Impact of Hormone Replacement Therapy on Risk of Ovarian Cancer in Postmenopausal Women with De Novo Endometriosis or a History of Endometriosis

SIMPLE SUMMARY: Endometriosis is a risk factor for ovarian cancer. Meta and pooled analyses have shown hormone replacement therapy (HRT) is significantly associated with an increased risk of ovarian cancer. A combination of estrogen and progesterone is currently recommended to improve menopausal symptoms in women with a history of endometriosis. However, the effect of HRT on the malignant transformation of postmenopausal endometriosis remains unclear. Therefore, this study investigated the impact of HRT on ovarian cancer occurrence in 20,608 postmenopausal women with de novo endometriosis or a history of endometriosis using the nationwide cohort study. With the exception of HRT using estrogen alone, HRT did not increase the risk of ovarian cancer in postmenopausal women with de novo endometriosis or a history of endometriosis. HRT, but not estrogen alone, can be used to improve the quality of life in symptomatic women with postmenopausal endometriosis. ABSTRACT: The effect of hormone replacement therapy (HRT) on the malignant transformation of postmenopausal endometriosis remains unclear. This study aimed to investigate the impact of HRT on ovarian cancer occurrence in postmenopausal women with de novo endometriosis or a history of endometriosis. A total of 10,304 women that received HRT (the HRT group) and 10,304 that did not (the control group) were selected by 1:1 matching those that met the study criteria. Incidences of ovarian cancer (0.3% in the HRT group and 0.5% in the control group) and cumulative incidence rates of ovarian cancer were similar in the two groups. The overall mean duration of HRT was 1.4 ± 2.2 years, but the duration of HRT in women with ovarian cancer was 2.2 ± 2.9 years. After adjusting for co-variables, receipt of HRT, duration of HRT, combined use of estrogen and progesterone, and tibolone were not found to be risk factors for ovarian cancer. However, the use of estrogen alone was found to be a significant risk factor for ovarian cancer (HR 2.898; 95% CI 1.251–6.715; p = 0.013). With the exception of HRT using estrogen alone, HRT did not increase the risk of ovarian cancer in postmenopausal women with a history of endometriosis or de novo endometriosis.