Menopausal Hormone Therapy and Ovarian and Endometrial Cancers: Long-Term Follow-Up of the Women's Health Initiative Randomized Trials

PURPOSE Menopausal hormone therapy's influence on ovarian and endometrial cancers remains unsettled. Therefore, we assessed the long-term influence of conjugated equine estrogen (CEE) plus medroxyprogesterone acetate (MPA) and CEE-alone on ovarian and endometrial cancer incidence and mortality in the Women's Health Initiative randomized, placebo-controlled clinical trials. MATERIALS AND METHODS Postmenopausal women, age 50-79 years, were entered on two randomized clinical trials evaluating different menopausal hormone therapy regimens. In 16,608 women with a uterus, 8,506 were randomly assigned to once daily 0.625 mg of CEE plus 2.5 mg once daily of MPA and 8,102 placebo. In 10,739 women with previous hysterectomy, 5,310 were randomly assigned to once daily 0.625 mg of CEE-alone and 5,429 placebo. Intervention was stopped for cause before planned 8.5-year intervention after 5.6 years (CEE plus MPA) and after 7.2 years (CEE-alone). Outcomes include incidence and mortality from ovarian and endometrial cancers and deaths after these cancers. RESULTS After 20-year follow-up, CEE-alone, versus placebo, significantly increased ovarian cancer incidence (35 cases [0.041%] v 17 [0.020%]; hazard ratio [HR], 2.04 [95% CI, 1.14 to 3.65]; P = .014) and ovarian cancer mortality ( P = .006). By contrast, CEE plus MPA, versus placebo, did not increase ovarian cancer incidence (75 cases [0.051%] v 63 [0.045%]; HR, 1.14 [95% CI, 0.82 to 1.59]; P = .44) or ovarian cancer mortality but did significantly lower endometrial cancer incidence (106 cases [0.073%] v 140 [0.10%]; HR, 0.72 [95% CI, 0.56 to 0.92]; P = .01). CONCLUSION In randomized clinical trials, CEE-alone increased ovarian cancer incidence and ovarian cancer mortality, while CEE plus MPA did not. By contrast, CEE plus MPA significantly reduced endometrial cancer incidence.