Mortality reductions due to mammography screening: Contemporary population-based data.

Our objective was to compare breast cancer mortality in two regions of the Republic of Ireland that introduced a screening programme eight years apart, and to estimate the steady-state mortality deficits the programme will produce. We carried out age- and year-matched between-region comparison of breast cancer mortality rates, and of incidence rates of stage 2-4 breast cancer, in the eligible cohorts. The regions comprised counties that, beginning in early 2000 (region 1) and late 2007 (region 2), invited women aged 50-64 to biennial mammography screening. The data were supplied by the National Cancer Registry, Central Statistics Office. As impact measures, we used age-and-year-matched mortality (from breast cancers diagnosed from 2000 onwards), rate ratios and incidence rate ratios in the compared regions from 2000 to 2013. Ratios were adjusted for between-region differences in background rates. In cohorts too old to be invited, death rates in regions 1 and 2 were 702 per 0.91 and 727 per 0.90 million women-years respectively (Ratio 0.96). In the eligible cohorts, they were 1027 per 2.9 and 1095 per 2.67 (Ratio 0.88). Thus, rates in cohorts that could have benefitted were 9% lower in region 1 than region 2: (95%CI: -20%, +4%). The incidence rates of stage 2-4 breast cancer were 7% lower in region 2 than region 1 over the entire 14 year period, and 20% lower in 2007, i.e., before the screening in region 2 began to narrow the difference. Since mortality reductions due to screening only manifest after several years, the full impact of screening has not yet been realized in region 1. The lower rate observed in that region is a conservative estimate of the steady state benefit. Additional deaths would have been averted had screening continued beyond age 64.