Breast cancer and mammography Study results.
Twenty five year follow-up for breast cancer incidence
and mortality of the Canadian National Breast
Screening Study: randomised screening trial
Anthony B Miller professor emeritus1, Claus Wall data manager1, Cornelia J Baines professor emerita1, Ping Sun statistician2, Teresa To senior scientist3, Steven A Narod professor1 2
1Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario M5T 3M7, Canada; 2Women’s College Research Institute, Women’s College Hospital, Toronto, Ontario M5G 1N8, Canada; 3Child Health Evaluative Services, The Hospital for Sick Children, Toronto, Ontario, Canada
Objective To compare breast cancer incidence and mortality up to 25 years in women aged 40-59 who did or did not undergo mammography screening.
Design Follow-up of randomised screening trial by centre coordinators, the study’s central office, and linkage to cancer registries and vital statistics databases.
Setting 15 screening centres in six Canadian provinces,1980-85 (Nova Scotia, Quebec, Ontario, Manitoba, Alberta, and British Columbia).
Participants 89 835 women, aged 40-59, randomly assigned to mammography (five annual mammography screens) or control (no mammography).
Interventions Women aged 40-49 in the mammography arm and all women aged 50-59 in both arms received annual physical breast examinations. Women aged 40-49 in the control arm received a single examination followed by usual care in the community.
Main outcome measure Deaths from breast cancer.
Results During the five year screening period, 666 invasive breast cancers were diagnosed in the mammography arm (n=44 925 participants) and 524 in the controls (n=44 910), and of these, 180 women in the mammography arm and 171 women in the control arm died of breast cancer during the 25 year follow-up period. The overall hazard ratio for death from breast cancer diagnosed during the screening period associated with mammography was 1.05 (95% confidence interval 0.85 to 1.30). The findings for women aged 40-49 and 50-59 were almost identical. During the entire study period, 3250 women in the mammography arm and 3133 in the control arm had a diagnosis of breast cancer, and 500 and 505, respectively, died of breast cancer. Thus the cumulative mortality from breast cancer was similar between women in the mammography arm and in the control arm (hazard ratio 0.99, 95% confidence interval 0.88 to 1.12). After 15 years of follow-up a residual excess of 106 cancers was observed in the mammography arm, attributable to over-diagnosis.