Introduction: Estrogen receptor (ER) and Progesterone receptor (PR) negative breast cancer (ER-PR-) is associated with poor prognosis. Little is known about modifiable risk factors, such as oral contraceptive use or breastfeeding, that can reduce risk of this cancer type; however, emerging evidence suggests that increased parity without breastfeeding may increase the risk of ER-PR- breast cancer, and that breastfeeding specifically will reduce this increased risk. Using population-based ascertained breast cancer cases from the Breast Cancer Family Registry (BCFR), we examined reproductive risk factors in relation to ER-PR- breast cancer.

Methods: Using data from 4,011 incident invasive breast cancer cases and 2,997 population-based controls from the BCFR, we evaluated the association between reproductive and hormonal risk factors with ER and PR status using unordered polytomous logistic regression, comparing two case groups, hormone receptor positive (HR+) breast cancers and ER-PR- breast cancer, to the control group.

Results: High parity without breastfeeding was positively associated only with ER-PR- tumors (odds ratio [OR] =1.57, 95% confidence interval [CI], 1.10-2.24, for ≥3 live births vs. nulliparity). High parity, when coupled with breastfeeding, was no longer associated with ER-PR- breast cancer (OR=0.93, 95%CI 0.71-1.22), and was inversely associated with all other HR+ cancers (OR=0.82, 95%CI 0.68-0.98). Parity (≥1 live birth vs. nulliparity), regardless of breastfeeding history, was protective against HR+ cancers only among postmenopausal women (OR=0.69, 95%CI 0.50-0.94). Oral contraceptive (OC) use was inversely associated with HR+ cancer (OR=0.82, 95%CI 0.69-0.96), but not associated with ER-PR- cancer. However, we found that OC use that commenced prior to 1975 was associated with an increased risk of ER-PR- cancer (OR=1.32, 95%CI 1.04-1.67), whereas OC use remained inversely associated with HR+ cancer only among women who began use in 1975 or later (OR=0.59, 95% CI 0.48-0.72).

Conclusion: As there are few modifiable factors for ER-PR- breast cancers, our findings lend further support to the potential that may be gained through greater promotion of breastfeeding to women at risk of ER-PR- breast cancers, and indicate that OC use is not a source of increased risk for these women.

Citation Format: Meghan E. Work, Esther M. John, John L. Hopper, Irene L. Andrulis, Mary Beth Terry. Parity, breastfeeding, and oral contraceptive use and risk of estrogen- and progesterone-negative breast cancer in the Breast Cancer Family Registry. [abstract]. In: Proceedings of the Eleventh Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2012 Oct 16-19; Anaheim, CA. Philadelphia (PA): AACR; Cancer Prev Res 2012;5(11 Suppl):Abstract nr B98.