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Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial

BMJ 2014; 348 doi: https://doi.org/10.1136/bmj.g366 (Published 11 February 2014) Cite this as: BMJ 2014;348:g366
  1. Anthony B Miller, professor emeritus1,
  2. Claus Wall, data manager1,
  3. Cornelia J Baines, professor emerita1,
  4. Ping Sun, statistician2,
  5. Teresa To, senior scientist3,
  6. Steven A Narod, professor12
  1. 1Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario M5T 3M7, Canada
  2. 2Women’s College Research Institute, Women’s College Hospital, Toronto, Ontario M5G 1N8, Canada
  3. 3Child Health Evaluative Services, The Hospital for Sick Children, Toronto, Ontario, Canada
  1. Correspondence to: A B Miller ab.miller{at}utoronto.ca
  • Accepted 16 January 2014

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.

Footnotes

  • Contributors: CW collated the data for analysis. PS performed the analysis. SAN, TT, and ABM planned the analysis. ABM and SAN drafted the manuscript. All authors checked the manuscript, tables, and figures for accuracy and completeness. ABM is guarantor.

  • Funding This study was supported by the Canadian Breast Cancer Research Alliance, Canadian Breast Cancer Research Initiative, Canadian Cancer Society, Health and Welfare Canada, National Cancer Institute of Canada, Alberta Heritage Fund for Cancer Research, Manitoba Health Services Commission, Medical Research Council of Canada, le Ministère de la Santé et des Services Soçiaux du Québec, Nova Scotia Department of Health, and Ontario Ministry of Health. ABM was supported in part by a national health scientist award from Health and Welfare Canada. The study sponsors (funders) had no role in the preparation, approval, or submission of this manuscript.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf and declare: no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Ethical approval: This study was originally approved by the University of Toronto’s Human Experimentation Committee. The extended follow-up and its analysis was approved by the Women’s College Hospital research ethics board (reference 2007-0025-B)

  • Data sharing: No additional data available.

  • Transparency: The lead author (the manuscript’s guarantor), ABM, affirms that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies are disclosed.

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