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Russell P. Harris, How Best to Determine the Mortality Benefit From Screening Mammography: Dueling Results and Methodologies From Canada, JNCI: Journal of the National Cancer Institute, Volume 106, Issue 11, November 2014, dju317, https://doi.org/10.1093/jnci/dju317
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In this issue of the Journal, Coldman et al. report results from the Pan-Canadian Study of Mammography Screening and Mortality from Breast Cancer, an analysis of observational data from seven of 12 provincial screening programs from 1990 to 2009 ( 1 ). They find that “participation in mammography screening programs in Canada was associated with substantially reduced breast cancer mortality,” with a relative risk reduction of 40%. Interestingly, five of the seven provinces involved in this study were also sites for the Canadian Breast Screening Study (CNBSS), a randomized controlled trial (RCT) that recently reported, after 25 years of follow-up (1980 to 2005), no reduction in breast cancer mortality from screening mammography ( 2 ). If we are to determine the mortality benefit from current breast cancer screening, and to monitor it over time, we need to understand the lessons from the fascinating contrast presented by these two studies.
In addition to the common setting, many aspects of the two studies were similar. In the Pan-Canadian provincial study, women ages 40 (five provinces started at 50) to 79 years (five provinces stopped at 69) self-referred for screening; in the CNBSS, women ages 40 to 59 years volunteered to be randomized. Mammography in the two studies appears to have been similar: two-view, single-read, analog mammography. CNBSS screened annually for five years; the provincial screening programs screened biennially (two provinces screened women aged 40–49 years annually). CNBSS included clinical breast examination (CBE) in some screening arms; some provinces used CBE and some didn’t. Referral to treatment in both studies was determined by primary care physicians.
