Smoking and risk of ovarian cancer by histological subtypes: an analysis among 300 000 Norwegian women

BACKGROUND: We prospectively investigated the association between different measures of smoking exposure and the risk of serous, mucinous, and endometrioid ovarian cancers (OC) in a cohort of more than 300 000 Norwegian women. METHODS: We followed 300 398 women aged 19–67 years at enrolment until 31 December 2013 for OC incidence through linkage to national registries. We used Cox proportional hazards models with attained age as the underlying time scale to estimate multivariable-adjusted hazard ratios (HRs) with 95% confidence intervals (CIs) adjusted for relevant confounders. RESULTS: During more than 5.9 million person-years and a median follow-up time of 19 years, 2336 primary invasive (1647, 71%) and borderline (689, 29%) OC were identified (53% serous, 19% mucinous). Compared with never smokers, current smokers who had smoked for ⩾10 years had a higher risk of mucinous OC (HR(10–19 years vs never)=1.73, 95% CI 1.24–2.42; HR(⩾20 vs never)=2.26, 95% CI 1.77–2.89, P(trend) <0.001). When stratified by invasiveness, current smokers had a higher risk of invasive mucinous OC (HR=1.78, 95% CI 1.20–2.64) and borderline mucinous OC (HR=2.26 95% CI, 1.71–2.97) (P(heterogeneity)=0.34) than never smokers. Smoking was not associated with serous or endometrioid OC. CONCLUSIONS: Using a very large cohort of women, the current analysis provides an important replication for a similar risk of invasive and borderline mucinous OC related to smoking.