. Immunostaining also can be helpful in differentiating cellular dermatofibromas in which cells stain positive for factor XIIIa. CD34 staining is negative.

Dermatofibrosarcoma protuberans usually appears as one or multiple firm, red to violaceous nodules or plaques. They most often occur on the trunk in middle-aged adults. Histopathologically, DFSP presents with a dense, hypercellular, spindle cell proliferation that demonstrates a typical storiform pattern. The tumor generally infiltrates into the deep dermis and subcutaneous adipose layer with characteristic adipocyte entrapment (Figure 3). Positive CD34 and negative factor XIIIa staining helps to differentiate DFSP from a cellular dermatofibroma. Immunohistochemically, it is more difficult to distinguish DFSP from SFT, as both are CD34⁺ spindle cell neoplasms that also stain positive for CD99 and BCL-2.² GRIA2 positivity also is seen in both SFT and DFSP.⁷ However, differentiation can be made on morphologic grounds alone, as DFSP has ill-defined tumor borders with adnexal and fat entrapment and SFT tends to be more circumscribed with prominent arborizing hyalinized vessels.⁸

Spindle cell lipoma (SCL) is an asymptomatic subcutaneous tumor commonly located on the back, neck, and shoulders in older patients, typically men. It often presents as a solitary lesion, though multiple lesions may occur. It is a well-circumscribed tumor of mature adipose tissue with areas of spindle cell proliferation and ropey collagen bundles (Figure 4). In early lesions, the spindle cell areas are myxoid with the presence of many mast cells.⁹ The spindle cells stain positive for CD34. Although spindle cell lipoma would be included in both the clinical and histopathologic differential diagnosis for SFT, its histopathologic features often are enough to differentiate SCL, which is highlighted by the aforementioned features as well as a relatively low cellularity and lack of ectatic vessels.⁸ However, discerning tumor variants, such as low-fat pseudoangiomatous SCL and lipomatous or myxoid SFT, might prove more challenging.

Nodular fasciitis typically presents as a rapidly growing subcutaneous nodule that may be tender. It is a benign reactive process usually affecting the arms and trunk of young to middle-aged adults, though it commonly involves the head and neck region in children.¹⁰ The tumor histopathologically appears as a well-circumscribed subcutaneous or fascial nodule with an angulated appearance. Spindle-shaped and stellate fibroblasts are loosely arranged in an edematous myxomatous stroma with a feathered appearance (Figure 5). Extravasated erythrocytes often are present. With time, collagen bundles become thicker and hyalinized. Immunohistochemical studies demonstrate positivity for vimentin, calponin, muscle-specific actin, and smooth muscle actin. Desmin, CD34, cytokeratin, and S-100 typically are negative.^10-12 Therefore, CD34 staining is one of the main differentiating factors between nodular fasciitis and SFTs.

The Diagnosis: Inverted Follicular Keratosis

The differential diagnosis for endophytic squamous neoplasms encompasses benign and malignant entities. The histologic findings of our patient's lesion were compatible with the diagnosis of inverted follicular keratosis (IFK), a benign neoplasm that usually presents as a keratotic papule on the head or neck. Histologically, IFK is characterized by an endophytic growth pattern with squamous eddies (quiz images). Inverted follicular keratosis may represent an irritated seborrheic keratosis or a distinct neoplasm derived from the infundibular portion of the hair follicle; the exact etiology is uncertain.^1,2 No relationship between IFK and human papillomavirus (HPV) has been established.³ Inverted follicular keratosis can mimic squamous cell carcinoma (SCC). Important clues to the diagnosis of IFK are the presence of squamous eddies and the lack of squamous pearls or cytologic atypia.⁴ Squamous eddies consist of whorled keratinocytes without keratinization or atypia. Superficial shave biopsies may fail to demonstrate the characteristic well-circumscribed architecture and may lead to an erroneous diagnosis.

Acantholytic SCC is characterized by atypical keratinocytes that have lost cohesive properties, resulting in acantholysis (Figure 1).⁵ This histologic variant was once categorized as an aggressive variant of SCC, but studies have failed to support this assertion.^5,6 Acantholytic SCC has a discohesive nature producing a pseudoglandular appearance sometimes mistaken for adenosquamous carcinoma or metastatic carcinoma. Recent literature has suggested that acantholytic SCCs, similar to IFKs, are derived from the follicular infundibulum.^5,6 Also similar to IFKs, acantholytic SCCs often are located on the face. The invasive architecture and atypical cytology of acantholytic SCCs can differentiate them from IFKs. Acantholytic SCCs can contain keratin pearls with concentric keratinocytes showing incomplete keratinization centrally, often with retained nuclei, but rare to no squamous eddies unless irritated.

Trichilemmoma is an endophytic benign neoplasm derived from the outer sheath of the pilosebaceous follicle characterized by lobules of clear cells hanging from the epidermis.⁷ A study investigating the relationship between HPV and trichilemmomas failed to definitively detect HPV in trichilemmomas and this relationship remains unclear.⁸ Desmoplastic trichilemmoma is a subtype histologically characterized by jagged islands of epithelial cells separated by dense pink stroma and encased in a glassy basement membrane (Figure 2). The presence of desmoplasia and a jagged growth pattern can mimic invasive SCC, but the absence of cytologic atypia and the surrounding basement membrane differs from SCC.^4,7 Trichilemmomas typically are solitary, but multiple lesions are associated with Cowden syndrome. Cowden syndrome is a rare autosomal-dominant condition characterized by the presence of benign hamartomas and a predisposition to the development of malignancies including breast, endometrial, and thyroid cancers.^9,10 There is no such association with desmoplastic trichilemmomas.¹¹

Pilar sheath acanthoma is a benign neoplasm that clinically presents as a solitary flesh-colored nodule with a central pore containing keratin.¹² Histologically, pilar sheath acanthoma is similar to a dilated pore of Winer with the addition of acanthotic epidermal projections (Figure 3).

Warty dyskeratoma (WD) is a benign endophytic neoplasm traditionally seen as a solitary lesion histologically similar to Darier disease. Warty dyskeratomas are known to occur both on the skin and oral mucosa.¹³ Histologically, WD is characterized as a cup-shaped lesion with numerous villi at the base of the lesion along with acantholysis and dyskeratosis (Figure 4). The dyskeratotic cells in WD consist of corps ronds, which are cells with abundant pink cytoplasm, and small nuclei along with grains, which are flattened basophilic cells. These dyskeratotic cells help differentiate WD from IFK. Although they are endophytic neoplasms, WDs are well circumscribed and should not be confused with SCC. Despite this entity's name and histologic similarity to verrucae, no relationship with HPV has been established.¹⁴

The Diagnosis: Inverted Follicular Keratosis

The differential diagnosis for endophytic squamous neoplasms encompasses benign and malignant entities. The histologic findings of our patient's lesion were compatible with the diagnosis of inverted follicular keratosis (IFK), a benign neoplasm that usually presents as a keratotic papule on the head or neck. Histologically, IFK is characterized by an endophytic growth pattern with squamous eddies (quiz images). Inverted follicular keratosis may represent an irritated seborrheic keratosis or a distinct neoplasm derived from the infundibular portion of the hair follicle; the exact etiology is uncertain.^1,2 No relationship between IFK and human papillomavirus (HPV) has been established.³ Inverted follicular keratosis can mimic squamous cell carcinoma (SCC). Important clues to the diagnosis of IFK are the presence of squamous eddies and the lack of squamous pearls or cytologic atypia.⁴ Squamous eddies consist of whorled keratinocytes without keratinization or atypia. Superficial shave biopsies may fail to demonstrate the characteristic well-circumscribed architecture and may lead to an erroneous diagnosis.

Acantholytic SCC is characterized by atypical keratinocytes that have lost cohesive properties, resulting in acantholysis (Figure 1).⁵ This histologic variant was once categorized as an aggressive variant of SCC, but studies have failed to support this assertion.^5,6 Acantholytic SCC has a discohesive nature producing a pseudoglandular appearance sometimes mistaken for adenosquamous carcinoma or metastatic carcinoma. Recent literature has suggested that acantholytic SCCs, similar to IFKs, are derived from the follicular infundibulum.^5,6 Also similar to IFKs, acantholytic SCCs often are located on the face. The invasive architecture and atypical cytology of acantholytic SCCs can differentiate them from IFKs. Acantholytic SCCs can contain keratin pearls with concentric keratinocytes showing incomplete keratinization centrally, often with retained nuclei, but rare to no squamous eddies unless irritated.

Trichilemmoma is an endophytic benign neoplasm derived from the outer sheath of the pilosebaceous follicle characterized by lobules of clear cells hanging from the epidermis.⁷ A study investigating the relationship between HPV and trichilemmomas failed to definitively detect HPV in trichilemmomas and this relationship remains unclear.⁸ Desmoplastic trichilemmoma is a subtype histologically characterized by jagged islands of epithelial cells separated by dense pink stroma and encased in a glassy basement membrane (Figure 2). The presence of desmoplasia and a jagged growth pattern can mimic invasive SCC, but the absence of cytologic atypia and the surrounding basement membrane differs from SCC.^4,7 Trichilemmomas typically are solitary, but multiple lesions are associated with Cowden syndrome. Cowden syndrome is a rare autosomal-dominant condition characterized by the presence of benign hamartomas and a predisposition to the development of malignancies including breast, endometrial, and thyroid cancers.^9,10 There is no such association with desmoplastic trichilemmomas.¹¹

Pilar sheath acanthoma is a benign neoplasm that clinically presents as a solitary flesh-colored nodule with a central pore containing keratin.¹² Histologically, pilar sheath acanthoma is similar to a dilated pore of Winer with the addition of acanthotic epidermal projections (Figure 3).

Warty dyskeratoma (WD) is a benign endophytic neoplasm traditionally seen as a solitary lesion histologically similar to Darier disease. Warty dyskeratomas are known to occur both on the skin and oral mucosa.¹³ Histologically, WD is characterized as a cup-shaped lesion with numerous villi at the base of the lesion along with acantholysis and dyskeratosis (Figure 4). The dyskeratotic cells in WD consist of corps ronds, which are cells with abundant pink cytoplasm, and small nuclei along with grains, which are flattened basophilic cells. These dyskeratotic cells help differentiate WD from IFK. Although they are endophytic neoplasms, WDs are well circumscribed and should not be confused with SCC. Despite this entity's name and histologic similarity to verrucae, no relationship with HPV has been established.¹⁴

The Diagnosis: Inverted Follicular Keratosis

The differential diagnosis for endophytic squamous neoplasms encompasses benign and malignant entities. The histologic findings of our patient's lesion were compatible with the diagnosis of inverted follicular keratosis (IFK), a benign neoplasm that usually presents as a keratotic papule on the head or neck. Histologically, IFK is characterized by an endophytic growth pattern with squamous eddies (quiz images). Inverted follicular keratosis may represent an irritated seborrheic keratosis or a distinct neoplasm derived from the infundibular portion of the hair follicle; the exact etiology is uncertain.^1,2 No relationship between IFK and human papillomavirus (HPV) has been established.³ Inverted follicular keratosis can mimic squamous cell carcinoma (SCC). Important clues to the diagnosis of IFK are the presence of squamous eddies and the lack of squamous pearls or cytologic atypia.⁴ Squamous eddies consist of whorled keratinocytes without keratinization or atypia. Superficial shave biopsies may fail to demonstrate the characteristic well-circumscribed architecture and may lead to an erroneous diagnosis.

Acantholytic SCC is characterized by atypical keratinocytes that have lost cohesive properties, resulting in acantholysis (Figure 1).⁵ This histologic variant was once categorized as an aggressive variant of SCC, but studies have failed to support this assertion.^5,6 Acantholytic SCC has a discohesive nature producing a pseudoglandular appearance sometimes mistaken for adenosquamous carcinoma or metastatic carcinoma. Recent literature has suggested that acantholytic SCCs, similar to IFKs, are derived from the follicular infundibulum.^5,6 Also similar to IFKs, acantholytic SCCs often are located on the face. The invasive architecture and atypical cytology of acantholytic SCCs can differentiate them from IFKs. Acantholytic SCCs can contain keratin pearls with concentric keratinocytes showing incomplete keratinization centrally, often with retained nuclei, but rare to no squamous eddies unless irritated.

Trichilemmoma is an endophytic benign neoplasm derived from the outer sheath of the pilosebaceous follicle characterized by lobules of clear cells hanging from the epidermis.⁷ A study investigating the relationship between HPV and trichilemmomas failed to definitively detect HPV in trichilemmomas and this relationship remains unclear.⁸ Desmoplastic trichilemmoma is a subtype histologically characterized by jagged islands of epithelial cells separated by dense pink stroma and encased in a glassy basement membrane (Figure 2). The presence of desmoplasia and a jagged growth pattern can mimic invasive SCC, but the absence of cytologic atypia and the surrounding basement membrane differs from SCC.^4,7 Trichilemmomas typically are solitary, but multiple lesions are associated with Cowden syndrome. Cowden syndrome is a rare autosomal-dominant condition characterized by the presence of benign hamartomas and a predisposition to the development of malignancies including breast, endometrial, and thyroid cancers.^9,10 There is no such association with desmoplastic trichilemmomas.¹¹

Pilar sheath acanthoma is a benign neoplasm that clinically presents as a solitary flesh-colored nodule with a central pore containing keratin.¹² Histologically, pilar sheath acanthoma is similar to a dilated pore of Winer with the addition of acanthotic epidermal projections (Figure 3).

