A model for determining the effect of mammography service screening.

In an overview, Swedish mammography screening trials demonstrated a 29% reduction in breast cancer mortality in women aged 50-69 and no effect on total mortality. Three Danish regions introduced mammography screening in the 1990s. The authors developed a method to evaluate the effect of mammography service screening on breast cancer mortality and validated it applying it to total mortality, where no effect was expected. Study groups and historical and national control groups were analysed using Poisson regression. Total mortality in the screening regions and periods was close to that expected in the absence of screening. A small (3%) excess risk, also seen in a non-screening area, probably results from regional differences changing over time. The effect of this is inseparable from the screening effect. The design is adequate for analysing the effect of mammography service screening on breast cancer mortality.