Postmenopausal cancer risk after self-reported endometriosis diagnosis in the Iowa Women's Health Study

BACKGROUND: Endometriosis, the presence of endometrial tissue outside the uterus, has an estimated prevalence between 4% and 10%. A recent study reported that women with a hospital discharge diagnosis of endometriosis were at increased risk of cancer at any site, breast cancer, ovarian cancer, and hematopoietic malignancies, especially non-Hodgkin lymphoma (NHL). METHODS: The authors examined whether self-reported diagnosis of endometriosis was associated with increased risk of various cancers in the Iowa Women's Health Study. Incident cancer cases were identified from 1986 through 1998. Cox proportional hazards regression models were used to calculate relative risks and 95% confidence intervals for the particular cancers of interest. RESULTS: Of 37,434 participants in this analysis, 3.8% reported a history of endometriosis at baseline in 1986. After 13 years of follow-up, 1795 breast, 188 ovarian, and 243 NHL cases were detected in the cohort. Endometriosis was not associated with risk of all cancers combined (age-adjusted relative risk [RR], 0.9; 95% confidence interval [CI], 0.7-1.2), breast carcinoma (RR, 1.0; 95% CI, 0.8-1.3), or ovarian carcinoma (RR, 0.8; 95% CI, 0.2-2.4). However, endometriosis was significantly associated with risk of NHL (age-adjusted RR, 1.8; 95% CI, 1.0-3.0), especially diffuse NHL (RR, 3.2; 95% CI, 1.8-5.6). Multivariate adjustment had minimal effect on the point estimates of risk (NHL RR, 1.7; 95% CI, 0.97-2.7; diffuse NHL RR, 3.3; 95% CI, 1.9-5.9). Endometriosis was not associated with elevated risk of lung, urinary tract, endometrial, melanoma, or colorectal carcinomas (RRs, 1.2, 0.8, 1.2, 0.7, and 0.7, respectively). CONCLUSIONS: These results corroborate a previously reported association between endometriosis and increased risk of NHL but not cancer at other sites.