Abstract 1169: Racial/ethnic differences in postmenopausal breast cancer incidence and risk factors: Results from the Multiethnic Cohort (MEC) Study
Authors: Sarink Danja , Le Marchand Loic , Loo Lenora W. , Park Song-Yi , White Kami K. , Setiawan V Wendy , Wu Anna H. , Wilkens Lynne R. , Merritt Melissa A. 
Published: September 2020

Abstract
The impact of hormone-related factors on breast cancer risk has been well-studied; however, data on racial/ethnic minorities, especially Native Hawaiian women, remain relatively scarce.
MEC participants completed a baseline questionnaire providing information on hormone-related factors. During a median follow-up of 20 years, 4,634 incident invasive breast cancer cases were identified among 81,511 postmenopausal women (20,003 White, 16,060 African American, 5,468 Native Hawaiian, 22,333 Japanese American, and 17,647 Latina). We used Cox proportional hazards regression models to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for associations of: (1) race/ethnicity with estrogen receptor positive (ER+) and negative (ER-) breast cancer risk and (2) hormone-related factors with ER+ and ER- breast cancer risk across race/ethnicity. Models were adjusted for baseline age and body mass index (BMI), parity, as well as ages at menarche, menopause, and first birth.
Compared to Whites, age-adjusted ER+ breast cancer risk was higher in Native Hawaiians (HR 1.42 [CI 1.26-1.60]), lower in African Americans (0.72 [0.66-0.80]) and Latinas (0.54 [0.49-0.60]), and similar in Japanese Americans (0.97 [0.89-1.05]). Risk for ER- disease was higher in African Americans (1.41 [1.15-1.72]) than in Whites, but no difference was observed in Native Hawaiians, Latinas or Japanese Americans. These results were unchanged after adjustment for hormone-related factors. In the full population, parity and a later age at natural menopause were associated with a decreased ER+ breast cancer risk, and postmenopausal hormone (PMH) use and a higher BMI with an increased risk. There was no significant interaction of these factors with race/ethnicity (p-value for interaction [p-int] ≥0.10). In race/ethnicity specific analyses, parity (vs nulliparity) was associated with lower ER+ breast cancer risk in Whites (0.72 [0.59-0.87]), Japanese Americans (0.57 [0.44-0.74]), and Latinas (0.63 [0.46-0.86]). Age at menopause (≥55 vs &lt;45 years) was inversely associated with ER+ disease risk in African Americans (0.69 [0.52-0.91]) and Japanese Americans (0.69 [0.57-0.84]). PMH use (ever vs never) increased ER+ breast cancer risk by 17-36% in all racial/ethnic groups except Native Hawaiians. BMI (≥30 vs &lt;25 kg/m2) was associated with a 22-66% increased risk of ER+ disease in all racial/ethnic groups. For ER- breast cancer, ever use of oral contraceptives was the only factor associated with risk in the full population (p-int 0.45) and we observed an increased risk of ER- disease in Whites (1.52 [1.09-2.12]) and African Americans (1.80 [1.30-2.51]).
Although we observed different associations across racial/ethnic groups for several hormone-related factors with risk of ER+ disease, these did not appear to explain the observed racial/ethnic differences in breast cancer incidence.
Citation Format: Danja Sarink, Loic Le Marchand, Lenora W. Loo, Song-Yi Park, Kami K. White, V Wendy Setiawan, Anna H. Wu, Lynne R. Wilkens, Melissa A. Merritt. Racial/ethnic differences in postmenopausal breast cancer incidence and risk factors: Results from the Multiethnic Cohort (MEC) Study [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1169.