Is mammographic density differentially associated with breast cancer according to receptor status? A meta-analysis

Mammographic density (MD) is a strong marker of breast cancer risk, but it is debated whether the association holds, and is of a similar magnitude, for different subtypes of breast cancer defined by receptor status or gene expression profiles. A literature search conducted in June 2012 was used to identify all studies that had investigated the association of MD with subtype-specific breast cancer, independent of age. 7 cohort/case-control and 12 case-only studies were included, comprising a total of >24,000 breast cancer cases. Random effects meta-analysis models were used to combine relative risks (RR) of MD with subtype-specific breast cancer for case-control studies, and in case-only studies to combine relative risk ratios (RRR) of receptor positive versus negative breast tumors. In case-control/cohort studies, relative to women in the lowest density category, women in the highest density category had 3.1-fold (95 % confidence interval [CI] 2.2, 4.2) and 3.2-fold (1.7, 5.9) increased risk of estrogen receptor positive (ER+) and ER- breast cancer, respectively. In case-only analyses, RRRs of breast tumors being ER+ versus ER- were 1.13 (95 % CI 0.89, 1.42) for medium versus minimal MD. MD remained associated with screen-detected ER+ tumors, despite the expectation of this association to be attenuated due to masking bias and overdiagnoses of ER+ tumors. In eight contributing studies, the association of MD did not differ by HER2 status. This combined evidence strengthens the importance of MD as a strong marker of overall and of subtype-specific risk, and confirms its value in overall breast cancer risk assessment and monitoring for both research and clinical purposes.