A prospective study of endometriosis and risk of benign breast disease

Given the altered hormonal and inflammatory environment of women with endometriosis, several studies have suggested a positive association between endometriosis and breast cancer, although findings have been mixed. This study investigates the relationship between endometriosis and benign breast disease (BBD), benign lesions that are associated with an increased risk of breast cancer.

Among women in the Nurses' Health Study II followed from 1991-2003 (n = 76,393), we investigated the association between laparoscopically confirmed endometriosis and biopsy-confirmed BBD. Cox proportional hazard models, adjusted for a priori potential confounding factors, were used to calculate hazard ratios (HR) and 95 % confidence intervals (CI). Across follow-up, 2011 BBD biopsies were collected and centrally reviewed by study pathologists (nonproliferative = 675, proliferative = 1336). Effect modification by infertility history and use of screening mammography was investigated.

Endometriosis was associated with a modest increased risk of biopsy-confirmed BBD in crude and multivariable adjusted models (HR 1.20, 95 % CI 1.00-1.43). When evaluating subtypes of BBD, we did not see different associations for nonproliferative or proliferative BBD lesions, as endometriosis was associated with a modest increased risk for both (HR nonproliferative 1.15, 95 % CI 0.84-1.57; HR proliferative 1.22, 95 % CI 0.98-1.52). The association between endometriosis and proliferative BBD appeared strongest among women who had ever experienced infertility (HR 1.50, 95 % CI 1.12-2.03; P value, test for heterogeneity = 0.05). Sensitivity analyses investigating screening behaviors between those with and without endometriosis did not significantly alter results.

Endometriosis was associated with a modest increased risk of both proliferative and nonproliferative BBD, although future work should replicate this novel finding.