The risk of breast cancer (BC) overdiagnosis attributed to mammography screening is an unresolved issue, complicated by heterogeneity in the methodology of quantifying its magnitude, and both political and scientific elements surrounding interpretation of the evidence on this phenomenon. Evidence from randomized trials and also from observational studies shows that mammography screening reduces the risk of BC death; similarly, these studies provide sufficient evidence that overdiagnosis represents a serious harm from population breast screening. For both these outcomes of screening, BC mortality reduction and overdiagnosis, estimates of magnitude vary between studies however overdiagnosis estimates are associated with substantial uncertainty. The trade-off between the benefit and the collective harms of BC screening, including false-positives and overdiagnosis, is more finely balanced than initially recognized, however the snapshot of evidence presented on overdiagnosis does not mean that breast screening is worthless. Future efforts should be directed towards (a) ensuring that any changes in the implementation of BC screening optimize the balance between benefit and harms, including assessing how planned or actual changes modify the risk of overdiagnosis; (b) informing women of all the outcomes that may affect them when they participate in screening using well-crafted and balanced information; and (c) investing in research that will help define and reduce the ensuing overtreatment of screen-detected BC.
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