Warty dyskeratoma (WD) is a benign endophytic neoplasm traditionally seen as a solitary lesion histologically similar to Darier disease. Warty dyskeratomas are known to occur both on the skin and oral mucosa.¹³ Histologically, WD is characterized as a cup-shaped lesion with numerous villi at the base of the lesion along with acantholysis and dyskeratosis (Figure 4). The dyskeratotic cells in WD consist of corps ronds, which are cells with abundant pink cytoplasm, and small nuclei along with grains, which are flattened basophilic cells. These dyskeratotic cells help differentiate WD from IFK. Although they are endophytic neoplasms, WDs are well circumscribed and should not be confused with SCC. Despite this entity's name and histologic similarity to verrucae, no relationship with HPV has been established.¹⁴

The Diagnosis: Rosai-Dorfman Disease

Rosai-Dorfman disease is a rare histiocytic proliferative disorder of unknown etiology. It has 2 forms: limited cutaneous and systemic. The systemic form, also known as sinus histiocytosis with massive lym­phadenopathy, affects the lymph nodes and other organs at times. The disease is characterized by a proliferation of histiocytes in the lymph nodes, most commonly in the cervical basin¹; however, the inguinal, axillary, mediastinal, or para-aortic nodes also may be affected.^1,2 The skin is the most common site of extranodal disease, seen in approximately 10% of cases.¹ Cutaneous involvement often is in the facial area but also can be found on the trunk, ears, neck, arms, legs, and genitals. Clinically, skin lesions appear as papules, plaques, and/or nodules.²

Histopathologic examination of Rosai-Dorfman disease generally shows a dense sheetlike dermal infiltrate of large polygonal histiocytes (Figure 1). Histiocytes may display pale pink or clear cytoplasm. The pathognomonic finding is emperipolesis, which consists of histiocytes with engulfed lymphocytes, erythrocytes, plasma cells, and/or granulocytes surrounded by a clear halo. Immunohistochemical staining also is characteristic, with lesional histiocytes showing expression of S-100 protein (Figure 1, inset) and CD68. The associated inflammatory infiltrate is mixed, containing primarily plasma cells but also lymphocytes, neutrophils, and eosinophils.

Blastomycosis (Figure 2) is a systemic infection due to inhalation of Blastomyces dermatitidis conidia. Primary infection occurs in the lungs, and with dissemination the skin is the most common subsequently involved organ.³ Cutaneous blastomycosis shows pseudoepitheliomatous hyperplasia with neutrophilic microabscesses and a dense dermal infiltrate containing suppurative granulomatous inflammation. The nonpigmented yeast phase typically is 8 to 15 µm in length with a refractile cell wall and characteristic single, broad-based budding.³

Granuloma faciale (Figure 3) is a rare disease with unknown etiology characterized by reddish brown plaques or nodules most commonly occurring on the face.^4,5 Histology shows a dense nodular dermal infiltrate with a grenz zone. The infiltrate is mixed, containing mostly neutrophils with leukocytoclasis and eosinophils. Leukocytoclastic vasculitis is present with associated extravasated erythrocytes. In chronic fibrosing granuloma faciale, lesions can demonstrate fibrosis and hemosiderin deposition, similar to erythema elevatum diutinum.

Juvenile xanthogranuloma (Figure 4) is a common histiocytic disease of early childhood, though adult cases have been reported.⁶ Tumors are found on the head and trunk and are typically firm, reddish yellow papules or nodules.^6,7 Histologic examination shows a nodular infiltrate of foamy histiocytes in the superficial dermis. Touton-type multinucleated giant cells with a peripheral rim of xanthomatized foamy cytoplasm and a wreathlike arrangement of nuclei are characteristic. Associated eosinophils are seen. No emperipolesis is present.

Reticulohistiocytoma (Figure 5) is a benign dermal lesion that presents as solitary or less commonly multiple red-brown papules or nodules.⁸ Lesions consist of well-delineated nodular aggregates of histiocytes containing a finely granular eosinophilic ground glass cytoplasm. Few, if any, eosinophils are found. The lack of Touton multinucleated giant cells or emperipolesis and lack of expression of S-100 protein helps to distinguish reticulohistiocytoma from other entities in the differential diagnosis.

The Diagnosis: Rosai-Dorfman Disease

Rosai-Dorfman disease is a rare histiocytic proliferative disorder of unknown etiology. It has 2 forms: limited cutaneous and systemic. The systemic form, also known as sinus histiocytosis with massive lym­phadenopathy, affects the lymph nodes and other organs at times. The disease is characterized by a proliferation of histiocytes in the lymph nodes, most commonly in the cervical basin¹; however, the inguinal, axillary, mediastinal, or para-aortic nodes also may be affected.^1,2 The skin is the most common site of extranodal disease, seen in approximately 10% of cases.¹ Cutaneous involvement often is in the facial area but also can be found on the trunk, ears, neck, arms, legs, and genitals. Clinically, skin lesions appear as papules, plaques, and/or nodules.²

Histopathologic examination of Rosai-Dorfman disease generally shows a dense sheetlike dermal infiltrate of large polygonal histiocytes (Figure 1). Histiocytes may display pale pink or clear cytoplasm. The pathognomonic finding is emperipolesis, which consists of histiocytes with engulfed lymphocytes, erythrocytes, plasma cells, and/or granulocytes surrounded by a clear halo. Immunohistochemical staining also is characteristic, with lesional histiocytes showing expression of S-100 protein (Figure 1, inset) and CD68. The associated inflammatory infiltrate is mixed, containing primarily plasma cells but also lymphocytes, neutrophils, and eosinophils.

Blastomycosis (Figure 2) is a systemic infection due to inhalation of Blastomyces dermatitidis conidia. Primary infection occurs in the lungs, and with dissemination the skin is the most common subsequently involved organ.³ Cutaneous blastomycosis shows pseudoepitheliomatous hyperplasia with neutrophilic microabscesses and a dense dermal infiltrate containing suppurative granulomatous inflammation. The nonpigmented yeast phase typically is 8 to 15 µm in length with a refractile cell wall and characteristic single, broad-based budding.³

Granuloma faciale (Figure 3) is a rare disease with unknown etiology characterized by reddish brown plaques or nodules most commonly occurring on the face.^4,5 Histology shows a dense nodular dermal infiltrate with a grenz zone. The infiltrate is mixed, containing mostly neutrophils with leukocytoclasis and eosinophils. Leukocytoclastic vasculitis is present with associated extravasated erythrocytes. In chronic fibrosing granuloma faciale, lesions can demonstrate fibrosis and hemosiderin deposition, similar to erythema elevatum diutinum.

Juvenile xanthogranuloma (Figure 4) is a common histiocytic disease of early childhood, though adult cases have been reported.⁶ Tumors are found on the head and trunk and are typically firm, reddish yellow papules or nodules.^6,7 Histologic examination shows a nodular infiltrate of foamy histiocytes in the superficial dermis. Touton-type multinucleated giant cells with a peripheral rim of xanthomatized foamy cytoplasm and a wreathlike arrangement of nuclei are characteristic. Associated eosinophils are seen. No emperipolesis is present.

Reticulohistiocytoma (Figure 5) is a benign dermal lesion that presents as solitary or less commonly multiple red-brown papules or nodules.⁸ Lesions consist of well-delineated nodular aggregates of histiocytes containing a finely granular eosinophilic ground glass cytoplasm. Few, if any, eosinophils are found. The lack of Touton multinucleated giant cells or emperipolesis and lack of expression of S-100 protein helps to distinguish reticulohistiocytoma from other entities in the differential diagnosis.

The Diagnosis: Rosai-Dorfman Disease

Rosai-Dorfman disease is a rare histiocytic proliferative disorder of unknown etiology. It has 2 forms: limited cutaneous and systemic. The systemic form, also known as sinus histiocytosis with massive lym­phadenopathy, affects the lymph nodes and other organs at times. The disease is characterized by a proliferation of histiocytes in the lymph nodes, most commonly in the cervical basin¹; however, the inguinal, axillary, mediastinal, or para-aortic nodes also may be affected.^1,2 The skin is the most common site of extranodal disease, seen in approximately 10% of cases.¹ Cutaneous involvement often is in the facial area but also can be found on the trunk, ears, neck, arms, legs, and genitals. Clinically, skin lesions appear as papules, plaques, and/or nodules.²

Histopathologic examination of Rosai-Dorfman disease generally shows a dense sheetlike dermal infiltrate of large polygonal histiocytes (Figure 1). Histiocytes may display pale pink or clear cytoplasm. The pathognomonic finding is emperipolesis, which consists of histiocytes with engulfed lymphocytes, erythrocytes, plasma cells, and/or granulocytes surrounded by a clear halo. Immunohistochemical staining also is characteristic, with lesional histiocytes showing expression of S-100 protein (Figure 1, inset) and CD68. The associated inflammatory infiltrate is mixed, containing primarily plasma cells but also lymphocytes, neutrophils, and eosinophils.

Blastomycosis (Figure 2) is a systemic infection due to inhalation of Blastomyces dermatitidis conidia. Primary infection occurs in the lungs, and with dissemination the skin is the most common subsequently involved organ.³ Cutaneous blastomycosis shows pseudoepitheliomatous hyperplasia with neutrophilic microabscesses and a dense dermal infiltrate containing suppurative granulomatous inflammation. The nonpigmented yeast phase typically is 8 to 15 µm in length with a refractile cell wall and characteristic single, broad-based budding.³

Granuloma faciale (Figure 3) is a rare disease with unknown etiology characterized by reddish brown plaques or nodules most commonly occurring on the face.^4,5 Histology shows a dense nodular dermal infiltrate with a grenz zone. The infiltrate is mixed, containing mostly neutrophils with leukocytoclasis and eosinophils. Leukocytoclastic vasculitis is present with associated extravasated erythrocytes. In chronic fibrosing granuloma faciale, lesions can demonstrate fibrosis and hemosiderin deposition, similar to erythema elevatum diutinum.

Juvenile xanthogranuloma (Figure 4) is a common histiocytic disease of early childhood, though adult cases have been reported.⁶ Tumors are found on the head and trunk and are typically firm, reddish yellow papules or nodules.^6,7 Histologic examination shows a nodular infiltrate of foamy histiocytes in the superficial dermis. Touton-type multinucleated giant cells with a peripheral rim of xanthomatized foamy cytoplasm and a wreathlike arrangement of nuclei are characteristic. Associated eosinophils are seen. No emperipolesis is present.

Reticulohistiocytoma (Figure 5) is a benign dermal lesion that presents as solitary or less commonly multiple red-brown papules or nodules.⁸ Lesions consist of well-delineated nodular aggregates of histiocytes containing a finely granular eosinophilic ground glass cytoplasm. Few, if any, eosinophils are found. The lack of Touton multinucleated giant cells or emperipolesis and lack of expression of S-100 protein helps to distinguish reticulohistiocytoma from other entities in the differential diagnosis.

