Revised estimates of overdiagnosis from the Canadian National Breast Screening Study.

We have re-estimated overdiagnosis of breast cancer from mammography screening by age group in the Canadian National Breast Screening Study (CNBSS), a randomized screening trial. In the CNBSS, participants were recruited in 15 centers. 89,835 women were randomized with informed consent, 50,430 age 40-49 and 39,405 age 50-59. Women aged 40-49 received annual mammography and physical examination (MA+PX) versus a single physical examination and usual care in the community (UC). Women aged 50-59 received (MA+PX) versus (PX-alone) annually. Individual randomization resulted in 44 almost identically distributed demographic and risk factors. Annual compliance over the five or four scheduled screens was 86-95%. The cumulative numbers of invasive and in situ breast cancers ascertained by year during screening and subsequent follow-up to 25 years post entry to the CNBSS in the mammography arm have been compared to those in the control arm. Estimates of overdiagnosis were derived using post-screening cessation cut-off points from 1 to 20years.⋯Overdiagnosis of invasive breast cancer at five years post cessation of screening for women aged 40-49 was estimated to be 32%, and 16% for women aged 50-59; 20years post cessation of screening 48% for women 40-49 and 5% for those 50-59. Including ductal carcinoma in situ with invasive cancer, estimates were 41%, 25%, 55% and 16%, respectively. We conclude that approximately 30% of invasive screen-detected breast cancers in women age 40-49 were overdiagnosed, and 20% of those screen-detected in women age 50-59. Including ductal carcinoma in situ, the estimates are 40% and 30%, respectively.