DiaLog: Screening Mammography Reduces Breast Cancer–Related Deaths
By Therese Bevers, M.D., Professor, Department of Clinical Cancer Prevention
The findings of the CNBSS contradict not only findings from other mammography trials but also a meta-analysis conducted by the U.S. Preventive Services Task Force, which reported that mammographic screening significantly reduced the relative risk for breast cancer–related mortality for women 39–69 years old.
The CNBSS has been plagued by criticisms dating back to the early 1990s. Among the most important criticisms is that patients were randomly assigned to the intervention (mammography) or control (no mammography) arm after the performance of a physical examination rather than at study entry. The knowledge of the clinical breast exam findings prior to patients’ assignment to the intervention or control arms had the potential to influence the randomization process. Indeed, in the CNBSS, the number of women 40–49 years old in the mammography arm who had breast cancers with four or more lymph node metastases exceeded that of the control group by 380%. Such a skewed allocation is unlikely to have occurred by chance and would minimize or eliminate any impact of mammographic screening on breast cancer–related mortality.
Additionally, concerns have been raised regarding the acquisition and interpretation of images for the CNBSS. According to an external review, more than half of the mammograms obtained in the first 4 years of the trial were judged as poor or unacceptable, but the image quality improved in the trial’s final 2 years. Also, technologists in the trial were not taught to position patients properly, and the radiologists were not experienced in the interpretation of mammographic images.
Concerns about the CNBSS negate its strengths and render its recommendations regarding the use of mammography for breast cancer screening unhelpful. At this time, our greatest tool for the early detection of breast cancer remains screening mammography.
American College of Radiology and Society of Breast Imaging. BMJ article on breast cancer screening effectiveness: incredibly flawed and misleading [PDF]. Society of Breast Imaging Web site. Posted February 11, 2014. Accessed April 16, 2014.
Miller AB, Wall C, Baines CJ, et al. Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial. Br Med J. 2014;348:g366.
Nelson HD, Tyne K, Naik A, et al. Screening for breast cancer: an update for the U.S. Preventive Services Task Force. Ann Intern Med. 2009;151:727–737.
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Other articles in OncoLog, May 2014 issue: