Screen shot.

Top Line: We invest a lot of societal capital in breast cancer screening programs, but conflicting data suggest improvements in treatments and not screening are the real reason for better outcomes.
The Study: This Danish study takes an interesting look at some of the inherent limitations of studies that analyze the effect of screening on mortality. Limitation 1: Retrospectively analyzing data for all women “of screening age.” In Copenhagen, regular biennial breast cancer screening was instituted in 1991—but not in all parts of Denmark. So the authors created birth-matched cohorts in Copenhagen where women were actually invited to breast cancer screening and other Danish regions where women were “pseudo” invited at the same ages. Limitation 2: Broadly associating breast cancer deaths with screening effects. The problem here is that measuring mortality events resulting from diagnoses before or after (not during) screening dilutes the effects of screening. Using three models of screening effects, the authors show that screening from age 50 to 70 appears to have a small effect on mortality. However, for women diagnosed during screening ages, the effect was magnified to a 20% reduction in breast cancer mortality.
Bottom Line: Breast cancer screening appears to reduce breast cancer mortality by as much as one-fifth in women undergoing active screening. | Beau, J Clin Oncol 2018


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