NEW YORK (Reuters) – About 15% of screen-detected breast cancers among women ages 50-74 would not have caused symptoms or signs in a woman’s remaining lifetime, which is less than previously thought, a modeling study suggests.
Identification of those cancers on screening is considered to be overdiagnosis.
“We have long known that the most prominent estimates of breast cancer overdiagnosis in the U.S. were inflated,” senior author Dr. Ruth Etzioni of the Fred Hutchinson Cancer Research Center, Seattle, told Reuters Health by email. “While our results confirm that breast cancer overdiagnosis is real, they also reassure that it is not as alarmingly frequent as the most prominent studies have suggested.”
Lead author Dr. Marc Ryser of Duke University, Durham, N.C., also commented by email. “While mammography has been shown in randomized trials to be effective at reducing the risk of dying from breast cancer, the overdiagnosis issue has created confusion for many women about the value of breast cancer screening.”
“Now that we have a more reliable number for the risk of overdiagnosis,” he said, “we hope it will make conversations between clinicians and patients about benefit and harm when screening for breast cancer a lot easier.”
“We were not able in this study to discern the percent overdiagnosed between in-situ and invasive breast cancers,” he added “But other studies that we and our coauthors have published show that, biologically, in-situ cancers are less likely to progress, and this would make them more likely to be overdiagnosed.”
AsAnnals of Internal Medicine, Dr. Etzioni, Dr. Ryser, and colleagues analyzed data from Breast Cancer Surveillance Consortium (BCSC) facilities from 2000-2018 to estimate the rate of breast cancer overdiagnosis in contemporary mammography practice. The cohort included close to 36,000 women, 82,677 mammograms, and 718 breast cancer diagnoses.
Among women of known race, 64.4% were White, 19.0% Asian, and 12.1% Black. Among women of known ethnicity, 11.0% identified as Hispanic.
The median age at first screen was 56, with a local peak at 65, corresponding to the starting age of Medicare eligibility. Each woman received 2.3 screening tests, on average.
The authors used data on screen and interval cancer incidence to estimate the underlying latency and fraction of indolent cancer. They then used these estimates together with life tables on the risk for death from causes other than breast cancer to predict the extent of overdiagnosis.
Among all preclinical cases, 4.5% were estimated to be nonprogressive. In a biennial screening program from ages 50 to 74, 15.4% of screen-detected cancer cases were estimated to be overdiagnosed, with 6.1% due to detecting indolent preclinical cancer and 9.3% due to detecting progressive preclinical cancer in women who would have died of an unrelated cause before clinical diagnosis.
The authors note that because women with a first mammography screen outside the BCSC were excluded, numbers were too low to produce models accounting for patient features such as race, ethnicity, and breast density, or histologic or molecular tumor subtypes.
Summing up, they state that “on the basis of an authoritative U.S. population data set, the analysis projected that among biennially screened women aged 50 to 74 years, about 1 in 7 cases of screen-detected cancer is overdiagnosed.”
Dr. Katrina Armstrong of Massachusetts General Hospital, Boston, coauthor of a related editorial, commented by email to Reuters Health: “The rate of overdiagnosis is lower than found in recent studies and should be reassuring to women considering mammography screening.”
“However,” she said, “the burden of overdiagnosis at the population level is large, given how many women undergo screening each year, and more should be done to reduce that risk by making better screening tools, better ways to predict what tumors will progress, and better approaches to implementing primary prevention.”
“While overdiagnosis receives a lot of attention in cancer screening, it is also a major issue for other diseases where we use screening tests in primary care, like diabetes, and is often overlooked in the enthusiasm for early-detection programs,” she added. “Finding a disease that was never going to cause a problem doesn’t help anyone.”
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