The Diagnosis: Grover Disease

Grover disease (also known as transient acantholytic dermatosis) was first described by Ralph W. Grover in 1970 as an idiopathic, acquired, monomorphous, papulovesicular eruption. Although originally characterized by solely transient acantholytic dermatosis, over time the term Grover disease has been expanded to include persistent acantholytic dermatoses. Grover disease chiefly affects white adults older than 40 years and is more prevalent in males than females. Cases generally are self-limited but correlate with age, as older adults are more likely to have prolonged eruptions.¹

Grover disease typically erupts with discrete, erythematous, edematous, acneform, red-brown or flesh-colored papules, papulovesicles, or keratotic papules that primarily are seen on the trunk and anterior portion of the chest. As the rash spreads, it can erupt on the neck and thighs. The etiology of Grover disease is unknown, but many factors have been associated with the condition in a limited number of patients, including exposure to UV radiation, excessive heat or sweating, use of sulfadoxine-pyrimethamine and recombinant human IL-4, and infection with Malassezia furfur and Demodex folliculorum.¹ Grover disease also has been associated with other conditions such as asteatotic eczema, allergic contact dermatitis, and atopic dermatitis.²

Histologically, Grover disease (Figure 1) is an acantholytic process that can exhibit dyskeratosis (corps ronds and grains). Foci often are small and multiple foci are seen on shave biopsy. There also may be spongiotic changes when associated with an eczematous element. A perivascular lymphohistiocytic infiltrate with eosinophils usually is seen.³ Basket weave keratin may be seen; however, as the lesions cause pruritus, erosions and ulcerations often are present.⁴

Grover disease has multiple histologic variants that may resemble Darier disease, Hailey-Hailey disease, pemphigus foliaceus, pemphigus vulgaris, and spongiotic dermatitis and can present in combination.⁵

The variant of Grover disease that has a Darier-like pattern is difficult to distinguish from Darier disease, an autosomal-dominant-inherited disorder classified by small papules that emerge in seborrheic areas during childhood and adolescence. Histologically, Darier disease (Figure 2) shows broad areas of dyskeratosis and acantholysis that lead to suprabasal cleavage. Follicular extension may be present. In addition, there often is prominent vertical parakeratosis in Darier disease.⁶ Histologic features that favor Darier disease over the Darier-like variant of Grover disease include a broad focus of acanthotic dyskeratosis with follicular extension; the presence of a hyperkeratotic stratum corneum; and a lack of spongiosis and eosinophils, which are notably absent in Darier disease but may be present in Grover disease.⁴

Another variant of Grover disease has a Hailey-Hailey-like pattern, which is characterized by Hailey-Hailey disease's dilapidated brick wall appearance or the diffuse suprabasal acantholysis of all epidermal layers without notable dyskeratosis.⁴ Hailey-Hailey disease, also known as familial benign pemphigus, is an autosomal-dominant disorder that presents with erythematous vesicular plaques in flexural areas. The plaques progress to flaccid bullae with rupture and crusting and spread peripherally.⁷ Pathology shows suprabasilar clefts and numerous acantholytic cells (Figure 3). Dyskeratotic keratinocytes are rare with infrequent corps ronds and rare grains. The epidermis also is less hyperplastic in Grover disease than in Hailey-Hailey disease.¹

Grover disease also may present histologically with a pemphiguslike pattern, mimicking pemphigus foliaceus and pemphigus vulgaris; however, direct immunofluorescence studies are negative in Grover disease.

Pemphigus foliaceus is an autoimmune disorder caused by autoantibodies to desmoglein 1, which are present on the surfaces of keratinocytes, and is characterized by scaly crusts and blisters.⁸ Histologically, pemphigus foliaceus (Figure 4) shows a superficial epidermal blistering process. The acantholysis may be subtle and is commonly localized to the stratum granulosum, extending into the stratum corneum. Complete loss of the stratum corneum can be seen, resulting in only scattered acantholytic cells. Spongiosis also may be seen. The dermis shows a perivascular infiltrate that often contains eosinophils. Pemphigus foliaceus is confirmed by direct immunofluorescence.⁹

Pemphigus vulgaris is an autoimmune blistering disorder that is characterized by IgG autoantibodies to desmoglein 3, a component of desmosomes that are involved in keratinocyte-to-keratinocyte adhesion. Clinically, patients present with flaccid fragile blisters on the skin and mucous membranes that rupture easily, leading to painful erosions.¹⁰ Intraepidermal blisters are seen histologically (Figure 5) with the loss of cohesion (acantholysis) seen classically in the lower portions of the epidermis where desmoglein 3 is most prominent. When only the basal layer remains, the histology has been likened to a tombstone row.¹¹ Extension of the blister along the adnexa is common. The underlying dermis shows a perivascular infiltrate with eosinophils. Early lesions may show only eosinophilic spongiosis. Direct immunofluorescence studies show IgG and C3 in an intercellular pattern that resembles a fish net or chicken wire.^4,11

The spongioticlike pattern of Grover disease is marked by epidermal edema with separation of the keratinocytes and the revelation of their intracellular bridges,⁴ which manifests as vesiculation in the stratum corneum or upper layers of the epidermis.¹²

Grover disease is self-limited and may spontaneously resolve; however, the disease may be responsive to topical and systemic steroids. Additionally, avoidance of aggravating factors such as sunlight, heat, and sweating can improve symptoms.²

The Diagnosis: Grover Disease

Grover disease (also known as transient acantholytic dermatosis) was first described by Ralph W. Grover in 1970 as an idiopathic, acquired, monomorphous, papulovesicular eruption. Although originally characterized by solely transient acantholytic dermatosis, over time the term Grover disease has been expanded to include persistent acantholytic dermatoses. Grover disease chiefly affects white adults older than 40 years and is more prevalent in males than females. Cases generally are self-limited but correlate with age, as older adults are more likely to have prolonged eruptions.¹

Grover disease typically erupts with discrete, erythematous, edematous, acneform, red-brown or flesh-colored papules, papulovesicles, or keratotic papules that primarily are seen on the trunk and anterior portion of the chest. As the rash spreads, it can erupt on the neck and thighs. The etiology of Grover disease is unknown, but many factors have been associated with the condition in a limited number of patients, including exposure to UV radiation, excessive heat or sweating, use of sulfadoxine-pyrimethamine and recombinant human IL-4, and infection with Malassezia furfur and Demodex folliculorum.¹ Grover disease also has been associated with other conditions such as asteatotic eczema, allergic contact dermatitis, and atopic dermatitis.²

Histologically, Grover disease (Figure 1) is an acantholytic process that can exhibit dyskeratosis (corps ronds and grains). Foci often are small and multiple foci are seen on shave biopsy. There also may be spongiotic changes when associated with an eczematous element. A perivascular lymphohistiocytic infiltrate with eosinophils usually is seen.³ Basket weave keratin may be seen; however, as the lesions cause pruritus, erosions and ulcerations often are present.⁴

Grover disease has multiple histologic variants that may resemble Darier disease, Hailey-Hailey disease, pemphigus foliaceus, pemphigus vulgaris, and spongiotic dermatitis and can present in combination.⁵

The variant of Grover disease that has a Darier-like pattern is difficult to distinguish from Darier disease, an autosomal-dominant-inherited disorder classified by small papules that emerge in seborrheic areas during childhood and adolescence. Histologically, Darier disease (Figure 2) shows broad areas of dyskeratosis and acantholysis that lead to suprabasal cleavage. Follicular extension may be present. In addition, there often is prominent vertical parakeratosis in Darier disease.⁶ Histologic features that favor Darier disease over the Darier-like variant of Grover disease include a broad focus of acanthotic dyskeratosis with follicular extension; the presence of a hyperkeratotic stratum corneum; and a lack of spongiosis and eosinophils, which are notably absent in Darier disease but may be present in Grover disease.⁴

Another variant of Grover disease has a Hailey-Hailey-like pattern, which is characterized by Hailey-Hailey disease's dilapidated brick wall appearance or the diffuse suprabasal acantholysis of all epidermal layers without notable dyskeratosis.⁴ Hailey-Hailey disease, also known as familial benign pemphigus, is an autosomal-dominant disorder that presents with erythematous vesicular plaques in flexural areas. The plaques progress to flaccid bullae with rupture and crusting and spread peripherally.⁷ Pathology shows suprabasilar clefts and numerous acantholytic cells (Figure 3). Dyskeratotic keratinocytes are rare with infrequent corps ronds and rare grains. The epidermis also is less hyperplastic in Grover disease than in Hailey-Hailey disease.¹

Grover disease also may present histologically with a pemphiguslike pattern, mimicking pemphigus foliaceus and pemphigus vulgaris; however, direct immunofluorescence studies are negative in Grover disease.

Pemphigus foliaceus is an autoimmune disorder caused by autoantibodies to desmoglein 1, which are present on the surfaces of keratinocytes, and is characterized by scaly crusts and blisters.⁸ Histologically, pemphigus foliaceus (Figure 4) shows a superficial epidermal blistering process. The acantholysis may be subtle and is commonly localized to the stratum granulosum, extending into the stratum corneum. Complete loss of the stratum corneum can be seen, resulting in only scattered acantholytic cells. Spongiosis also may be seen. The dermis shows a perivascular infiltrate that often contains eosinophils. Pemphigus foliaceus is confirmed by direct immunofluorescence.⁹

Pemphigus vulgaris is an autoimmune blistering disorder that is characterized by IgG autoantibodies to desmoglein 3, a component of desmosomes that are involved in keratinocyte-to-keratinocyte adhesion. Clinically, patients present with flaccid fragile blisters on the skin and mucous membranes that rupture easily, leading to painful erosions.¹⁰ Intraepidermal blisters are seen histologically (Figure 5) with the loss of cohesion (acantholysis) seen classically in the lower portions of the epidermis where desmoglein 3 is most prominent. When only the basal layer remains, the histology has been likened to a tombstone row.¹¹ Extension of the blister along the adnexa is common. The underlying dermis shows a perivascular infiltrate with eosinophils. Early lesions may show only eosinophilic spongiosis. Direct immunofluorescence studies show IgG and C3 in an intercellular pattern that resembles a fish net or chicken wire.^4,11

The spongioticlike pattern of Grover disease is marked by epidermal edema with separation of the keratinocytes and the revelation of their intracellular bridges,⁴ which manifests as vesiculation in the stratum corneum or upper layers of the epidermis.¹²

Grover disease is self-limited and may spontaneously resolve; however, the disease may be responsive to topical and systemic steroids. Additionally, avoidance of aggravating factors such as sunlight, heat, and sweating can improve symptoms.²

The Diagnosis: Grover Disease

Grover disease (also known as transient acantholytic dermatosis) was first described by Ralph W. Grover in 1970 as an idiopathic, acquired, monomorphous, papulovesicular eruption. Although originally characterized by solely transient acantholytic dermatosis, over time the term Grover disease has been expanded to include persistent acantholytic dermatoses. Grover disease chiefly affects white adults older than 40 years and is more prevalent in males than females. Cases generally are self-limited but correlate with age, as older adults are more likely to have prolonged eruptions.¹

Grover disease typically erupts with discrete, erythematous, edematous, acneform, red-brown or flesh-colored papules, papulovesicles, or keratotic papules that primarily are seen on the trunk and anterior portion of the chest. As the rash spreads, it can erupt on the neck and thighs. The etiology of Grover disease is unknown, but many factors have been associated with the condition in a limited number of patients, including exposure to UV radiation, excessive heat or sweating, use of sulfadoxine-pyrimethamine and recombinant human IL-4, and infection with Malassezia furfur and Demodex folliculorum.¹ Grover disease also has been associated with other conditions such as asteatotic eczema, allergic contact dermatitis, and atopic dermatitis.²

Histologically, Grover disease (Figure 1) is an acantholytic process that can exhibit dyskeratosis (corps ronds and grains). Foci often are small and multiple foci are seen on shave biopsy. There also may be spongiotic changes when associated with an eczematous element. A perivascular lymphohistiocytic infiltrate with eosinophils usually is seen.³ Basket weave keratin may be seen; however, as the lesions cause pruritus, erosions and ulcerations often are present.⁴

Grover disease has multiple histologic variants that may resemble Darier disease, Hailey-Hailey disease, pemphigus foliaceus, pemphigus vulgaris, and spongiotic dermatitis and can present in combination.⁵

The variant of Grover disease that has a Darier-like pattern is difficult to distinguish from Darier disease, an autosomal-dominant-inherited disorder classified by small papules that emerge in seborrheic areas during childhood and adolescence. Histologically, Darier disease (Figure 2) shows broad areas of dyskeratosis and acantholysis that lead to suprabasal cleavage. Follicular extension may be present. In addition, there often is prominent vertical parakeratosis in Darier disease.⁶ Histologic features that favor Darier disease over the Darier-like variant of Grover disease include a broad focus of acanthotic dyskeratosis with follicular extension; the presence of a hyperkeratotic stratum corneum; and a lack of spongiosis and eosinophils, which are notably absent in Darier disease but may be present in Grover disease.⁴

Another variant of Grover disease has a Hailey-Hailey-like pattern, which is characterized by Hailey-Hailey disease's dilapidated brick wall appearance or the diffuse suprabasal acantholysis of all epidermal layers without notable dyskeratosis.⁴ Hailey-Hailey disease, also known as familial benign pemphigus, is an autosomal-dominant disorder that presents with erythematous vesicular plaques in flexural areas. The plaques progress to flaccid bullae with rupture and crusting and spread peripherally.⁷ Pathology shows suprabasilar clefts and numerous acantholytic cells (Figure 3). Dyskeratotic keratinocytes are rare with infrequent corps ronds and rare grains. The epidermis also is less hyperplastic in Grover disease than in Hailey-Hailey disease.¹

Grover disease also may present histologically with a pemphiguslike pattern, mimicking pemphigus foliaceus and pemphigus vulgaris; however, direct immunofluorescence studies are negative in Grover disease.

