Canadian National Breast Screening Study-2: 13-year results of a randomized trial in women aged 50-59 years

Background: Screening for breast cancer with mammography in women aged 50 y ears or more has been shown to reduce mortality from breast cancer. However , the extent to which mammography contributes to the reduction of mortality in women who also undergo physical examination of the breasts is not known . This study was designed to compare breast cancer mortality following annu al screening consisting of two-view mammography and physical examination of the breasts with mortality following annual screening by physical examinat ion only. Breast self-examination was taught to all participants. Methods: This trial randomly and individually assigned 39405 women aged 50-59 years, recruited from January 1980 through March 1985, to one of the study arms. The women were followed by record linkage with the Canadian National Cancer Registry and National Mortality Database to December 31, 1993, and by acti ve follow-up of breast cancer patients to June 30, 1996, Results: Randomiza tion achieved virtually equal distribution of demographic and breast cancer risk variables. At the first annual screen, 21% of the cancers found by ma mmography alone (in the mammography plus physical examination group) were 2 0 mm or more in size compared with 46% of those found by physical examinati on in the mammography plus physical examination group and 56% in the physic al examination-only group. The corresponding percentages for screens 2-5 we re 10%, 42%, and 50%, respectively. Screening detected 267 invasive breast cancers in the mammography plus physical examination group compared with 14 8 in the physical examination-only group. By December 31, 1993, 622 invasiv e and 71 in situ breast carcinomas were ascertained in the mammography plus physical examination group, and 610 and 16 were ascertained in the physica l examination-only group. At 13-year follow-up, with 107 and 105 deaths fro m breast cancer in the respective groups, the cumulative rate ratio was 1.0 2 (95% confidence interval = 0.78-1.33). Conclusion: In women aged 50-59 ye ars, the addition of annual mammography screening to physical examination h as no impact on breast cancer mortality.