Risk factors for epithelial ovarian cancer in Japan - results from the Japan Public Health Center-based Prospective Study cohort

The aim of this study was to evaluate the risk factors for invasive primary epithelial ovarian cancer among Japanese women. In 1990-1994, 45,748 women aged 40-69 years were enrolled in the Japan Public Health Center-based Prospective Study cohort. Only 86 epithelial ovarian cancer cases were diagnosed during follow-up through 2008, reflecting the low ovarian cancer incidence rates in Japan. Cox proportional hazard models were used to estimate hazard ratios (HR) and 95% confidence intervals (CI) according to the exposure of interest. The median age at epithelial ovarian cancer diagnosis was 59 years, with a median follow-up before diagnosis of 7.6 years. There were no statistically significant associations for age at menarche or first birth, breastfeeding, use of exogenous hormones, menopausal status at cohort enrollment, height, body mass index, smoking status, second-hand smoke, alcohol consumption, physical activity and family history of cancer in a first-degree relative. The linear decrease in HR associated with each additional birth was 0.75 (95% CI 0.56-0.99). Among women who usually slept >7 h per day, an HR of 0.4 (95% CI 0.2-0.9) emerged compared to those who slept <6 h. This study did not confirm risk factors for epithelial ovarian cancer among Japanese women that have been reported in studies carried out elsewhere. Usual sleep duration of >7 h per day was inversely associated with epithelial ovarian cancer risk, which is a novel finding that needs to be confirmed in other studies.