Risk of endometrial cancer among women with benign ovarian tumors - A Danish nationwide cohort study

Objective. The few studies on the association between benign ovarian tumors and endometrial cancer have been inconclusive. Using data from a large Danish register-based cohort study, we assessed the overall and type-specific risk of endometrial cancer among women with a benign ovarian tumor. Methods. We identified all Danish women diagnosed with a benign ovarian tumor during 1978-2016 in the Danish National Patient Register (n - 149,807). The study population was followed for subsequent development of endometrial cancer by linkage to the Danish Cancer Register and standardized incidence ratios (SIRS) with corresponding 95% confidence intervals (Cis) were calculated after correction for hysterectomy. Results. After a one-year delayed study entry, women with benign ovarian tumors had a decreased incidence of endometrial cancer (SIR - 0.74, 95% CI: 0.68-0.81) compared with women in the general Danish population. Both solid benign ovarian tumors (SIR - 0.79, 95% CI 0.70-0.88) and cystic benign ovarian tumors (SIR - 0.68. 95% CI 0.58-0.78) were associated with decreased incidences of endometrial cancer. Likewise, women with benign ovarian tumors had decreased incidences of both type I and type II endometrial cancer. The incidence of endometrial cancer was decreased to virtually the same magnitude irrespective of the age at diagnosis of a benign ovarian tumor and the reduction persisted throughout the follow-up period. Conclusions. The risk of endometrial cancer was decreased beyond the first year after a benign ovarian tumor and the decrease persisted for 20 or more years. The possible underlying mechanisms are not known and should be investigated further. (C) 2020 Elsevier Inc. All rights reserved.