1. About 1 in 7 cases of screen-detected cancer is overdiagnosed in the US population.

2. The rate of breast cancer overdiagnosis in US population-based mammography screening was not as high as suggested by previous excess-incidence studies.

Evidence Rating Level: 1 (Excellent)

Study Rundown: The US Preventive Services Task Force cites overdiagnosis as one of the major potential harms associated with mammography screening because of the burden and adverse effects of unnecessary treatments. However, the risk for breast cancer overdiagnosis remains unclear, with estimates ranging from 0% to 54%, as many factors such as overdiagnosis definitions, study settings, and estimation methods influence the rate. Therefore, there is a gap in knowledge as to estimating the rate of breast cancer overdiagnosis in contemporary mammography practice accounting for the detection of nonprogressive cancer. This study found that based on the US population, breast cancer overdiagnosis was unlikely to be as high as suggested by previous excess-incidence studies. This study was limited by including only those women who had their first screening examination within the Breast Cancer Surveillance Consortium (BCSC), as well as the potential for model misspecification. Nevertheless, these study’s findings are significant, as they demonstrate that the rate of overdiagnosis of breast cancer is not as high as previously estimated, which helps clarify the risk for such events in contemporary screening practice.

Click to read the study in AIM

Relevant Reading: Reducing the Burden of Overdiagnosis in Breast Cancer Screening and Beyond

In-Depth [population cohort study]: This population cohort study followed a cohort of women who received 1 or more screening mammograms at a BCSC facility between 2000 and 2018. This included 35,986 women, 82,677 mammograms, and 718 breast cancer diagnoses. Overdiagnosis was estimated through Bayesian inference. Patients who were aged 50 to 74 years and had the first screen at a BCSC facility were eligible for the study. Patients who had received a mammogram or breast cancer diagnosis before their first mammogram in the BCSC registry were excluded from the study. The primary outcome was screening mammograms and screen-detected or interval breast cancer. Outcomes in the primary analysis were conducted via Bayesian inference of the natural history of breast cancer using individual screening and diagnosis records, combined with fitted natural history models with life-table data to predict the rate of overdiagnosis under biennial screening. Based on the analysis, through biennial screening, 15.4% (95% uncertainty interval [UI], 9.4% to 26.5%) of screen-detected cancers were estimated to be overdiagnosed. Of these cases, 6.1% (95% UI, 0.2% to 20.1%) were due to detecting indolent preclinical cancer and 9.3% (95% UI, 5.5% to 13.5%) were due to detecting progressive preclinical cancer in patients who would have died of an unrelated cause before the breast cancer clinical diagnosis. Furthermore, doubling the screening frequency from biennial to annual resulted in an overdiagnosis rate of 14.6% (95% prediction interval, 9.4% to 23.9%) with patterns similar to those under biennial screening. Overall, this study demonstrated that breast cancer overdiagnosis is unlikely to be as high as previously suggested by excess-incidence studies based on the US population.

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