MedicalResearch.com: What are the main findings of the study?
Prof. Miller: The study involved 89,835 women aged 40 to 59. All underwent an annual physical breast examination, while half were randomly assigned to undergo annual mammograms for five years, beginning in 1980.
During the five-year screening period, 666 invasive breast cancers were diagnosed in the mammography arm and 524 in the controls. Over the 25 year follow-up 180 women in the mammography arm and 171 women in the control arm died of breast cancer. The overall hazard ratio for death from breast cancer diagnosed during the screening period associated with mammography was 1.05 (95% CI: 0.85 – 1.30). The findings for women aged 40-49 and aged 50-59 were almost identical.
After 15 years of follow-up an excess of 106 cancers was observed in the mammography arm, attributable to over-diagnosis, i.e. 22% of screen-detected invasive breast cancers, half of those detected by mammography alone. This represents one over-diagnosed breast cancer for every 424 women screened by mammography.
By 2005, 3,250 of the 44,925 women in the mammography arm of the study were diagnosed with breast cancer, and 500 had died of it. The control group of 44,910 women had 3,133 breast cancer diagnoses and 505 breast cancer deaths.
We conclude that 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.
MedicalResearch.com: Were any of the findings unexpected?
Prof. Miller: Yes, in that when we began the study we expected to find a benefit from mammography screening. However, during the course of the study other research suggested that because of advances in the treatment of breast cancer the benefits from mammography were likely to be small, and that the harms from mammography screening, false positive detections and over-diagnosis, could overwhelm any benefits from mammography screening
MedicalResearch.com: What should clinicians and patients take away from your report?
Prof. Miller: Women should reconsider the value of mammography, and concentrate more on being aware of their breasts and ensuring that if any potential problem, e.g. a lump or change in shape of a breast occurs, they are examined by a skilled physician. Mammography then has an important role in diagnosis. Clinicians should ensure they allocate sufficient time to breast examinations, and understand and seek the signs of early breast cancer.
MedicalResearch.com: What recommendations do you have for future research as a result of this study?
- Careful re-evaluation of all breast cancer screening trials, taking fully into consideration the advances in breast cancer treatment that have occurred.
- Research into biomarkers (especially tissue markers) that can help inform the probable prognosis of detected breast cancers.
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: http://dx.doi.org/10.1136/bmj.g366 (Published 11 February 2014)
Cite this as: BMJ 2014;348:g366