Overdiagnosis in Mammographic Screening because of Competing Risk of Death.

BACKGROUND Different definitions and estimates of overdiagnosis in mammographic screening reflect a substantial need to investigate and understand the complexity of the issue. This modeling study aims to estimate the number of overdiagnosed women, defined as those diagnosed with breast cancer who die from any cause within the lead-time period. METHODS We used numbers from incidence and death statistics available online and published estimates of lead-time. Postulated cohorts of screened and not screened women ages 50 to 51 were followed for a period corresponding to 10 biennial screening exams during 20 years, and a further 10 years, to ages 78 to 79. The increase in breast cancer incidence because of screening was estimated based on lead-time. The proportion of women diagnosed with breast cancer who died within the lead-time period was assessed based on the differences in the cumulative number of breast cancer diagnosed in a nonscreened and screened cohort. RESULTS The proportion of inevitable overdiagnosed women in a screened versus nonscreened cohort was 1.9% for England and Wales and 1.8% for Norway. Sensitivity analyses using various assumptions increased the estimates up to a maximum of 4%. CONCLUSION The proportion of women with breast cancer diagnosed after participation in a screening program who died within the estimated lead-time period was less than 4%. This inevitable proportion of overdiagnosis should be emphasized in the definition and communication of the issue. IMPACT The issue of overdiagnosis is complex and estimates should be interpreted with substantial care. Cancer Epidemiol Biomarkers Prev; 25(5); 759-65. ©2016 AACR.