“… in technically advanced countries, our results support the views of some commentators that the rationale for screening by mammography should be urgently reassessed by policy makers …”
Abstract
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.
Conclusion
Annual mammography in women aged 40-59 does not reduce mortality from breast cancer beyond that of physical examination or usual care when adjuvant therapy for breast cancer is freely available. Overall, 22% (106/484) of screen detected invasive breast cancers were over-diagnosed, representing one over-diagnosed breast cancer for every 424 women who received mammography screening in the trial.
Sources and Press releases:
- Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial, BMJ 2014;348:g366 11 February 2014 – PDF
- Too much mammography, BMJ 2014;348:g1403, 11 February 2014
- Vast Study Casts Doubts on Value of Mammograms
NewYorkTimes, 11 February 2014 - Study raises awareness of risks of mammograms
USA Today, 11 February 2014 - The Swiss Medical Board recommends a reform in breast cancer screening and fewer mammograms, 6 February 2014 – full report
- Tags Breast Cancer – Mammograms – Overdiagnosis – Screening