Pemphigus foliaceus is an autoimmune disorder caused by autoantibodies to desmoglein 1, which are present on the surfaces of keratinocytes, and is characterized by scaly crusts and blisters.⁸ Histologically, pemphigus foliaceus (Figure 4) shows a superficial epidermal blistering process. The acantholysis may be subtle and is commonly localized to the stratum granulosum, extending into the stratum corneum. Complete loss of the stratum corneum can be seen, resulting in only scattered acantholytic cells. Spongiosis also may be seen. The dermis shows a perivascular infiltrate that often contains eosinophils. Pemphigus foliaceus is confirmed by direct immunofluorescence.⁹

Pemphigus vulgaris is an autoimmune blistering disorder that is characterized by IgG autoantibodies to desmoglein 3, a component of desmosomes that are involved in keratinocyte-to-keratinocyte adhesion. Clinically, patients present with flaccid fragile blisters on the skin and mucous membranes that rupture easily, leading to painful erosions.¹⁰ Intraepidermal blisters are seen histologically (Figure 5) with the loss of cohesion (acantholysis) seen classically in the lower portions of the epidermis where desmoglein 3 is most prominent. When only the basal layer remains, the histology has been likened to a tombstone row.¹¹ Extension of the blister along the adnexa is common. The underlying dermis shows a perivascular infiltrate with eosinophils. Early lesions may show only eosinophilic spongiosis. Direct immunofluorescence studies show IgG and C3 in an intercellular pattern that resembles a fish net or chicken wire.^4,11

The spongioticlike pattern of Grover disease is marked by epidermal edema with separation of the keratinocytes and the revelation of their intracellular bridges,⁴ which manifests as vesiculation in the stratum corneum or upper layers of the epidermis.¹²

Grover disease is self-limited and may spontaneously resolve; however, the disease may be responsive to topical and systemic steroids. Additionally, avoidance of aggravating factors such as sunlight, heat, and sweating can improve symptoms.²

The Diagnosis: Dermatofibroma

Dermatofibroma (DF) is a commonly encountered lesion. Although usually a straightforward clinical diagnosis, histopathological diagnosis is sometimes required. Conventional histologic findings of DF are hyperkeratosis, induction of the epidermis with acanthosis, and basal layer hyperpigmentation.^1,2 Within the dermis there usually is proliferation of fibroblasts, histiocytes, and blood vessels that sometimes spares the overlying papillary dermis. Nomenclature of specific variants may be assigned based on the predominant component (eg, nodular subepidermal fibrosis, histiocytoma, sclerosing hemangioma) or histologic findings (eg, fibrocollagenous, sclerotic, cellular, histiocytic, lipidized, angiomatous, aneurysmal, clear cell, monster cell, myxoid, keloidal, palisading, osteoclastic, epithelioid).^3-5 Of the histologic variants, fibrocollagenous is most common, but knowledge of other variants is important for accurate diagnosis, especially to exclude malignancy.

The sclerosing hemangioma variant of DF may pre-sent a diagnostic dilemma. In addition to typical features of DF, pseudovascular spaces, abundant hemosiderin, and reactive-appearing spindled cells are histologically demonstrated. The marked sclerosis and pigment deposition may mimic a blue nevus, and the dilated pseudovascular spaces may be reminiscent of a vascular neoplasm such as angiosarcoma or Kaposi sarcoma. However, the presence of characteristic features such as peripheral collagen trapping and overlying epidermal hyperplasia provide important clues for correct diagnosis. 

Angiosarcomas (Figure 1) are malignant neoplasms with vascular differentiation. Cutaneous angiosarcomas present as purple plaques or nodules on the head and/or neck in elderly individuals as well as in patients with chronic lymphedema or prior radiation exposure.^6-9 They are aggressive neoplasms with high rates of recurrence and metastases. Microscopically, the tumor is composed of anastomosing vascular channels lined by atypical endothelial cells with a multilayered appearance. There is frequent red blood cell extravasation, and substantial hemosiderin deposition may be noted in long-standing lesions. Neoplastic cells are positive for vascular markers (CD34, CD31, ETS-related gene transcription factor). Notably, cases associated with radiation exposure and chronic lymphedema are positive for MYC.¹⁰

Blue nevi (Figure 2) are benign melanocytic tumors that occur most frequently in children but may pre-sent in any age group. Clinical presentation is a blue to black, slightly raised papule that may be found on any site of the body. Biopsy typically shows a wedge-shaped infiltrate of spindled melanocytes with elongated dendritic processes in a sclerotic collagenous stroma. There frequently is a striking population of heavily pigmented melanophages. The melanocytes are positive for melanoma antigen recognized by T cells (MART-1)/melan-A, S-100, and transcription factor SOX-10. In contrast to other benign nevi, human melanoma black-45 will be positive in the dermal component.

Dermatofibrosarcoma protuberans (Figure 3) is a dermal-based tumor of intermediate malignant potential with a high rate of local recurrence and potential for sarcomatous transformation. Dermatofibrosarcoma protuberans most commonly presents in young adults as firm, pink to brown plaques and can occur on any site of the body. Histologically, they show a dermal proliferation of spindled cells that infiltrate in a storiform fashion into the subcutaneous adipose tissue,¹¹ which imparts a honeycomb or Swiss cheese pattern. The tumor characteristically demonstrates positive staining for CD34. Loss of CD34 staining, increased mitoses, nuclear atypia, and fascicular growth are features suggestive of sarcomatous transformation.^11,12 Dermatofibrosarcoma protuberans is associated with chromosomal abnormalities of chromosomes 17 and 22, resulting in COL1A1 (collagen type 1 alpha 1 chain) and PDGF-β (platelet-derived growth factor subunit B) gene fusion.¹³

Sclerotic fibromas (also known as storiform collagenomas)(Figure 4) may represent regressed DFs and are frequently associated with prior trauma to the affected area.^14,15 They usually appear as flesh-colored papules or nodules on the face and trunk. The presence of multiple sclerotic fibromas is associated with Cowden syndrome.^16,17 Histologically, the lesions present as well-demarcated, nonencapsulated, dermal nodules composed of a storiform or whorled arrangement of collagen with spindled fibroblasts. The sclerotic collagen bundles often are separated by small clefts imparting a plywoodlike pattern.¹⁶

The differential diagnosis for DF expands once atypical clinical and histopathological findings are present. In this case, the nodule was much larger and darker than the usual appearance of DF (3-10 mm).^2,4 Given the lesion's nodularity, the clinical dimple sign on lateral compression could not be seen. On biopsy, the predominance of blood vessels and sclerosis further complicated the diagnostic picture. In unusual cases such as this one, correlation of clinical history, histology, and immunophenotype is ever important.

The Diagnosis: Dermatofibroma

Dermatofibroma (DF) is a commonly encountered lesion. Although usually a straightforward clinical diagnosis, histopathological diagnosis is sometimes required. Conventional histologic findings of DF are hyperkeratosis, induction of the epidermis with acanthosis, and basal layer hyperpigmentation.^1,2 Within the dermis there usually is proliferation of fibroblasts, histiocytes, and blood vessels that sometimes spares the overlying papillary dermis. Nomenclature of specific variants may be assigned based on the predominant component (eg, nodular subepidermal fibrosis, histiocytoma, sclerosing hemangioma) or histologic findings (eg, fibrocollagenous, sclerotic, cellular, histiocytic, lipidized, angiomatous, aneurysmal, clear cell, monster cell, myxoid, keloidal, palisading, osteoclastic, epithelioid).^3-5 Of the histologic variants, fibrocollagenous is most common, but knowledge of other variants is important for accurate diagnosis, especially to exclude malignancy.

The sclerosing hemangioma variant of DF may pre-sent a diagnostic dilemma. In addition to typical features of DF, pseudovascular spaces, abundant hemosiderin, and reactive-appearing spindled cells are histologically demonstrated. The marked sclerosis and pigment deposition may mimic a blue nevus, and the dilated pseudovascular spaces may be reminiscent of a vascular neoplasm such as angiosarcoma or Kaposi sarcoma. However, the presence of characteristic features such as peripheral collagen trapping and overlying epidermal hyperplasia provide important clues for correct diagnosis. 

Angiosarcomas (Figure 1) are malignant neoplasms with vascular differentiation. Cutaneous angiosarcomas present as purple plaques or nodules on the head and/or neck in elderly individuals as well as in patients with chronic lymphedema or prior radiation exposure.^6-9 They are aggressive neoplasms with high rates of recurrence and metastases. Microscopically, the tumor is composed of anastomosing vascular channels lined by atypical endothelial cells with a multilayered appearance. There is frequent red blood cell extravasation, and substantial hemosiderin deposition may be noted in long-standing lesions. Neoplastic cells are positive for vascular markers (CD34, CD31, ETS-related gene transcription factor). Notably, cases associated with radiation exposure and chronic lymphedema are positive for MYC.¹⁰

Blue nevi (Figure 2) are benign melanocytic tumors that occur most frequently in children but may pre-sent in any age group. Clinical presentation is a blue to black, slightly raised papule that may be found on any site of the body. Biopsy typically shows a wedge-shaped infiltrate of spindled melanocytes with elongated dendritic processes in a sclerotic collagenous stroma. There frequently is a striking population of heavily pigmented melanophages. The melanocytes are positive for melanoma antigen recognized by T cells (MART-1)/melan-A, S-100, and transcription factor SOX-10. In contrast to other benign nevi, human melanoma black-45 will be positive in the dermal component.

Dermatofibrosarcoma protuberans (Figure 3) is a dermal-based tumor of intermediate malignant potential with a high rate of local recurrence and potential for sarcomatous transformation. Dermatofibrosarcoma protuberans most commonly presents in young adults as firm, pink to brown plaques and can occur on any site of the body. Histologically, they show a dermal proliferation of spindled cells that infiltrate in a storiform fashion into the subcutaneous adipose tissue,¹¹ which imparts a honeycomb or Swiss cheese pattern. The tumor characteristically demonstrates positive staining for CD34. Loss of CD34 staining, increased mitoses, nuclear atypia, and fascicular growth are features suggestive of sarcomatous transformation.^11,12 Dermatofibrosarcoma protuberans is associated with chromosomal abnormalities of chromosomes 17 and 22, resulting in COL1A1 (collagen type 1 alpha 1 chain) and PDGF-β (platelet-derived growth factor subunit B) gene fusion.¹³

Sclerotic fibromas (also known as storiform collagenomas)(Figure 4) may represent regressed DFs and are frequently associated with prior trauma to the affected area.^14,15 They usually appear as flesh-colored papules or nodules on the face and trunk. The presence of multiple sclerotic fibromas is associated with Cowden syndrome.^16,17 Histologically, the lesions present as well-demarcated, nonencapsulated, dermal nodules composed of a storiform or whorled arrangement of collagen with spindled fibroblasts. The sclerotic collagen bundles often are separated by small clefts imparting a plywoodlike pattern.¹⁶

The differential diagnosis for DF expands once atypical clinical and histopathological findings are present. In this case, the nodule was much larger and darker than the usual appearance of DF (3-10 mm).^2,4 Given the lesion's nodularity, the clinical dimple sign on lateral compression could not be seen. On biopsy, the predominance of blood vessels and sclerosis further complicated the diagnostic picture. In unusual cases such as this one, correlation of clinical history, histology, and immunophenotype is ever important.

The Diagnosis: Dermatofibroma

Dermatofibroma (DF) is a commonly encountered lesion. Although usually a straightforward clinical diagnosis, histopathological diagnosis is sometimes required. Conventional histologic findings of DF are hyperkeratosis, induction of the epidermis with acanthosis, and basal layer hyperpigmentation.^1,2 Within the dermis there usually is proliferation of fibroblasts, histiocytes, and blood vessels that sometimes spares the overlying papillary dermis. Nomenclature of specific variants may be assigned based on the predominant component (eg, nodular subepidermal fibrosis, histiocytoma, sclerosing hemangioma) or histologic findings (eg, fibrocollagenous, sclerotic, cellular, histiocytic, lipidized, angiomatous, aneurysmal, clear cell, monster cell, myxoid, keloidal, palisading, osteoclastic, epithelioid).^3-5 Of the histologic variants, fibrocollagenous is most common, but knowledge of other variants is important for accurate diagnosis, especially to exclude malignancy.

The sclerosing hemangioma variant of DF may pre-sent a diagnostic dilemma. In addition to typical features of DF, pseudovascular spaces, abundant hemosiderin, and reactive-appearing spindled cells are histologically demonstrated. The marked sclerosis and pigment deposition may mimic a blue nevus, and the dilated pseudovascular spaces may be reminiscent of a vascular neoplasm such as angiosarcoma or Kaposi sarcoma. However, the presence of characteristic features such as peripheral collagen trapping and overlying epidermal hyperplasia provide important clues for correct diagnosis. 

Angiosarcomas (Figure 1) are malignant neoplasms with vascular differentiation. Cutaneous angiosarcomas present as purple plaques or nodules on the head and/or neck in elderly individuals as well as in patients with chronic lymphedema or prior radiation exposure.^6-9 They are aggressive neoplasms with high rates of recurrence and metastases. Microscopically, the tumor is composed of anastomosing vascular channels lined by atypical endothelial cells with a multilayered appearance. There is frequent red blood cell extravasation, and substantial hemosiderin deposition may be noted in long-standing lesions. Neoplastic cells are positive for vascular markers (CD34, CD31, ETS-related gene transcription factor). Notably, cases associated with radiation exposure and chronic lymphedema are positive for MYC.¹⁰

Blue nevi (Figure 2) are benign melanocytic tumors that occur most frequently in children but may pre-sent in any age group. Clinical presentation is a blue to black, slightly raised papule that may be found on any site of the body. Biopsy typically shows a wedge-shaped infiltrate of spindled melanocytes with elongated dendritic processes in a sclerotic collagenous stroma. There frequently is a striking population of heavily pigmented melanophages. The melanocytes are positive for melanoma antigen recognized by T cells (MART-1)/melan-A, S-100, and transcription factor SOX-10. In contrast to other benign nevi, human melanoma black-45 will be positive in the dermal component.

