SAN DIEGO – Patients treated with ionizing radiation were 2.65 to 3.78 times more likely to develop basal cell carcinoma than were controls, and exposure at younger ages or relatively high doses further increased that risk, according to a pooled analysis presented at the annual meeting of the American Society for Dermatologic Surgery.
“Concomitant exposure to ultraviolet radiation may potentiate this effect,” said Dr. Min Deng, a dermatology resident at the University of Chicago. “With newer treatment protocols and improved shielding, it would be interesting to see if the effect of ionizing radiation has changed, or whether we as dermatologists should more actively screen this at-risk population.”
Basal cell carcinoma (BCC) is the most common skin cancer worldwide, but relatively few dermatology papers have assessed the effects of ionizing radiation on the incidence of BCC, said Dr. Deng and coauthor Dr. Diana Bolotin, also of the University of Chicago.
To better understand the link, the researchers searched PubMed for controlled studies on the topic by using the terms “radiation,” “risk,” and “basal cell carcinoma.” They excluded case reports, animal studies, studies published in languages other than English, and trials of radiation as a treatment of BCC, they said. They also excluded studies of atomic bomb survivors, because exposure was uncontrolled and methods to estimate exposure in this group have changed over time, they noted.
In all, six studies published between 1991 and 2012 met the inclusion criteria, and all six showed a statistically significant relative risk or odds of BCC with ionizing radiation exposure, the investigators reported. Three analyses calculated the relative risk of BCC in patients who received radiation for tinea capitis or who underwent total-body irradiation before hematopoietic cell transplantation, they said. Two studies evaluated the odds of BCC in patients with a past history of radiation exposure, and one study assessed time to subsequent BCCs in patients with a history of BCC.
For the three studies that calculated relative risk, the researchers calculated a pooled RR of BCC after ionizing radiation treatment of 2.65 (95% confidence interval, 1.22-5.72) compared with controls. The study of total-body irradiation (TBI) did not report or control for the primary disease for which patients were treated, “which may have confounded the results,” they said. When they excluded that study from their calculation, the overall RR rose to 3.78 (95% CI, 2.62 to 5.44).
Notably, in the two studies of patients with tinea capitis, the relative risk of BCC fell by 12% to 16% for every additional year of age at which patients received radiation treatment, the researchers said. Similarly, risk of BCC dropped by 10.9% for every 1-year increase in age at total-body irradiation prior to cell transplantation.
The combined odds ratio for the next two studies was 4.28 (1.45-12.63). “Likewise, both studies found an elevated odds ratio with younger age at radiation exposure,” the researchers added. One study that stratified patients by type of medical condition detected a “markedly elevated” 8.7 odds of BCC after radiation treatment for acne (2.0 to 38.0), they noted.
The sixth study was a nested case-control analysis that found a statistically significant increase in the odds of BCC starting at a 1-Gy dose of ionizing radiation, and rising linearly up to doses of 35-63.3 Gy. “The risk for developing multiple BCCs also appears to be elevated in patients with a history of radiation therapy,” the researchers said. Patients who had been exposed to ionizing ratio were 2.3 times more likely to develop new BCCs compared with unexposed patients (1.7 to 3.1), they said.
The investigators reported no funding sources or conflicts of interest.