Risk of ovarian cancer in women with histological proven endometriosis. The ENOCA a population-based cohort study

Background and objectives: Several studies have suggested that endometriosis is associated with an increased risk of ovarian cancer. However previous studies lack sufficient power, follow-up data, or diagnostic certainty. We used a large nationwide pathology database to assess whether there is an association between endometriosis and ovarian cancer.Methods: We identified all women with a histological diagnosis of endometriosis between 1990 and 2015 from the Dutch nationwide registry of histopathology and cytopathology (PALGA), and matched with women with a benign dermal nevus. Histology results for ovarian cancer, fallopian tubes, and peritoneum between January, 1990 and July, 2017 were retrieved. Incidence rate ratios (IRR) were estimated for ovarian cancer and its subtypes.Results: The crude IRR for ovarian cancer in endometriosis patients was 4.80 (95%CI 4.34 to 5.32), and the age-adjusted IRR was 7.22 (95% CI 6.20 to 8.41) (table 1). Median age at ovarian cancer diagnosis was significantly different, 56 years (IQR 49 to 63) and 60 years (IQR 53 to 67) in the endometriosis and nevus group respectively. After excluding cases with less than a half-year of follow-up, the age-adjusted IRR was 1.09 (95% CI 0.88 to 1.35). In both analyses endometrioid and clear cell cancers were significantly more associated with endometriosis (figure 1).Conclusions: These findings show a five to seven times higher prevalence of ovarian cancer in endometriosis patients. To our knowledge, this is the largest study to investigate the risk of ovarian cancer in women with endometriosis and the first study to only include patients with histological proven endometriose. Women with endometriosis are on average four years younger at ovarian cancer diagnosis. We found a large proportion of women with synchronous endometriosis and ovarian cancer after the average menopausal age, this suggests that the risk of endometriosis remains, even when clinical symptoms are no longer present. Disclosures: no disclosures of interest.