Dermatofibrosarcoma protuberans (Figure 3) is a dermal-based tumor of intermediate malignant potential with a high rate of local recurrence and potential for sarcomatous transformation. Dermatofibrosarcoma protuberans most commonly presents in young adults as firm, pink to brown plaques and can occur on any site of the body. Histologically, they show a dermal proliferation of spindled cells that infiltrate in a storiform fashion into the subcutaneous adipose tissue,¹¹ which imparts a honeycomb or Swiss cheese pattern. The tumor characteristically demonstrates positive staining for CD34. Loss of CD34 staining, increased mitoses, nuclear atypia, and fascicular growth are features suggestive of sarcomatous transformation.^11,12 Dermatofibrosarcoma protuberans is associated with chromosomal abnormalities of chromosomes 17 and 22, resulting in COL1A1 (collagen type 1 alpha 1 chain) and PDGF-β (platelet-derived growth factor subunit B) gene fusion.¹³

Sclerotic fibromas (also known as storiform collagenomas)(Figure 4) may represent regressed DFs and are frequently associated with prior trauma to the affected area.^14,15 They usually appear as flesh-colored papules or nodules on the face and trunk. The presence of multiple sclerotic fibromas is associated with Cowden syndrome.^16,17 Histologically, the lesions present as well-demarcated, nonencapsulated, dermal nodules composed of a storiform or whorled arrangement of collagen with spindled fibroblasts. The sclerotic collagen bundles often are separated by small clefts imparting a plywoodlike pattern.¹⁶

The differential diagnosis for DF expands once atypical clinical and histopathological findings are present. In this case, the nodule was much larger and darker than the usual appearance of DF (3-10 mm).^2,4 Given the lesion's nodularity, the clinical dimple sign on lateral compression could not be seen. On biopsy, the predominance of blood vessels and sclerosis further complicated the diagnostic picture. In unusual cases such as this one, correlation of clinical history, histology, and immunophenotype is ever important.

Branchial Cleft Cyst

Cystic lesions present in a myriad of ways and often require histopathologic examination for definitive diagnosis. Correct identification of the cells comprising the lining of the cyst and the composition of the surrounding tissue are utilized to classify these lesions.

Branchial cleft cysts (quiz image, Figure 1) most commonly present as a soft tissue swelling of the lateral neck anterior to the sternocleidomastoid; they also can present in the preauricular or mandibular region.^1,2 Although the cyst is present at birth, it typically is not clinically apparent until the second or third decades of life. The origin of branchial cleft cysts is subject to some debate; however, the prevailing theory is that they result from failure of obliteration of the second branchial arch during development.¹ Histopathologically, branchial cleft cysts are characterized by a stratified squamous epithelial lining and abundant lymphoid tissue with germinal centers.^3,4 Infection is a common reason for presentation and excision is curative.

Bronchogenic cysts (Figure 2) present as midline lesions in the suprasternal notch and can present clinically due to compression of the airway.⁵ They develop as anomalies of the primitive foregut, budding off of the tracheobronchial tree. Similar to respiratory tissue, they are lined with a ciliated pseudostratified columnar epithelium and contain goblet cells. Concentric smooth muscle often surrounds the cyst and cartilage may be present.⁴ Excision is curative and recommended if the cyst encroaches on vital structures.

Median raphe cysts occur most commonly on the ventral surface of the penis on or near the glans (Figure 3). These cysts are thought to result from anomalous budding from the urethral epithelium, though they do not maintain contact with the urethra.³ The lining varies in thickness from 1 to 4 cell layers and mimics the transitional epithelium of the urethra. Amorphous debris often is seen within the cyst, and surrounding genital tissue can be appreciated by identification of delicate collagen, smooth muscle, and numerous small nerves and vessels.^3,4 Excision is curative and often is sought when the cyst becomes irritated or cosmetically bothersome.

Steatocystomas can present as solitary (steatocystoma simplex) or multiple lesions (steatocystoma multiplex)(Figure 4). They present as small, well-defined, yellow cystic papules most commonly on the chest, axilla, or groin.² Their lining is composed of a stratified squamous epithelium that lacks a granular layer and contains a distinct overlying corrugated "shark tooth" eosinophilic cuticle. Sebaceous lobules are characteristically present along or within the cyst wall.^3,4 Excision is curative and treatment often is sought for cosmetic purposes.

Similar to bronchogenic cysts, thyroglossal duct cysts (Figure 5) present on the midline neck, though they characteristically move with swallowing. The thyroglossal duct develops as the thyroid migrates from the floor of the pharynx to the anterior neck. Remnants of this duct result in the thyroglossal duct cyst.² These cysts contain a respiratory-type epithelial lining and are distinguished by distinct thyroid follicles and lymphoid aggregates surrounding the cyst wall. Unlike bronchogenic cysts, they do not contain smooth muscle.^3,4 Excision is curative.

Branchial Cleft Cyst

Cystic lesions present in a myriad of ways and often require histopathologic examination for definitive diagnosis. Correct identification of the cells comprising the lining of the cyst and the composition of the surrounding tissue are utilized to classify these lesions.

Branchial cleft cysts (quiz image, Figure 1) most commonly present as a soft tissue swelling of the lateral neck anterior to the sternocleidomastoid; they also can present in the preauricular or mandibular region.^1,2 Although the cyst is present at birth, it typically is not clinically apparent until the second or third decades of life. The origin of branchial cleft cysts is subject to some debate; however, the prevailing theory is that they result from failure of obliteration of the second branchial arch during development.¹ Histopathologically, branchial cleft cysts are characterized by a stratified squamous epithelial lining and abundant lymphoid tissue with germinal centers.^3,4 Infection is a common reason for presentation and excision is curative.

Bronchogenic cysts (Figure 2) present as midline lesions in the suprasternal notch and can present clinically due to compression of the airway.⁵ They develop as anomalies of the primitive foregut, budding off of the tracheobronchial tree. Similar to respiratory tissue, they are lined with a ciliated pseudostratified columnar epithelium and contain goblet cells. Concentric smooth muscle often surrounds the cyst and cartilage may be present.⁴ Excision is curative and recommended if the cyst encroaches on vital structures.

Median raphe cysts occur most commonly on the ventral surface of the penis on or near the glans (Figure 3). These cysts are thought to result from anomalous budding from the urethral epithelium, though they do not maintain contact with the urethra.³ The lining varies in thickness from 1 to 4 cell layers and mimics the transitional epithelium of the urethra. Amorphous debris often is seen within the cyst, and surrounding genital tissue can be appreciated by identification of delicate collagen, smooth muscle, and numerous small nerves and vessels.^3,4 Excision is curative and often is sought when the cyst becomes irritated or cosmetically bothersome.

Steatocystomas can present as solitary (steatocystoma simplex) or multiple lesions (steatocystoma multiplex)(Figure 4). They present as small, well-defined, yellow cystic papules most commonly on the chest, axilla, or groin.² Their lining is composed of a stratified squamous epithelium that lacks a granular layer and contains a distinct overlying corrugated "shark tooth" eosinophilic cuticle. Sebaceous lobules are characteristically present along or within the cyst wall.^3,4 Excision is curative and treatment often is sought for cosmetic purposes.

Similar to bronchogenic cysts, thyroglossal duct cysts (Figure 5) present on the midline neck, though they characteristically move with swallowing. The thyroglossal duct develops as the thyroid migrates from the floor of the pharynx to the anterior neck. Remnants of this duct result in the thyroglossal duct cyst.² These cysts contain a respiratory-type epithelial lining and are distinguished by distinct thyroid follicles and lymphoid aggregates surrounding the cyst wall. Unlike bronchogenic cysts, they do not contain smooth muscle.^3,4 Excision is curative.

Branchial Cleft Cyst

Cystic lesions present in a myriad of ways and often require histopathologic examination for definitive diagnosis. Correct identification of the cells comprising the lining of the cyst and the composition of the surrounding tissue are utilized to classify these lesions.

Branchial cleft cysts (quiz image, Figure 1) most commonly present as a soft tissue swelling of the lateral neck anterior to the sternocleidomastoid; they also can present in the preauricular or mandibular region.^1,2 Although the cyst is present at birth, it typically is not clinically apparent until the second or third decades of life. The origin of branchial cleft cysts is subject to some debate; however, the prevailing theory is that they result from failure of obliteration of the second branchial arch during development.¹ Histopathologically, branchial cleft cysts are characterized by a stratified squamous epithelial lining and abundant lymphoid tissue with germinal centers.^3,4 Infection is a common reason for presentation and excision is curative.

Bronchogenic cysts (Figure 2) present as midline lesions in the suprasternal notch and can present clinically due to compression of the airway.⁵ They develop as anomalies of the primitive foregut, budding off of the tracheobronchial tree. Similar to respiratory tissue, they are lined with a ciliated pseudostratified columnar epithelium and contain goblet cells. Concentric smooth muscle often surrounds the cyst and cartilage may be present.⁴ Excision is curative and recommended if the cyst encroaches on vital structures.

Median raphe cysts occur most commonly on the ventral surface of the penis on or near the glans (Figure 3). These cysts are thought to result from anomalous budding from the urethral epithelium, though they do not maintain contact with the urethra.³ The lining varies in thickness from 1 to 4 cell layers and mimics the transitional epithelium of the urethra. Amorphous debris often is seen within the cyst, and surrounding genital tissue can be appreciated by identification of delicate collagen, smooth muscle, and numerous small nerves and vessels.^3,4 Excision is curative and often is sought when the cyst becomes irritated or cosmetically bothersome.

Steatocystomas can present as solitary (steatocystoma simplex) or multiple lesions (steatocystoma multiplex)(Figure 4). They present as small, well-defined, yellow cystic papules most commonly on the chest, axilla, or groin.² Their lining is composed of a stratified squamous epithelium that lacks a granular layer and contains a distinct overlying corrugated "shark tooth" eosinophilic cuticle. Sebaceous lobules are characteristically present along or within the cyst wall.^3,4 Excision is curative and treatment often is sought for cosmetic purposes.

Similar to bronchogenic cysts, thyroglossal duct cysts (Figure 5) present on the midline neck, though they characteristically move with swallowing. The thyroglossal duct develops as the thyroid migrates from the floor of the pharynx to the anterior neck. Remnants of this duct result in the thyroglossal duct cyst.² These cysts contain a respiratory-type epithelial lining and are distinguished by distinct thyroid follicles and lymphoid aggregates surrounding the cyst wall. Unlike bronchogenic cysts, they do not contain smooth muscle.^3,4 Excision is curative.

Cutaneous Larva Migrans 

Cutaneous larva migrans (CLM) is caused by the larval migration of animal hookworms. Ancylostoma braziliense, Ancylostoma ceylanicum, and Ancylostoma caninum are the species most commonly associated with the disease. The hookworm is endemic to tropical and subtropical climates in areas such as Africa, Southeast Asia, South America, and the southeastern United States.¹ Although cats and dogs are most commonly affected, humans can be infected if they are exposed to sand or soil containing hookworm larvae, often due to contamination from animal feces.² Cutaneous larva migrans is characterized by pruritic erythematous papules and linear or serpiginous, reddish brown, elevated tracks most commonly appearing on the feet, buttocks, thighs, and lower legs; however, lesions can appear anywhere. In human hosts, the larvae travel in the epidermis and are unable to invade the dermis; it is speculated that they lack the collagenase enzymes required to penetrate the basement membrane before invading the dermis.² 

On histopathology, there typically are small cavities in the epidermis corresponding to the track of the larvae.³ There often is a spongiotic dermatitis with a mixed inflammatory infiltrate following the larvae with scattered eosinophils. The migrating larvae may be up to 1 mm in size and have bilateral double alae, or winglike projections, on the side of the body (Figure 1).⁴ The larvae are difficult to find on histopathology because they often travel beyond the areas that demonstrate clinical findings. The diagnosis of CLM is mostly clinical, but if a biopsy is performed, the specimen should be taken ahead of the track. 

Disseminated strongyloidiasis is caused by Strongyloides stercoralis. When filariform larvae migrate out of the intestinal tract into the skin, they can cause an urticarial rash and serpiginous patterns on the skin that can move 5 to 15 cm per hour, a clinical condition known as larva currens. In immunocompromised individuals, there can be hyperinfection with diffuse petechial thumbprint purpura seen clinically, which characteristically radiate from the periumbilical area.¹ On pathology, there may be numerous larvae found between the dermal collagen bundles, measuring 9 to 15 µm in diameter. Rarely, they also can be found in small blood vessels.³ They often are accompanied by extravasated red blood cells in the tissues (Figure 2).

