Endometrioid and clear cell ovarian cancers: a comparative analysis of risk factors.

Endometrioid and clear cell subtypes of ovarian cancer are both known to be closely associated with endometriosis and endometrial pathology, and so have often been combined in studies of causation. We have examined these ovarian cancers separately for potentially distinct risk factors in our population-based, Australia-wide case control study of 142 women with incident invasive endometrioid, 90 with clear cell ovarian cancers and 1508 population controls. Multivariate logistic regression was used to calculated odds ratios (ORs) and 95% confidence intervals (CIs). Increasing parity, and hormonal contraceptive use for > or = 5 years, strongly decreased the risks of both subtypes. Breast feeding and tubal ligation were also inversely associated, but significantly so only for the endometrioid subtype. As expected endometriosis increased the risk of both subtypes (OR 2.2, 95% CI 1.2-3.9 for endometrioid and OR 3.0, 95% CI 1.5-5.9 for clear cell). Obesity was associated only with clear cell cancers, where we observed a two-fold increased risk (OR 2.2, 95% CI 1.2-4.1). Also a significant trend of decreasing risk with increasing intensity of smoking (p trend 0.02) and education beyond high school was associated with decreased development of clear cell cancers only. Endometrioid and clear cell ovarian cancers have some shared as well as some distinct risk factors, and therefore should be considered separately in studies of ovarian cancer.