Abstract P1-07-03: Reproductive factors and subtype specific breast cancer risk

Introduction: Reproductive history and breast cancer risk reportedly differ by the estrogen receptor (ER±) and by the joint expression of ER and the human epidermal growth factor-2 receptor (ER±/HER2±). However, large sample sizes are needed to identify risk factor associations for the relatively less common ER- subtypes. Material and Methods: We, therefore, linked two large-scale and population-based Danish registries to assess the associations for parity, number of live births, and age at first live birth (AFLB) with receptor-specific breast cancer risk. Relative risks (RRs) and 95% confidence intervals (CIs) for associations were estimated with Poisson regression models. Results: With nearly 31 million women-years of follow-up, there were 45786 Danish women between the ages 20-84 years who developed an invasive breast cancer during the study period 1992-2011. Parity significantly reduced risk for ER+ and ER+/HER2- subtypes (RR for ER+/HER2- = 0.92; 0.87, 0.98) and suggestively increased risk for ER- and ER-/HER2- subtypes (RR for ER-/HER2- = 1.16; 0.99, 1.36). RRs increased with advancing AFLB for ER+ cancers, especially among premenopausal women; and were elevated for ER- cancers among age groups 12-19 years and 30-34 years compared to the reference age group 20-24 years. Conclusion: Associations of breast cancer risk and reproductive history varied among Danish women by ER± and by ER±/HER2±, consistent with receptor-specific etiological heterogeneity. Risk estimates for ER+ and ER+/HER2- cancers were similar to the well-established associations for breast cancer overall, whereas relative risks for ER- and ER/HER2- cancers tended to be null or the inverse of ER+ associations. Citation Format: Anderson WF, Pfeiffer RM, Wohlfahrt J, Ejlertsen B, Jensen M-B, Kroman NT. Reproductive factors and subtype specific breast cancer risk. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P1-07-03.