Myiasis represents the largest pathogen in the differential diagnosis for CLM. In myiasis, fly larvae will infest human tissue, usually by forming a small cavity in the dermis or subcutaneous tissue. The larvae are visible to the human eye and can be up to several centimeters in length. In the skin, the histology of myiasis usually is accompanied by a heavy mixed inflammatory cell infiltrate with many eosinophils. Fragments of the larvae are seen encased by a thick chitinous cuticle with widely spaced spines or pigmented setae (Figure 3) on the surface of the cuticle.⁵ Layers of striated muscle and internal organs may be seen beneath the cuticle.³

Onchocerciasis, or river blindness, is a parasitic disease caused by Onchocerca volvulus that is most often seen in sub-Saharan Africa. It may cause the skin finding of an onchocercoma, a subcutaneous nodule made up of Onchocerca nematodes. However, when the filaria disseminate, it may cause onchocerciasis with cutaneous findings of an eczematous dermatitis with itching and lichenification.¹ In onchocercal dermatitis, microfilariae may be found in the dermis and there may be a mild dermal chronic inflammatory infiltrate with eosinophils.³ These microfilariae are smaller than Strongyloides larvae (Figure 4).

Sarcoptes scabiei are mites that are pathologically found limited to the stratum corneum. There often is a spongiotic dermatitis as the mite travels with an accompanying mixed cell inflammatory infiltrate with many eosinophils. One or more mites may be seen with or without eggs and excreta or scybala (Figure 5). Pink pigtails may be seen connected to the stratum corneum, representing egg fragments or casings left behind after the mite hatches.³ The female mite measures up to 0.4 mm in length.³

Cutaneous Larva Migrans 

Cutaneous larva migrans (CLM) is caused by the larval migration of animal hookworms. Ancylostoma braziliense, Ancylostoma ceylanicum, and Ancylostoma caninum are the species most commonly associated with the disease. The hookworm is endemic to tropical and subtropical climates in areas such as Africa, Southeast Asia, South America, and the southeastern United States.¹ Although cats and dogs are most commonly affected, humans can be infected if they are exposed to sand or soil containing hookworm larvae, often due to contamination from animal feces.² Cutaneous larva migrans is characterized by pruritic erythematous papules and linear or serpiginous, reddish brown, elevated tracks most commonly appearing on the feet, buttocks, thighs, and lower legs; however, lesions can appear anywhere. In human hosts, the larvae travel in the epidermis and are unable to invade the dermis; it is speculated that they lack the collagenase enzymes required to penetrate the basement membrane before invading the dermis.² 

On histopathology, there typically are small cavities in the epidermis corresponding to the track of the larvae.³ There often is a spongiotic dermatitis with a mixed inflammatory infiltrate following the larvae with scattered eosinophils. The migrating larvae may be up to 1 mm in size and have bilateral double alae, or winglike projections, on the side of the body (Figure 1).⁴ The larvae are difficult to find on histopathology because they often travel beyond the areas that demonstrate clinical findings. The diagnosis of CLM is mostly clinical, but if a biopsy is performed, the specimen should be taken ahead of the track. 

Disseminated strongyloidiasis is caused by Strongyloides stercoralis. When filariform larvae migrate out of the intestinal tract into the skin, they can cause an urticarial rash and serpiginous patterns on the skin that can move 5 to 15 cm per hour, a clinical condition known as larva currens. In immunocompromised individuals, there can be hyperinfection with diffuse petechial thumbprint purpura seen clinically, which characteristically radiate from the periumbilical area.¹ On pathology, there may be numerous larvae found between the dermal collagen bundles, measuring 9 to 15 µm in diameter. Rarely, they also can be found in small blood vessels.³ They often are accompanied by extravasated red blood cells in the tissues (Figure 2).

Myiasis represents the largest pathogen in the differential diagnosis for CLM. In myiasis, fly larvae will infest human tissue, usually by forming a small cavity in the dermis or subcutaneous tissue. The larvae are visible to the human eye and can be up to several centimeters in length. In the skin, the histology of myiasis usually is accompanied by a heavy mixed inflammatory cell infiltrate with many eosinophils. Fragments of the larvae are seen encased by a thick chitinous cuticle with widely spaced spines or pigmented setae (Figure 3) on the surface of the cuticle.⁵ Layers of striated muscle and internal organs may be seen beneath the cuticle.³

Onchocerciasis, or river blindness, is a parasitic disease caused by Onchocerca volvulus that is most often seen in sub-Saharan Africa. It may cause the skin finding of an onchocercoma, a subcutaneous nodule made up of Onchocerca nematodes. However, when the filaria disseminate, it may cause onchocerciasis with cutaneous findings of an eczematous dermatitis with itching and lichenification.¹ In onchocercal dermatitis, microfilariae may be found in the dermis and there may be a mild dermal chronic inflammatory infiltrate with eosinophils.³ These microfilariae are smaller than Strongyloides larvae (Figure 4).

Sarcoptes scabiei are mites that are pathologically found limited to the stratum corneum. There often is a spongiotic dermatitis as the mite travels with an accompanying mixed cell inflammatory infiltrate with many eosinophils. One or more mites may be seen with or without eggs and excreta or scybala (Figure 5). Pink pigtails may be seen connected to the stratum corneum, representing egg fragments or casings left behind after the mite hatches.³ The female mite measures up to 0.4 mm in length.³

Cutaneous Larva Migrans 

Cutaneous larva migrans (CLM) is caused by the larval migration of animal hookworms. Ancylostoma braziliense, Ancylostoma ceylanicum, and Ancylostoma caninum are the species most commonly associated with the disease. The hookworm is endemic to tropical and subtropical climates in areas such as Africa, Southeast Asia, South America, and the southeastern United States.¹ Although cats and dogs are most commonly affected, humans can be infected if they are exposed to sand or soil containing hookworm larvae, often due to contamination from animal feces.² Cutaneous larva migrans is characterized by pruritic erythematous papules and linear or serpiginous, reddish brown, elevated tracks most commonly appearing on the feet, buttocks, thighs, and lower legs; however, lesions can appear anywhere. In human hosts, the larvae travel in the epidermis and are unable to invade the dermis; it is speculated that they lack the collagenase enzymes required to penetrate the basement membrane before invading the dermis.² 

On histopathology, there typically are small cavities in the epidermis corresponding to the track of the larvae.³ There often is a spongiotic dermatitis with a mixed inflammatory infiltrate following the larvae with scattered eosinophils. The migrating larvae may be up to 1 mm in size and have bilateral double alae, or winglike projections, on the side of the body (Figure 1).⁴ The larvae are difficult to find on histopathology because they often travel beyond the areas that demonstrate clinical findings. The diagnosis of CLM is mostly clinical, but if a biopsy is performed, the specimen should be taken ahead of the track. 

Disseminated strongyloidiasis is caused by Strongyloides stercoralis. When filariform larvae migrate out of the intestinal tract into the skin, they can cause an urticarial rash and serpiginous patterns on the skin that can move 5 to 15 cm per hour, a clinical condition known as larva currens. In immunocompromised individuals, there can be hyperinfection with diffuse petechial thumbprint purpura seen clinically, which characteristically radiate from the periumbilical area.¹ On pathology, there may be numerous larvae found between the dermal collagen bundles, measuring 9 to 15 µm in diameter. Rarely, they also can be found in small blood vessels.³ They often are accompanied by extravasated red blood cells in the tissues (Figure 2).

Myiasis represents the largest pathogen in the differential diagnosis for CLM. In myiasis, fly larvae will infest human tissue, usually by forming a small cavity in the dermis or subcutaneous tissue. The larvae are visible to the human eye and can be up to several centimeters in length. In the skin, the histology of myiasis usually is accompanied by a heavy mixed inflammatory cell infiltrate with many eosinophils. Fragments of the larvae are seen encased by a thick chitinous cuticle with widely spaced spines or pigmented setae (Figure 3) on the surface of the cuticle.⁵ Layers of striated muscle and internal organs may be seen beneath the cuticle.³

Onchocerciasis, or river blindness, is a parasitic disease caused by Onchocerca volvulus that is most often seen in sub-Saharan Africa. It may cause the skin finding of an onchocercoma, a subcutaneous nodule made up of Onchocerca nematodes. However, when the filaria disseminate, it may cause onchocerciasis with cutaneous findings of an eczematous dermatitis with itching and lichenification.¹ In onchocercal dermatitis, microfilariae may be found in the dermis and there may be a mild dermal chronic inflammatory infiltrate with eosinophils.³ These microfilariae are smaller than Strongyloides larvae (Figure 4).

Sarcoptes scabiei are mites that are pathologically found limited to the stratum corneum. There often is a spongiotic dermatitis as the mite travels with an accompanying mixed cell inflammatory infiltrate with many eosinophils. One or more mites may be seen with or without eggs and excreta or scybala (Figure 5). Pink pigtails may be seen connected to the stratum corneum, representing egg fragments or casings left behind after the mite hatches.³ The female mite measures up to 0.4 mm in length.³

Primary Cutaneous B-cell Lymphoma

Cutaneous B-cell lymphomas (CBCLs) are a diverse but rare group of cutaneous lympho­proliferative neoplasms that make up approximately 20% of the total number of hematolymphoid neoplasms primary to the skin.¹ These lymphomas are comprised of neoplastic B cells in various stages of differentiation. As a whole, they are rare neoplasms that primarily involve the head, neck, trunk, arms, or legs.¹ Clinically, patients present with nontender, compressible, solitary, red to violaceous papules or nodules. Most CBCLs are considered low-grade malignancies with nonaggressive behavior and excellent prognosis; however, the diffuse large B-cell lymphomas, including but not limited to intravascular and leg type; lymphomatoid granulomatosis; and B-cell lymphoblastic lymphoma can act more aggressively.¹

Histopathologic examination of primary CBCL generally reveals a relatively normal epidermis accompanied by a nodular to diffuse monomorphic lymphocytic cellular infiltrate in the dermis that can occasionally extend into the subcutaneous tissue (quiz image). Although not specific for CBCLs, oftentimes there is an acellular portion of the superficial papillary dermis known as a grenz zone that can serve as a histopathologic clue to the diagnosis of a cutaneous lymphoproliferative disorder. The list of malignant B-cell neoplasms is extensive (eg, cutaneous marginal zone B-cell lymphoma, primary cutaneous follicle center lymphoma, diffuse large B-cell lymphoma, intravascular large B-cell lymphoma), and few are seen in the skin.

The most common type of CBCL is marginal zone B-cell lymphoma, which is considered to be a tumor of mucosa-associated (or skin-associated) lymphoid tissue. It is characterized by a monomorphous population of small mature lymphocytes showing characteristics of the B cells of the marginal zone of the lymph node. Some cells have the features of centrocytes/centroblasts (Figure 1) demonstrated by slightly irregular or indented nuclei and generous amounts of cytoplasm. Larger and more pleomorphic cells such as immunoblasts are similarly noted (Figure 1). The quiz image and Figure 1 demonstrate a cutaneous marginal zone B-cell lymphoma. A histomorphologic clue supporting a diagnosis of marginal zone B-cell lymphoma over reactive lymphoid hyperplasia is a B-cell predominate (B- to T-cell ratio of at least 3 to 1) infiltrate that is comprised of marginal zone-type cells. Immunohistochemistry demonstrating fewer differentiated B cells with light chain restriction may provide additional evidence that supports a clonal and potentially malignant process.

Erythematous to violaceous nodules on the head and neck of older individuals are characteristic of both granuloma faciale and CBCL. Histologically, granuloma faciale is characterized by a dense cellular infiltrate, often with a nodular outline, occupying the mid dermis.² Granuloma faciale typically spares the immediate subepidermis and hair follicles, forming a grenz zone. The cellular infiltrate is polymorphic and consists of eosinophils and neutrophils with scattered plasma cells, mast cells, and lymphocytes in a vasculocentric distribution, eventually with chronic concentric fibrosis (Figure 2).

Leukemia cutis demonstrates a dermal infiltrate that contains atypical mononuclear cells (myeloblasts and myelocytes)(Figure 3).³ These markedly atypical mononuclear cells can have kidney bean-shaped nuclei and percolate through the dermal collagen, resembling single-file cells. They have increased nuclear to cytoplasmic ratios and occasionally have prominent nucleoli. Correlation with immunophenotypic and cytochemical studies is required for specific typing of the leukemic infiltrate.

Similar to primary CBCL, lymphomatoid papulosis (LyP) consists of erythematous papules or nodules that can occur anywhere on the body. In contrast to CBCL, the lesions of LyP classically self-resolve. However, approximately 10% to 20% of patients develop a malignant lymphoma, with mycosis fungoides, Hodgkin disease, and anaplastic large cell lymphoma being the most commonly associated.

