Context

The goal of screening mammography is to decrease breast cancer mortality by diagnosing tumours at an earlier, more treatable stage. Meta-analyses of randomised clinical trials have estimated that breast cancer screening results in approximately a 20% decrease in breast cancer mortality.1 However, breast cancer incidence and treatment, as well as performance of screening mammography, have evolved since the conclusion of most of these clinical trials and it is unclear whether screening in the modern era confers the same benefit. Moreover, substantial uncertainty still exists regarding the extent of over diagnosis, the diagnosis of tumours that would not have become clinically apparent within a woman's lifetime in the absence of …