Rotating night shift work and risk of ovarian cancer.

BACKGROUND Night shift work has been associated with higher risks of breast and endometrial cancer, but few studies have evaluated associations with other reproductive cancers. METHODS We examined the association between rotating night shift work and risk of ovarian cancer during 20 years of follow-up in 181,548 women participating in two large cohort studies, the Nurses' Health Study (NHS) and NHSII. Number of years of rotating night shift work was queried in 1988 for NHS and in 1989, 1991, 1993, 2001, and 2005 for NHSII. We used Cox proportional hazards regression to model HRs and 95% CIs of ovarian cancer for each shift work category (1-2, 3-5, 6-9, 10-14, 15-19, and 20+ years). RESULTS We confirmed 718 incident cases of ovarian cancer over 2,974,672 person-years of follow-up. Rotating shift work was not associated with ovarian cancer risk in either cohort individually. Combining both cohorts, compared with women without any night work, the HR for 15 to 19 years of rotating night shift work was 1.28 (95% CI: 0.84-1.94) and for 20+ years 0.80 (95% CI: 0.51-1.23). CONCLUSIONS In this large prospective study, there was no association between duration of rotating night shift work and risk of ovarian cancer. IMPACT Although associated with other cancers, night shift work does not appear to be associated with increased risk of ovarian cancer. However, further exploration of the association between melatonin and risk of ovarian cancer is warranted.