Objective: To investigate the association between laparoscopically confirmed endometriosis and the risk of breast cancer. Previous research on endometriosis and breast cancer has reported mixed results.
Methods: Our prospective cohort study included 116,430 women from the Nurses' Health Study II cohort followed from 1989 until 2013. Our primary analysis investigated the association between self-reported laparoscopically confirmed endometriosis and the risk of breast cancer. Breast cancer diagnosis was verified through medical records. Multivariable adjusted Cox proportional hazard models were used to calculate hazard ratios (HRs) and 95% confidence intervals (CIs). Breast cancer was further classified by menopausal status at the time of diagnosis and tumor hormone receptor status verified through tissue microarrays when available and medical records.
Results: At baseline, 5,389 (5%) women reported laparoscopically confirmed endometriosis. Over 24 years of follow-up, 4,979 (3%) incident breast cancer cases were diagnosed. Women with endometriosis were not at higher risk for overall (adjusted HR 0.96, 95% CI 0.88-1.06), premenopausal (adjusted HR 1.05, 95% CI 0.89-1.23), or postmenopausal breast cancer (adjusted HR 0.93, 95% CI 0.80-1.07). However, associations varied by tumor hormone receptor status (P value, test for heterogeneity: .001), although women with endometriosis were not at increased risk of estrogen- and progesterone receptor-positive (ER+/PR+) tumors (adjusted HR 1.00, 95% CI 0.87-1.14) or ER- and PR- tumors (adjusted HR 0.90, 95% CI 0.67-1.21). Women with endometriosis reported 2.87 ER+/PR- breast cancer cases per 10,000 person-years compared with women without endometriosis (1.32/10,000 person-years), which resulted in nearly a twofold increased risk of ER+/PR- breast cancers (adjusted HR 1.90, 95% CI 1.44-2.50).
Conclusion: Endometriosis was not found to be associated with overall risk of breast cancer in this study; however, endometriosis was significantly associated with an increased risk of ER+/PR- breast tumors, which should be interpreted cautiously.