Hormone therapy and ovarian borderline tumors: a national cohort study

Little is known about the influence of postmenopausal hormone therapy on the risk of ovarian borderline tumors. We aimed at assessing the influence of different hormone therapies on this risk. A total of 909,875 Danish women 50-79 years old without previous hormone-sensitive cancers or bilateral oophorectomy were followed in this nationwide cohort study 1995-2005. The National Register of Medicinal Product Statistics provided exposure information on all women who redeemed prescriptions on hormone therapy. The National Cancer and Pathology Register provided data on borderline ovarian tumors. Information on confounding factors was available from other national registers. Poisson regression analyses provided risk estimates with hormone exposures as time-dependent covariates. In an average of 8.0 years of follow-up, 703 incident ovarian borderline tumors were detected. Compared with never users, hormone use for more than 4 years increased the risk of borderline tumors: relative risk (RR) 1.40; 95% confidence interval (CI), 1.09-1.81. Combined estrogen and progestin therapy for more than 4 years increased the risk: RR 1.49 (1.10-2.01), with no difference between cyclic and continuous combined therapy (p = 0.83); RR 1.56 (1.08-2.25) and 1.45 (0.87-2.43), respectively. The RR with estrogen therapy did not differ significantly from RR with combined therapy (p = 0.58): RR 1.27 (0.82-1.98). Disregarding the type of hormone therapy, hormone use for 4 years or less did not increase the risk of borderline tumors. Combined hormone therapy for more than 4 years increases the risk of ovarian borderline tumors.