Effect on breast cancer mortality of a mammography screening program initiated in 1998 was assessed. Two effect estimates were obtained for each of three groups: participants, eligible women and all women of the target age group including those with prior breast cancer diagnosis. Methods: Four approaches were used: 1) observed and projected breast cancer mortality trends for 1998-2004 were compared, 2) breast cancer mortality in the first five years of the program (1998-2003) was compared to mortality for the five years prior to program initiation (1992-1997) restricting numerators of rates to breast cancer deaths occurring among incident cases, 3) observed number of breast cancer deaths among 523,830 program participants was compared to expected number based on breast cancer incidence and survival of non-participants, 4) nested case-control study (873 cases, 8730 controls) was done within the 1,054,620 women eligible for screening. Results: Among participants, estimates of breast cancer mortality reduction associated with screening were 35% (95% confidence interval (CI): 23% to 48%) and 41% (95% CI: 28% to 52%). Among eligible women, estimates were 11% (95% CI: 1% to 21%) and 7% (95% CI: 1% to 13%). Among all women of the target age group, estimates were 3% (95% CI: -1% to 15%) and 3% (95% CI: -1% to 6%). Conclusion: The four approaches used provided effect estimates that were consistent within and between groups studied. Effect estimates and their consistency support the view that initiation of this screening program led to a significant reduction in breast cancer mortality within five years of its initiation.
A population-based mammography screening program was initiated in Québec, Canada, in 1998. This program invites by letter all women aged 50-69 for biennial screening mammography in designated screening centers. Such community screening programs have been associated with a reduction of breast cancer mortality using a variety of epidemiologic methods that have different strengths and limitations. Possible variation in extent of residual bias of these methods could lead to differences and non-comparability of estimates of screening effects. Thus, in order to estimate the possible effect on breast cancer mortality of the Québec program in the first five years after its initiation, we carried-out four comparisons that provided two estimates of effect for each of three groups of women. Two estimates were obtained for women who participated in the program (participants). These are the only women whose breast cancer mortality can be affected directly by program screening. Among participants, the expected breast cancer mortality reduction associated with screening could reach 35%. Two estimates were also obtained for women who were eligible to the program (eligible women). This group which includes participants as well as non-participants, is of interest because it is the one targeted by the program. The aim of this screening program was to reduce breast cancer mortality in eligible women by 25% after 10 years of operation. Finally, two estimates were obtained for the entire population of women in the target age group (all women of the target age group). This group is of interest because screening is expected to be reflected in overall population age-specific breast cancer mortality statistics. This group includes eligible women as well as women in the target age group who already had a breast cancer diagnosis before program initiation or before becoming eligible to the program once the program had started. Women with prior diagnosis of breast cancer will be responsible for a substantial proportion of breast cancer deaths in the first years after program initiation.