ABSTRACT
There is little doubt that widespread introduction of mammographic screening in Europe and North America has made a signifi cant contribution to the reduction in breast cancer mortality that has been observed during the last twenty years. An overview of published breast screening trials in 2002 documented a reduction in breast cancer mortality of 21% in women who attended for mammographic screening (3). The commonly accepted explanation for this survival benefi t is that screening picks up breast cancers at an earlier stage in their natural history. This stage shift at diagnosis is a refl ection of lead-time bias, which is the time interval between cancer detection by mammography and the time the cancer would have been detected in the absence of screening (4). Another factor that contributes to the improved prognosis of patients with a screen-detected cancer is length bias as screening is more likely to detect slow-growing cancers that are associated with a better survival, as they remain asymptomatic for longer (5).
