Cigarette smoking and risk of borderline and invasive epithelial ovarian cancer

Studies regarding the association between smoking and risk of epithelial ovarian cancer (EOC) are inconsistent. The purpose of this study was to examine the association between smoking and EOC, overall and according to invasiveness and histological subtype in a cohort of women with a high proportion of smokers at enrollment. We followed 103,081 women, aged 30-50 years in 1991/1992, from the Norwegian-Swedish Women's Lifestyle and Health cohort. The women completed a questionnaire on personal characteristics and exposures at enrollment and were subsequently followed with linkages to national registers through December 31, 2004. We used Cox proportional hazard regression models to estimate hazard ratio (RR) of EOC with 95% confidence intervals (CIs) associated with different measures of smoking exposures adjusting for confounding variables. Altogether 343 241 (70%) invasive and 102(30%) borderline incident EOC cases were identified. Former HR = 2.2(95% CI 1.0-4.7) and current HR = 2.7(95% CI 1.2-5.7) smokers had a more than doubling in risk for borderline tumors compared to never smokers. Women who had smoked for more than 20 years had 3 times HR = 3.1(95% CI 1.5-6.7) the risk of borderline tumors compared to never smokers. A test for trend according to smoking status was almost significant for mucinous tumors (p-trend = 0.05). A significant dose response relationship was found according to smoking intensity pack-years; (0-9, 0-14, >or= 15) and duration number of years; (0-10, 11-20, >or= 20) for borderline and serous tumors (p-trends < 0.05). In conclusion, smoking may increase the risk of borderline EOC.