Breast cancer: how often does overdiagnosis occur?

A new modelling study from the US found that previous estimates of breast cancer overdiagnosis may have been overestimated. The study, based on data from the Breast Cancer Screening Consortium, suggests that overdiagnosis, or the finding of tumours that may never have progressed or caused harm in a woman’s lifetime, occurs in about 15% of screen-detected cancers.

The US Preventive Services Task Force (USPSTF) cites overdiagnosis as one of the chief potential harms associated with mammography screening because of the burden and adverse consequences of unnecessary treatments. Therefore, knowledge about overdiagnosis is critical for supporting shared decision-making about screening. However, the risk for breast cancer overdiagnosis in contemporary screening programs remains uncertain, with the most widely cited estimates reaching about 30%.

Researchers from Duke University and the Fred Hutchinson Cancer Research Center studied data from Breast Cancer Surveillance Consortium facilities to estimate the rate of breast cancer overdiagnosis in contemporary mammography practice for a cohort including 35,986 women, 82,677 mammograms and 718 breast cancer diagnoses. To estimate overdiagnosis, the authors considered a cohort of women whose parameters for disease natural history were given by the best-fitting parameter combinations and who had annual or biennial screening.

In a program of biennial screening of women aged 50 to 74 years, which corresponds to USPSTF recommendations for average risk women, approximately 1 in 7 screen-detected cases would be overdiagnosed, the research found. Increasing the screening interval to annual screening did not seem to affect this number.

An editorial from Massachusetts General Hospital says that these findings may help women who are considering having mammography screening better understand the risk of overdiagnosis. Given that approximately 7 in 1000 women will be diagnosed with invasive or non-invasive breast cancer on the basis of a screening mammogram, women should be told that approximately 1 in 1000 women who undergo mammography will be found to have a cancer that would never have caused problems. The findings are published in Annals of Internal Medicine.

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