Breast cancer screening effect across breast density strata: A case–control study

Breast cancer screening is known to reduce breast cancer mortality. A high breast density may affect this reduction. We assessed the effect of screening on breast cancer mortality in women with dense and fatty breasts separately. Analyses were performed within the Nijmegen (Dutch) screening programme (1975–2008), which invites women (aged 50–74 years) biennially. Performance measures were determined. Furthermore, a case–control study was performed for women having dense and women having fatty breasts. Breast density was assessed visually with a dichotomized Wolfe scale. Breast density data were available for cases. The prevalence of dense breasts among controls was estimated with age‐specific rates from the general population. Sensitivity analyses were performed on these estimates. Screening performance was better in the fatty than in the dense group (sensitivity 75.7% vs 57.8%). The mortality reduction appeared to be smaller for women with dense breasts, with an odds ratio (OR) of 0.87 (95% CI 0.52–1.45) in the dense and 0.59 (95% CI 0.44–0.79) in the fatty group. We can conclude that high density results in lower screening performance and appears to be associated with a smaller mortality reduction. Breast density is thus a likely candidate for risk‐stratified screening. More research is needed on the association between density and screening harms. What's new? High breast density is known to increase breast cancer risk and decrease sensitivity of mammographic screening. As a result, women with dense breasts may not benefit from screening to the same extent as women with lower breast density. Here, using data from the Nijmegen (Dutch) screening programme, differences in screening effect on breast cancer mortality were assessed among women with either dense or fatty breasts. Compared to fatty breasts, high breast density was associated with reduced screening performance and reduced effects on mortality. The findings suggest that modifications in effect are relevant for risk‐stratified and personalized screening.