The Canadian National Breast Screening Study conducted in the 1980s and led by researchers at the University of Toronto evaluated the efficacy of breast cancer screening in reducing mortality from breast cancer. Because the research was supposedly a “gold standard” randomized controlled trial, its results, published in academic journals and reported in the media, have influenced public perceptions and informed policy on mammography screening in several countries.
However, over the past decades, flaws in this study have come to light. My colleagues and I learned of failures in randomization, and we and other researchers have found other serious problems. We think these flaws strongly suggest the publications of CNBSS results should be retracted. Despite being informed of the flaws in this study in 2021, the University of Toronto has not adequately or appropriately addressed these issues.
The CNBSS was configured as two separate randomized clinical trials, one for women in their 40s at entry and the other for women in their 50s. In CNBSS1, 50,000 women ages 40-49 were supposedly randomly assigned to the intervention arm, in which they would receive up to five annual screens with two view film mammography plus clinical breast examination by a nurse, or to the control arm, where they received a single clinical examination at entry and usual care (essentially, no screening) afterwards. In CNBSS2 for 40,000 women, the randomization was between the intervention of mammography plus a clinical breast exam versus clinical exam only.
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