Estrogen receptor positive tumors: Do reproductive factors explain differences in incidence between black and white women?
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The incidence of estrogen receptor positive (ER+) breast cancer is higher among white women relative to black women. In two large prospective cohorts, the Black Women’s Health Study (BWHS) and the Nurses’ Health Study II (NHSII), we investigated whether reproductive factors explain the difference.
During 1,582,083 person-years of follow-up of 140,914 women observed from 1995 to 2007, 327 ER+ breast cancers were identified among black women in BWHS and NHSII and 1,179 among white women in NHSII. Cox proportional hazards regression models, stratified by race and pooled, were used to estimate hazard ratios (HR) and 95 % confidence intervals (CI) for the association of race, parity, age at first birth, and lactation in relation to risk of ER+ cancer with adjustment for age and other breast cancer risk factors.
Age at first birth differed markedly in the two groups, with 66 % of parous black women having their first child before age 25 as compared with 36 % of white women. Each additional year of age at first birth was associated with a 4 % increased risk of ER+ breast cancer among both racial groups. Relative to nulliparous women, parous women were at decreased risk of ER+ breast cancer (HR 0.59, 95 % CI 0.20, 1.77), in black women and (HR 0.63, 95 % CI 0.45, 0.87) in white women. The HR for the association of black race with ER+ cancer was 0.67 (95 % CI 0.53, 0.84) in a model that adjusted for age only, 0.77 (95 % CI 0.61, 0.99) in a model that controlled for parity, age at first birth, and other reproductive/hormonal factors, and 0.83 (95 % CI 0.70, 0.98) in a model that additionally controlled for other breast cancer risk factors such as alcohol consumption and use of hormone supplements. Similar associations were seen among premenopausal women and in an analysis restricted to ER+PR+ tumors.
Reproductive factors explained some of the higher incidence of ER+ tumors among white women as compared to black women.
KeywordsBreast carcinoma Race Estrogen receptor Reproductive factors Incidence Black African American
Erica T. Warner was supported by National Cancer Institute grant number 5T32CA009001-36 and National Institute of General Medical Sciences grant number 5R25GM055353-14. Dr. Colditz is supported in part by an American Cancer Society Cissy Hornung Clinical Research Professorship. Black Women’s Health Study is supported by National Cancer Institute grant R01 CA058420. The Nurses’ Health Study II is supported by National Cancer Institute grant number R01 CA50385. We thank the participants of the Black Women’s Health Study and Nurses’ Health Study II. The content of this paper is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health. Data on breast cancer pathology were obtained from several state cancer registries (AZ, CA, CO, CT, DE, DC, FL, GA, IL, IN, KY, LA, MD, MA, MI, NJ, NY, NC, OK, PA, SC, TN, TX, VA) and results reported do not necessarily represent their views.
Conflict of interest
The authors declare that they have no conflict of interest.
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