Uterine leiomyomata (UL), benign tumors of the myometrium, are influenced by sex steroid hormones. A history of UL diagnosis has been associated with a higher risk of uterine malignancies. The relation between UL and breast cancer, another hormonally responsive cancer, has not been studied.MethodsWe investigated the association between self-reported physician-diagnosed UL and incidence of breast cancer in the Black Women’s Health Study, a prospective cohort study. We followed 57,747 participants without a history of breast cancer from 1995 to 2013. UL diagnoses were reported at baseline and biennially. Breast cancer was reported on biennial questionnaires and confirmed by pathology data from medical records or cancer registries. Cox regression was used to derive incidence rate ratios (IRRs) and 95 % confidence intervals (CI) and adjust for potential confounders.ResultsThere were 2,276 incident cases of breast cancer (1,699 invasive, 394 in situ, and 183 unknown) during 879,672 person-years of follow-up. The multivariable IRR for the overall association between history of UL and breast cancer incidence was 0.99 (95 % CI 0.90–1.08), with similar results for ER + (IRR = 1.03) and ER − breast cancer (IRR = 1.05). IRRs for early diagnosis of UL (before age 30) were slightly above 1.0, with IRRs of 1.14 (95 % CI 0.99–1.31) for overall breast cancer, 1.14 (95 % CI 0.93–1.40) for ER + breast cancer, and 1.20 (95 % CI 0.89–1.61) for ER − breast cancer. IRRs for early diagnosis of UL were elevated for breast cancer diagnosed before 40 years of age (IRR = 1.39, 95 % CI 0.97–1.99) and premenopausal breast cancer (IRR = 1.26, 95 % CI 1.01–1.58). No consistent patterns in risk were observed across estrogen receptor subtypes, and IRRs did not differ appreciably within strata of BMI, female hormone use, mammography recency, or family history of breast cancer.ConclusionsThe present study of US black women suggests that a history of UL diagnosis is unrelated to the incidence of breast cancer overall. The positive associations observed for early diagnosed UL with breast cancer before age 40 and with premenopausal breast cancer require confirmation in future studies.

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