Andrew M. Kaunitz, MD reviewing Welch HG et al. N Engl J Med 2016 Oct 13.
Long-term analysis suggests overdiagnosis is more common than early detection and that the bulk of reduced breast cancer mortality in recent decades reflects better treatment.
Screening mammography is based on the premise that detecting small, nonpalpable tumors can lower incidence of larger tumors. To assess screening mammography's effects on overdiagnosis (detection of tumors that will not lead to symptoms) and breast cancer mortality, investigators analyzed population-based cancer registry data in women aged ≥40 (representing about 10% of the U.S. population), with a focus on tumor size. Data from 1975 to 1979 (prior to widespread screening) and 2000 to 2002 (the most recent period for which 10-year follow-up data were available) were compared.
With the advent of widespread screening, incidence of small (<2 cm) in situ and invasive breast tumors increased by 162 cases per 100,000 women, whereas incidence of large tumors (≥ 2cm) decreased by 30 cases per 100,000. Assuming that the underlying incidence of breast cancer was stable throughout the study period, these observations indicate that 132 cases per 100,000 (i.e., 162 minus 30) reflect overdiagnosis. Based on the supposition that screening's impact on incidence of large cancers reflects its ability to lower mortality, the authors estimated that screening was responsible for no more than one third of the reduction in breast cancer mortality, with the balance a function of improved treatment.
Welch HG et al. Breast-cancer tumor size, overdiagnosis, and mammography screening effectiveness. N Engl J Med 2016 Oct 13; 375:1438.