Data from Association of Fibroids, Endometriosis, and Gynecologic Surgeries with Breast Cancer Incidence and Hormone Receptor Subtypes

AbstractBackground:Fibroids and endometriosis are sex hormone–mediated and exhibit cancer-like behavior. Breast cancer may be more common in women who have had these conditions, but the literature is conflicting and does not always address factors like hysterectomy/oophorectomy status, race/ethnicity, menopause, and hormone receptor subtypes.Methods:Data are from the Sister Study, a cohort of 50,884 U.S. women enrolled in 2003 to 2009 and followed through 2020. Cox proportional hazards models with time-varying exposures and covariates assessed the relationship of fibroids or endometriosis with breast cancer. Logistic regression examined the association with estrogen receptor (ER) status among cases.Results:Fibroids (19,932 cases) were positively associated with breast cancer [fully adjusted HR: 1.07; 95% confidence interval (CI): 1.01–1.14], notably among Black participants (HR: 1.34; 95% CI: 1.07–1.69) and women who had a hysterectomy (HR: 1.18; 95% CI: 1.05–1.31). Endometriosis (3,970 cases) was not associated with breast cancer (HR: 0.99; 95% CI: 0.91–1.08). Among 4,419 breast cancer cases, fibroids were positively associated with ER+ subtypes (OR: 1.34; 95% CI: 1.10–1.65), while endometriosis was negatively associated with ER+ subtypes (OR: 0.78; 95% CI: 0.61–1.01).Conclusions:We observed a modest positive association between fibroids and breast cancer, particularly ER+ breast cancer. No relationship with endometriosis and breast cancer incidence was found.Impact:Fibroids, even in those with a family history of breast cancer, might modify breast cancer risk stratification tools. Future studies should further assess this link and interrogate shared risk factors.