Histologic examination of lesions of LyP classically demonstrates a wedge-shaped dermal infiltrate with variable epidermal changes (Figure 4). The wedge-shaped infiltrate is composed of large atypical cells. Three main types of lesions have been delineated: types A, B, and C. Type A is characterized by an increased number of cells with large vesicular nuclei with clumped chromatin, prominent nucleoli, and pronounced cytoplasm. Reed-Sternberg-like cells with an admixture of inflammatory cells including small lymphocytes, macrophages, neutrophils, and eosinophils also are present. Type B neoplastic cells vary in size and feature hyperchromatic, convoluted, or cerebriform nuclei. The infiltrate can be dense and bandlike with fewer cells resembling mycosis fungoides; type B LyP has neoplastic cells, not inflammatory cells. Finally, type C demonstrates solid sheets of large atypical cells resembling anaplastic large cell lymphoma. Immunohistochemically, the atypical cells often are CD4⁺ and CD8^- with variable loss of pan-T-cell antigens. The atypical cells of types A and C express CD30 reactivity.⁴

Merkel cell carcinoma (MCC) is a primary neuroendocrine carcinoma of the skin that usually arises on sun-exposed skin in elderly patients with lesions that histologically and clinically resemble cutaneous lymphoma.⁵ It classically is composed of small, round to oval, basophilic cells with a vesicular nucleus and multiple small nucleoli. Apoptotic cells and mitoses often are present.⁶ One key finding that helps to differentiate MCC from lymphoma is the presence of finely dispersed salt-and-pepper chromatin and molded nuclear contour in MCC (Figure 5).

Immunophenotyping is important in the differentiation of these diagnoses. The atypical cells of LyP are positive for CD3, CD4, and CD30 but are negative for CD8. However, in type B LyP, the large CD30⁺ cells seen in the other types are not commonly seen. In contrast, MCC expresses reactivity with cytokeratins, in particular cytokeratin 20 and CAM5.2, classically in a paranuclear dotlike pattern. In keeping with MCC's neuroendocrine differentiation, the tumor cells will demonstrate reactivity with synaptophysin, chromogranin, and CD56. The immunohistochemistry for leukemia cutis varies depending on the type of leukemia. Acute myelomonocytic leukemia is positive for myeloperoxidase, CD13, CD33, and CD68. The immunophenotype of these marginal zone lymphoma cells is as follows: positive for CD20, CD79a, and Bcl-2; negative for Bcl-6, CD5, CD10, CD23, and cyclin D1 (Bcl-1).⁷ 

Primary Cutaneous B-cell Lymphoma

Cutaneous B-cell lymphomas (CBCLs) are a diverse but rare group of cutaneous lympho­proliferative neoplasms that make up approximately 20% of the total number of hematolymphoid neoplasms primary to the skin.¹ These lymphomas are comprised of neoplastic B cells in various stages of differentiation. As a whole, they are rare neoplasms that primarily involve the head, neck, trunk, arms, or legs.¹ Clinically, patients present with nontender, compressible, solitary, red to violaceous papules or nodules. Most CBCLs are considered low-grade malignancies with nonaggressive behavior and excellent prognosis; however, the diffuse large B-cell lymphomas, including but not limited to intravascular and leg type; lymphomatoid granulomatosis; and B-cell lymphoblastic lymphoma can act more aggressively.¹

Histopathologic examination of primary CBCL generally reveals a relatively normal epidermis accompanied by a nodular to diffuse monomorphic lymphocytic cellular infiltrate in the dermis that can occasionally extend into the subcutaneous tissue (quiz image). Although not specific for CBCLs, oftentimes there is an acellular portion of the superficial papillary dermis known as a grenz zone that can serve as a histopathologic clue to the diagnosis of a cutaneous lymphoproliferative disorder. The list of malignant B-cell neoplasms is extensive (eg, cutaneous marginal zone B-cell lymphoma, primary cutaneous follicle center lymphoma, diffuse large B-cell lymphoma, intravascular large B-cell lymphoma), and few are seen in the skin.

The most common type of CBCL is marginal zone B-cell lymphoma, which is considered to be a tumor of mucosa-associated (or skin-associated) lymphoid tissue. It is characterized by a monomorphous population of small mature lymphocytes showing characteristics of the B cells of the marginal zone of the lymph node. Some cells have the features of centrocytes/centroblasts (Figure 1) demonstrated by slightly irregular or indented nuclei and generous amounts of cytoplasm. Larger and more pleomorphic cells such as immunoblasts are similarly noted (Figure 1). The quiz image and Figure 1 demonstrate a cutaneous marginal zone B-cell lymphoma. A histomorphologic clue supporting a diagnosis of marginal zone B-cell lymphoma over reactive lymphoid hyperplasia is a B-cell predominate (B- to T-cell ratio of at least 3 to 1) infiltrate that is comprised of marginal zone-type cells. Immunohistochemistry demonstrating fewer differentiated B cells with light chain restriction may provide additional evidence that supports a clonal and potentially malignant process.

Erythematous to violaceous nodules on the head and neck of older individuals are characteristic of both granuloma faciale and CBCL. Histologically, granuloma faciale is characterized by a dense cellular infiltrate, often with a nodular outline, occupying the mid dermis.² Granuloma faciale typically spares the immediate subepidermis and hair follicles, forming a grenz zone. The cellular infiltrate is polymorphic and consists of eosinophils and neutrophils with scattered plasma cells, mast cells, and lymphocytes in a vasculocentric distribution, eventually with chronic concentric fibrosis (Figure 2).

Leukemia cutis demonstrates a dermal infiltrate that contains atypical mononuclear cells (myeloblasts and myelocytes)(Figure 3).³ These markedly atypical mononuclear cells can have kidney bean-shaped nuclei and percolate through the dermal collagen, resembling single-file cells. They have increased nuclear to cytoplasmic ratios and occasionally have prominent nucleoli. Correlation with immunophenotypic and cytochemical studies is required for specific typing of the leukemic infiltrate.

Similar to primary CBCL, lymphomatoid papulosis (LyP) consists of erythematous papules or nodules that can occur anywhere on the body. In contrast to CBCL, the lesions of LyP classically self-resolve. However, approximately 10% to 20% of patients develop a malignant lymphoma, with mycosis fungoides, Hodgkin disease, and anaplastic large cell lymphoma being the most commonly associated.

Histologic examination of lesions of LyP classically demonstrates a wedge-shaped dermal infiltrate with variable epidermal changes (Figure 4). The wedge-shaped infiltrate is composed of large atypical cells. Three main types of lesions have been delineated: types A, B, and C. Type A is characterized by an increased number of cells with large vesicular nuclei with clumped chromatin, prominent nucleoli, and pronounced cytoplasm. Reed-Sternberg-like cells with an admixture of inflammatory cells including small lymphocytes, macrophages, neutrophils, and eosinophils also are present. Type B neoplastic cells vary in size and feature hyperchromatic, convoluted, or cerebriform nuclei. The infiltrate can be dense and bandlike with fewer cells resembling mycosis fungoides; type B LyP has neoplastic cells, not inflammatory cells. Finally, type C demonstrates solid sheets of large atypical cells resembling anaplastic large cell lymphoma. Immunohistochemically, the atypical cells often are CD4⁺ and CD8^- with variable loss of pan-T-cell antigens. The atypical cells of types A and C express CD30 reactivity.⁴

Merkel cell carcinoma (MCC) is a primary neuroendocrine carcinoma of the skin that usually arises on sun-exposed skin in elderly patients with lesions that histologically and clinically resemble cutaneous lymphoma.⁵ It classically is composed of small, round to oval, basophilic cells with a vesicular nucleus and multiple small nucleoli. Apoptotic cells and mitoses often are present.⁶ One key finding that helps to differentiate MCC from lymphoma is the presence of finely dispersed salt-and-pepper chromatin and molded nuclear contour in MCC (Figure 5).

Immunophenotyping is important in the differentiation of these diagnoses. The atypical cells of LyP are positive for CD3, CD4, and CD30 but are negative for CD8. However, in type B LyP, the large CD30⁺ cells seen in the other types are not commonly seen. In contrast, MCC expresses reactivity with cytokeratins, in particular cytokeratin 20 and CAM5.2, classically in a paranuclear dotlike pattern. In keeping with MCC's neuroendocrine differentiation, the tumor cells will demonstrate reactivity with synaptophysin, chromogranin, and CD56. The immunohistochemistry for leukemia cutis varies depending on the type of leukemia. Acute myelomonocytic leukemia is positive for myeloperoxidase, CD13, CD33, and CD68. The immunophenotype of these marginal zone lymphoma cells is as follows: positive for CD20, CD79a, and Bcl-2; negative for Bcl-6, CD5, CD10, CD23, and cyclin D1 (Bcl-1).⁷ 

Primary Cutaneous B-cell Lymphoma

Cutaneous B-cell lymphomas (CBCLs) are a diverse but rare group of cutaneous lympho­proliferative neoplasms that make up approximately 20% of the total number of hematolymphoid neoplasms primary to the skin.¹ These lymphomas are comprised of neoplastic B cells in various stages of differentiation. As a whole, they are rare neoplasms that primarily involve the head, neck, trunk, arms, or legs.¹ Clinically, patients present with nontender, compressible, solitary, red to violaceous papules or nodules. Most CBCLs are considered low-grade malignancies with nonaggressive behavior and excellent prognosis; however, the diffuse large B-cell lymphomas, including but not limited to intravascular and leg type; lymphomatoid granulomatosis; and B-cell lymphoblastic lymphoma can act more aggressively.¹

Histopathologic examination of primary CBCL generally reveals a relatively normal epidermis accompanied by a nodular to diffuse monomorphic lymphocytic cellular infiltrate in the dermis that can occasionally extend into the subcutaneous tissue (quiz image). Although not specific for CBCLs, oftentimes there is an acellular portion of the superficial papillary dermis known as a grenz zone that can serve as a histopathologic clue to the diagnosis of a cutaneous lymphoproliferative disorder. The list of malignant B-cell neoplasms is extensive (eg, cutaneous marginal zone B-cell lymphoma, primary cutaneous follicle center lymphoma, diffuse large B-cell lymphoma, intravascular large B-cell lymphoma), and few are seen in the skin.

The most common type of CBCL is marginal zone B-cell lymphoma, which is considered to be a tumor of mucosa-associated (or skin-associated) lymphoid tissue. It is characterized by a monomorphous population of small mature lymphocytes showing characteristics of the B cells of the marginal zone of the lymph node. Some cells have the features of centrocytes/centroblasts (Figure 1) demonstrated by slightly irregular or indented nuclei and generous amounts of cytoplasm. Larger and more pleomorphic cells such as immunoblasts are similarly noted (Figure 1). The quiz image and Figure 1 demonstrate a cutaneous marginal zone B-cell lymphoma. A histomorphologic clue supporting a diagnosis of marginal zone B-cell lymphoma over reactive lymphoid hyperplasia is a B-cell predominate (B- to T-cell ratio of at least 3 to 1) infiltrate that is comprised of marginal zone-type cells. Immunohistochemistry demonstrating fewer differentiated B cells with light chain restriction may provide additional evidence that supports a clonal and potentially malignant process.

Erythematous to violaceous nodules on the head and neck of older individuals are characteristic of both granuloma faciale and CBCL. Histologically, granuloma faciale is characterized by a dense cellular infiltrate, often with a nodular outline, occupying the mid dermis.² Granuloma faciale typically spares the immediate subepidermis and hair follicles, forming a grenz zone. The cellular infiltrate is polymorphic and consists of eosinophils and neutrophils with scattered plasma cells, mast cells, and lymphocytes in a vasculocentric distribution, eventually with chronic concentric fibrosis (Figure 2).

Leukemia cutis demonstrates a dermal infiltrate that contains atypical mononuclear cells (myeloblasts and myelocytes)(Figure 3).³ These markedly atypical mononuclear cells can have kidney bean-shaped nuclei and percolate through the dermal collagen, resembling single-file cells. They have increased nuclear to cytoplasmic ratios and occasionally have prominent nucleoli. Correlation with immunophenotypic and cytochemical studies is required for specific typing of the leukemic infiltrate.

Similar to primary CBCL, lymphomatoid papulosis (LyP) consists of erythematous papules or nodules that can occur anywhere on the body. In contrast to CBCL, the lesions of LyP classically self-resolve. However, approximately 10% to 20% of patients develop a malignant lymphoma, with mycosis fungoides, Hodgkin disease, and anaplastic large cell lymphoma being the most commonly associated.

Histologic examination of lesions of LyP classically demonstrates a wedge-shaped dermal infiltrate with variable epidermal changes (Figure 4). The wedge-shaped infiltrate is composed of large atypical cells. Three main types of lesions have been delineated: types A, B, and C. Type A is characterized by an increased number of cells with large vesicular nuclei with clumped chromatin, prominent nucleoli, and pronounced cytoplasm. Reed-Sternberg-like cells with an admixture of inflammatory cells including small lymphocytes, macrophages, neutrophils, and eosinophils also are present. Type B neoplastic cells vary in size and feature hyperchromatic, convoluted, or cerebriform nuclei. The infiltrate can be dense and bandlike with fewer cells resembling mycosis fungoides; type B LyP has neoplastic cells, not inflammatory cells. Finally, type C demonstrates solid sheets of large atypical cells resembling anaplastic large cell lymphoma. Immunohistochemically, the atypical cells often are CD4⁺ and CD8^- with variable loss of pan-T-cell antigens. The atypical cells of types A and C express CD30 reactivity.⁴

Merkel cell carcinoma (MCC) is a primary neuroendocrine carcinoma of the skin that usually arises on sun-exposed skin in elderly patients with lesions that histologically and clinically resemble cutaneous lymphoma.⁵ It classically is composed of small, round to oval, basophilic cells with a vesicular nucleus and multiple small nucleoli. Apoptotic cells and mitoses often are present.⁶ One key finding that helps to differentiate MCC from lymphoma is the presence of finely dispersed salt-and-pepper chromatin and molded nuclear contour in MCC (Figure 5).

