To examine the association of sedentary behavior with risk of ovarian cancer overall, by tumor subtype, and by participant characteristics in the Nurses’ Health Study (NHS) and NHSII. A total of 69,558 NHS (1992-2016) and 104,130 NHSII (1991-2015) participants were included in analysis who reported sitting time at home, work, and watching television, which included 884 histologically-confirmed ovarian cancer cases. Multivariable adjusted Cox proportional hazards models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for ovarian cancer by time spent sitting (no mutual adjustment of individual types of sitting in primary analyses). We examined potential heterogeneity by tumor histotype (type I and II), body mass index (BMI; <25, ≥25), and total physical activity (<15, ≥15 metabolic equivalent-hours/week). We observed an increased risk of ovarian cancer for women who sat at work 10-19 (HR=1.25, 95%CI: 1.04-1.51) and ≥20 (HR=1.40, 95%CI: 1.14-1.71) versus <5 hours/week. This association did not vary by BMI, physical activity, or histotype (P-heterogeneity≥0.43).
No associations were observed for overall sitting, sitting while watching TV, or other sitting at home. Longer sitting time at work was associated with an elevated risk of ovarian cancer. Further investigations are required to confirm these findings and elucidate underlying mechanisms.