Objective: We examined the relationship between breast cancer family history and mammographic breast density.
Methods: Participants included 35,019 postmenopausal women aged >or=40 years enrolled in a population-based mammography screening program. We collected data on the number and type of 1st and 2nd degree female relatives with a history of breast cancer and their ages at diagnosis. We used the Breast Imaging Reporting and Data System breast density categories to identify women with fatty (1 = almost entirely fatty or 2 = scattered fibroglandular tissue) and dense (3 = heterogeneously dense or 4 = extremely dense) breasts. We used logistic regression to calculate odds ratios (OR) and 95% confidence intervals for dense (N = 18,111) compared to fatty breasts (N = 16,908).
Results: The odds of having dense breasts were 17% greater for women with affected 1st degree relatives than women with no family history. The odds increased with more affected 1st degree relatives [>or=3 vs. none (OR = 1.46; 1.05-2.01)] and among women with >or=1 affected 1st degree relative diagnosed <50 years (OR = 1.22; 1.10-1.34).
Conclusions: Having a family history of breast cancer was more strongly associated with mammographic breast density when the affected relatives were more genetically similar. There may be common, yet undiscovered, genetic elements that affect breast cancer and mammographic breast density.