Differential Patterns of Risk Factors for Early-Onset Breast Cancer by ER Status in African American Women

Background: Given the disproportionately high incidence of early-onset breast cancer and aggressive subtypes, such as estrogen receptor (ER)-negative tumors, in African American (AA) women, elucidation of risk factors for early onset of specific subtypes of breast cancer is needed. Methods: We evaluated associations of reproductive, anthropometric, and other factors with incidence of invasive breast cancer by age at onset (<45, ≥45) in 57,708 AA women in the prospective Black Women's Health Study. From 1995 to 2013, we identified 529 invasive breast cancers among women <45 years of age (151 ER−, 219 ER+) and 1,534 among women ≥45 years (385 ER−, 804 ER+). We used multivariable Cox proportional hazards regression to estimate hazard ratios (HRs) for associations by age and ER status. Results: Higher parity, older age at first birth, never having breastfed, and abdominal adiposity were associated with increased risk of early-onset ER− breast cancer: HRs were 1.71 for ≥3 births versus one birth; 2.29 for first birth after age 25 versus <20 years; 0.61 for ever having breastfed versus never; and 1.64 for highest versus lowest tertile of waist-to-hip ratio. These factors were not associated with ER− cancer in older women or with ER+ cancer regardless of age. Conclusions: Differences in risk factors by ER subtype were observed for breast cancer diagnosed before the age of 45 years. Impact: Etiological heterogeneity by tumor subtype in early-onset breast cancer, in combination with a higher prevalence of the risk factors in AA women, may explain, in part, racial disparities in breast cancer incidence. Cancer Epidemiol Biomarkers Prev; 26(2); 270–7. ©2016 AACR.