Objectives Endometriosis is associated with an increased ovarian cancer incidence. Surgical treatment of endometriosis might reduce this risk. Therefore, we assessed the ovarian cancer incidence in women with endometriosis after bilateral salpingo-oophorectomy (BSO).

Methods All women with histological proven endometriosis between 1990 and 2015 in the Netherlands were identified. Women with a BSO without ovarian cancer at time of surgery were selected as cases (n=14,410). We selected two control cohorts; 1) women with histological proven endometriosis without BSO or with ovarian cancer at time of BSO (n=115,323), and 2) women with a benign dermal nevus (n=132,654). Histological diagnoses of ovarian or extra-ovarian cancers were retrieved. Incidence rate ratios (IRR) were estimated for (extra) ovarian cancer.

Results We identified 13 (0.09%) extra-ovarian cancers in the BSO cohort and 2,036 (1.8%) and 471 (0.4%) ovarian cancers in the endometriosis and nevus cohort, respectively. We found an age-adjusted IRR of 0.02 (95%CI 0.01–0.04) when the BSO cohort was compared with the endometriosis cohort and an age-adjusted IRR of 0.20 (95%CI 0.11–0.37) when comparing the BSO to the nevus cohort (table 1). Median age at cancer diagnosis was 61 (IQR 56–74) in the BSO cohort, 55 (IQR 48–63) in the endometriosis cohort and 58 years (IQR 51–65) in the nevus cohort (both p<0.05).

Conclusions We found a significantly reduced (extra-)ovarian cancer incidence in women with endometriosis and a BSO when compared to both controls with endometriosis without BSO, and controls without histological proven endometriosis.