Incidence of endometrioid and clear-cell ovarian cancer in histological proven endometriosis: the ENOCA population-based cohort study.

BACKGROUND: Several studies have suggested that endometriosis is associated with an increased risk of ovarian cancer, especially for the clear-cell and endometrioid subtypes. However, previous studies lack sufficient power or diagnostic certainty. OBJECTIVE: The objective of the study was to assess the association between histologically proven endometriosis and ovarian cancer in a large population-based cohort study. STUDY DESIGN: We identified 131,450 women with a histological diagnosis of endometriosis between 1990 and 2015 from the Dutch nationwide registry of histopathology and cytopathology (PALGA). For the control cohort 132,654 women with a benign dermal nevus were matched on age and inclusion year with the endometriosis cases. Histological diagnoses of ovarian, fallopian tubes, and peritoneal cancers between January 1990 and July 2017 were retrieved. Incidence rate ratios were estimated for ovarian cancer and its subtypes for the whole follow-up period as well as for women with more than 1 person-year at risk. RESULTS: We found a crude incidence rate ratio of 4.79 (95% confidence interval, 4.33-5.31) and an age-adjusted incidence rate ratio of 7.18 (95% confidence interval, 6.17-8.36) for ovarian cancer overall. Endometrioid and clear-cell ovarian cancer had the highest age-adjusted incidence rate ratio of 29.06 (95% confidence interval, 20.66-40.87) and 21.34 (95% confidence interval, 14.01-32.51), respectively. Median age at ovarian cancer diagnosis was 56 years (interquartile range, 49-63) for the endometriosis cohort and 60 years (interquartile range, 53-67) for the nevus cohort, (P < .05). After excluding women with less than 1 person-year at risk following an endometriosis diagnosis, we found a crude incidence rate ratio of 1.04 (95% confidence interval, 0.91-1.19) and an age-adjusted incidence rate ratio of 1.08 (95% confidence interval, 0.87-1.35) for ovarian cancer overall. However, statistically significant age-adjusted incidence rate ratios of 2.29 (95% confidence interval, 1.24-4.20) for clear-cell ovarian cancer and 2.56 (95% confidence interval, 1.47-4.47) for endometrioid ovarian cancer were found. CONCLUSION: A significantly higher incidence of clear-cell and endometrioid ovarian cancer was found in women with histologically proven endometriosis. Additionally, we found an increased incidence of all ovarian cancer subtypes in histologically proven endometriosis; however, in many of these women, endometriosis and ovarian cancer were diagnosed synchronously after the average menopausal age, which may suggest that the risk of ovarian cancer in endometriosis patients remains, even when clinical endometriosis symptoms are no longer present.