Background: Increase in breast cancer incidence associated with mammography screening diffusion may have attenuated risk associations between family history and breast cancer. Methods: The proportions of women ages 40 to 74 years reporting a first-degree family history of breast cancer were estimated in the Breast Cancer Surveillance Consortium cohort (BCSC: N ¼ 1,170,900; 1996–2012) and the Collaborative Breast Cancer Study (CBCS: cases N ¼ 23,400; controls N ¼ 26,460; 1987–2007). Breast cancer (ductal carcinoma in situ and invasive) relative risk estimates and 95% confidence intervals (CI) associated with family history were calculated using multivariable Cox proportional hazard and logistic regression models. Results: The proportion of women reporting a first-degree family history increased from 11% in the 1980s to 16% in 2010 to 2013. Family history was associated with a >60% increased risk of breast cancer in the BCSC (HR, 1.61; 95% CI, 1.55–1.66) and CBCS (OR, 1.64; 95% CI, 1.57–1.72). Relative risks decreased slightly with age. Consistent trends in relative risks were not observed over time or across stage of disease at diagnosis in both studies, except among older women (ages 60–74) where estimates were attenuated from about 1.7 to 1.3 over the last 20 years (P trend ¼ 0.08 for both studies). Conclusions: Although the proportion of women with a first-degree family history of breast cancer increased over time and by age, breast cancer risk associations with family history were nonetheless fairly constant over time for women under age 60. Impact: First-degree family history of breast cancer remains an important breast cancer risk factor, especially for younger women, despite its increasing prevalence in the mammography screening era.
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