The Fundamental Flaws of the CNBSS Trials: A Scientific Review

Although the two Canadian National Breast Screening Study (CNBSS) trials were performed 40 years ago, their negative findings continue to heavily influence screening policies around the world. These policies, based on underestimates of the mortality reduction attributable to mammography particularly for women in the 40-49-year age range, contribute to increased mortality and morbidity from breast cancer. This review summarizes principles of a randomized controlled trial (RCT) and evaluates the compliance of the CNBSS1 and CNBSS2 RCTs in the context of these principles. We describe the fundamental flaws of the CNBSS trials, which failed to demonstrate mortality benefit of screening mammography and contribute to their being the only two outlier studies of eight screening mammography RCTs. The most significant flaws of the trials are (1) inadequate power to detect significant differences in breast cancer mortality; (2) very poor quality mammography with low sensitivity and cancer detection rates; (3) inclusion of women with symptoms of breast cancer; and (4) study design that allowed for violation of the randomization of the allocation process. Finally, we demonstrate that the conditions of the screening intervention in the CNBSS do not reflect the environment of modern population-based screening mammography programs.