Major failings of trial procedures and quality of screening fatally compromise the results of the Canadian National Breast Screening Studies

Despite overwhelming evidence of a major reduction in deaths, the debate about the efficacy of breast cancer screening has continued for over 50 years. The poor results in the Canadian National Breast Screening Studies (CNBSS) have been used to challenge the benefits shown by the other randomized, controlled trials. They continue to be used in assessing the value of breast cancer screening despite their unblinded allocation process, which first identified women with breast abnormalities and then assigned them on open lists allowing for nonrandom assignment, compromising the trials and rendering their results unreliable. There were, statistically significantly, more women with advanced cancers who were assigned to the screening arm in CNBSS1. The early results for CNBSS1 showed an excess of women dying in the screening arm, and an (otherwise inexplicable) greater than 90%, 5-year survival for the control women. The failure of random assignment also explains why the clinically evident cancers were larger in the screening arms than the cancers in the “usual care” arms, despite the fact that the screened women underwent very intense clinical breast examinations each year by highly skilled examiners. The claim that balanced demographic factors prove random assignment is also false. Nonrandom allocation of a hundred or more women with clinically evident abnormalities would have no detectable influence on the distribution of demographic factors. In summary, policy decisions about mammography should not be influenced by the results of the CNBSS.