Immunophenotyping is important in the differentiation of these diagnoses. The atypical cells of LyP are positive for CD3, CD4, and CD30 but are negative for CD8. However, in type B LyP, the large CD30⁺ cells seen in the other types are not commonly seen. In contrast, MCC expresses reactivity with cytokeratins, in particular cytokeratin 20 and CAM5.2, classically in a paranuclear dotlike pattern. In keeping with MCC's neuroendocrine differentiation, the tumor cells will demonstrate reactivity with synaptophysin, chromogranin, and CD56. The immunohistochemistry for leukemia cutis varies depending on the type of leukemia. Acute myelomonocytic leukemia is positive for myeloperoxidase, CD13, CD33, and CD68. The immunophenotype of these marginal zone lymphoma cells is as follows: positive for CD20, CD79a, and Bcl-2; negative for Bcl-6, CD5, CD10, CD23, and cyclin D1 (Bcl-1).⁷ 

Epidermodysplasia Verruciformis

Epidermodysplasia verruciformis (EV) is a rare hereditary disorder that predisposes affected individuals to widespread infection with various forms of human papillomavirus (HPV). It is inherited in an autosomal-recessive pattern.¹ The first manifestations generally are seen in childhood. The clinical appearance of lesions can vary, at times mimicking other disease processes. Patients can present with flat wartlike papules resembling verruca plana distributed in sun-exposed areas. Another distinct presentation is multiple salmon-colored, hyperpigmented, or hypopigmented macules, papules, or plaques with overlying scale that can resemble tinea versicolor.^1,2 A large percentage of patients will go on to develop actinic keratosis and squamous cell carcinoma by 40 years of age.² The malignancies most commonly develop in sun-exposed areas, suggesting UV radiation as an important contributor to development along with HPV infection. Mutations in the EVER1 and EVER2 genes that code for transmembrane proteins on the endoplasmic reticulum that are involved in zinc transport lead to EV. The mutations lead to decreased zinc movement into the cytoplasm, which is thought to play a role in preventing HPV infection. The decreased protection against HPV leads to infections from both common subtypes and those that immunocompetent individuals would be resistant to, namely the β-genus HPV-5, -8, -9 and -20.^1,2 Immunosuppressed individuals, such as those with human immunodeficiency virus/AIDS, also are at an increased risk for infection with these HPV subtypes and generally have similar clinical and histological presentations.¹ It is important to promote sunscreen use for preventive care in patients with EV due to the increased risk for squamous cell carcinoma.

Histologically, the lesions in EV are composed of acanthosis and hyperkeratosis with keratinocytes arranged in clusters.^1,3 There is orthokeratosis and parakeratosis.¹ Scattered or clustered keratinocytes in the granular layer or upper stratum spinosum appear swollen with foamy blue-gray cytoplasm (quiz image and Figure 1).^1,4 The keratinocytes may become atypical and progress to squamous cell carcinoma, particularly in sun-exposed regions. Cell differentiation becomes disorganized and nuclei become enlarged and hyperchromatic.¹

Condyloma acuminatum will have pronounced acanthosis and hyperkeratosis with exophytic growth. Rounded parakeratosis is visible. The characteristic cell is the koilocyte, a keratinocyte that has an enlarged nucleus with areas of surrounding clearing, increased dark color in the nucleus, and wrinkled nuclear membrane (Figure 2).^1,3 True koilocytes may be rare in condyloma acuminatum.⁴ Other distinct features include coarse hypergranulosis and a compact stratum corneum.

Herpesvirus lesions typically demonstrate ballooning degeneration of keratinocytes.¹ They will become pale and fuse to form multinucleated giant cells, a feature not found in verruca. The nuclei will be slate gray with margination of the chromatin, which can be identified due to its increased basophilic appearance (Figure 3).^1,4 Inclusion bodies can be found, but unlike molluscum contagiosum (MC), these bodies are intranuclear as opposed to cytoplasmic.¹

The telltale Henderson-Patterson (molluscum) bodies can identify MC histologically.⁴ Located within keratinocytes, these cytoplasmic inclusions can vary in both color and size as they mature. As the keratinocytes develop outward, the molluscum bodies grow larger and become more eosinophilic (Figure 4).^1,4 Another feature of MC that can be used to differentiate it from EV is the scalloped borders located on lesions.⁴

On histology, verruca vulgaris will have pronounced acanthosis with orthokeratosis and vertical tiers of parakeratosis.^3,4 Growth is exophytic. The granular layer will have large irregular basophilic granules. Koilocytes may be seen. A distinctive feature is the papillomatosis with inward bending of rete ridges.^3,4 It is common to see invasion of tortuous blood vessels into the exophytic projections.³ In myrmecia (palmoplantar warts) it is common to see thrombosis of these vessels and inclusions of red cytoplasmic bodies (Figure 5).^1,4

Epidermodysplasia Verruciformis

Epidermodysplasia verruciformis (EV) is a rare hereditary disorder that predisposes affected individuals to widespread infection with various forms of human papillomavirus (HPV). It is inherited in an autosomal-recessive pattern.¹ The first manifestations generally are seen in childhood. The clinical appearance of lesions can vary, at times mimicking other disease processes. Patients can present with flat wartlike papules resembling verruca plana distributed in sun-exposed areas. Another distinct presentation is multiple salmon-colored, hyperpigmented, or hypopigmented macules, papules, or plaques with overlying scale that can resemble tinea versicolor.^1,2 A large percentage of patients will go on to develop actinic keratosis and squamous cell carcinoma by 40 years of age.² The malignancies most commonly develop in sun-exposed areas, suggesting UV radiation as an important contributor to development along with HPV infection. Mutations in the EVER1 and EVER2 genes that code for transmembrane proteins on the endoplasmic reticulum that are involved in zinc transport lead to EV. The mutations lead to decreased zinc movement into the cytoplasm, which is thought to play a role in preventing HPV infection. The decreased protection against HPV leads to infections from both common subtypes and those that immunocompetent individuals would be resistant to, namely the β-genus HPV-5, -8, -9 and -20.^1,2 Immunosuppressed individuals, such as those with human immunodeficiency virus/AIDS, also are at an increased risk for infection with these HPV subtypes and generally have similar clinical and histological presentations.¹ It is important to promote sunscreen use for preventive care in patients with EV due to the increased risk for squamous cell carcinoma.

Histologically, the lesions in EV are composed of acanthosis and hyperkeratosis with keratinocytes arranged in clusters.^1,3 There is orthokeratosis and parakeratosis.¹ Scattered or clustered keratinocytes in the granular layer or upper stratum spinosum appear swollen with foamy blue-gray cytoplasm (quiz image and Figure 1).^1,4 The keratinocytes may become atypical and progress to squamous cell carcinoma, particularly in sun-exposed regions. Cell differentiation becomes disorganized and nuclei become enlarged and hyperchromatic.¹

Condyloma acuminatum will have pronounced acanthosis and hyperkeratosis with exophytic growth. Rounded parakeratosis is visible. The characteristic cell is the koilocyte, a keratinocyte that has an enlarged nucleus with areas of surrounding clearing, increased dark color in the nucleus, and wrinkled nuclear membrane (Figure 2).^1,3 True koilocytes may be rare in condyloma acuminatum.⁴ Other distinct features include coarse hypergranulosis and a compact stratum corneum.

Herpesvirus lesions typically demonstrate ballooning degeneration of keratinocytes.¹ They will become pale and fuse to form multinucleated giant cells, a feature not found in verruca. The nuclei will be slate gray with margination of the chromatin, which can be identified due to its increased basophilic appearance (Figure 3).^1,4 Inclusion bodies can be found, but unlike molluscum contagiosum (MC), these bodies are intranuclear as opposed to cytoplasmic.¹

The telltale Henderson-Patterson (molluscum) bodies can identify MC histologically.⁴ Located within keratinocytes, these cytoplasmic inclusions can vary in both color and size as they mature. As the keratinocytes develop outward, the molluscum bodies grow larger and become more eosinophilic (Figure 4).^1,4 Another feature of MC that can be used to differentiate it from EV is the scalloped borders located on lesions.⁴

On histology, verruca vulgaris will have pronounced acanthosis with orthokeratosis and vertical tiers of parakeratosis.^3,4 Growth is exophytic. The granular layer will have large irregular basophilic granules. Koilocytes may be seen. A distinctive feature is the papillomatosis with inward bending of rete ridges.^3,4 It is common to see invasion of tortuous blood vessels into the exophytic projections.³ In myrmecia (palmoplantar warts) it is common to see thrombosis of these vessels and inclusions of red cytoplasmic bodies (Figure 5).^1,4

Epidermodysplasia Verruciformis

Epidermodysplasia verruciformis (EV) is a rare hereditary disorder that predisposes affected individuals to widespread infection with various forms of human papillomavirus (HPV). It is inherited in an autosomal-recessive pattern.¹ The first manifestations generally are seen in childhood. The clinical appearance of lesions can vary, at times mimicking other disease processes. Patients can present with flat wartlike papules resembling verruca plana distributed in sun-exposed areas. Another distinct presentation is multiple salmon-colored, hyperpigmented, or hypopigmented macules, papules, or plaques with overlying scale that can resemble tinea versicolor.^1,2 A large percentage of patients will go on to develop actinic keratosis and squamous cell carcinoma by 40 years of age.² The malignancies most commonly develop in sun-exposed areas, suggesting UV radiation as an important contributor to development along with HPV infection. Mutations in the EVER1 and EVER2 genes that code for transmembrane proteins on the endoplasmic reticulum that are involved in zinc transport lead to EV. The mutations lead to decreased zinc movement into the cytoplasm, which is thought to play a role in preventing HPV infection. The decreased protection against HPV leads to infections from both common subtypes and those that immunocompetent individuals would be resistant to, namely the β-genus HPV-5, -8, -9 and -20.^1,2 Immunosuppressed individuals, such as those with human immunodeficiency virus/AIDS, also are at an increased risk for infection with these HPV subtypes and generally have similar clinical and histological presentations.¹ It is important to promote sunscreen use for preventive care in patients with EV due to the increased risk for squamous cell carcinoma.

Histologically, the lesions in EV are composed of acanthosis and hyperkeratosis with keratinocytes arranged in clusters.^1,3 There is orthokeratosis and parakeratosis.¹ Scattered or clustered keratinocytes in the granular layer or upper stratum spinosum appear swollen with foamy blue-gray cytoplasm (quiz image and Figure 1).^1,4 The keratinocytes may become atypical and progress to squamous cell carcinoma, particularly in sun-exposed regions. Cell differentiation becomes disorganized and nuclei become enlarged and hyperchromatic.¹

Condyloma acuminatum will have pronounced acanthosis and hyperkeratosis with exophytic growth. Rounded parakeratosis is visible. The characteristic cell is the koilocyte, a keratinocyte that has an enlarged nucleus with areas of surrounding clearing, increased dark color in the nucleus, and wrinkled nuclear membrane (Figure 2).^1,3 True koilocytes may be rare in condyloma acuminatum.⁴ Other distinct features include coarse hypergranulosis and a compact stratum corneum.

Herpesvirus lesions typically demonstrate ballooning degeneration of keratinocytes.¹ They will become pale and fuse to form multinucleated giant cells, a feature not found in verruca. The nuclei will be slate gray with margination of the chromatin, which can be identified due to its increased basophilic appearance (Figure 3).^1,4 Inclusion bodies can be found, but unlike molluscum contagiosum (MC), these bodies are intranuclear as opposed to cytoplasmic.¹

The telltale Henderson-Patterson (molluscum) bodies can identify MC histologically.⁴ Located within keratinocytes, these cytoplasmic inclusions can vary in both color and size as they mature. As the keratinocytes develop outward, the molluscum bodies grow larger and become more eosinophilic (Figure 4).^1,4 Another feature of MC that can be used to differentiate it from EV is the scalloped borders located on lesions.⁴

On histology, verruca vulgaris will have pronounced acanthosis with orthokeratosis and vertical tiers of parakeratosis.^3,4 Growth is exophytic. The granular layer will have large irregular basophilic granules. Koilocytes may be seen. A distinctive feature is the papillomatosis with inward bending of rete ridges.^3,4 It is common to see invasion of tortuous blood vessels into the exophytic projections.³ In myrmecia (palmoplantar warts) it is common to see thrombosis of these vessels and inclusions of red cytoplasmic bodies (Figure 5).^1,4