Outdoor light at night and risk of endometrial cancer in the NIH-AARP diet and health study.

PURPOSE: Outdoor light at night (LAN) can result in circadian disruption and hormone dysregulation and is a suspected risk factor for some cancers. Our study is the first to evaluate the association between LAN and risk of endometrial cancer, a malignancy with known relationship to circulating estrogen levels. METHODS: We linked enrollment addresses (1996) for 97,677 postmenopausal women in the prospective NIH-AARP cohort to satellite imagery of nighttime radiance to estimate LAN exposure. Multivariable Cox models estimated hazard ratios (HR) and 95% confidence intervals (95% CI) for LAN quintiles and incident endometrial cancer overall (1,669 cases) and endometrioid adenocarcinomas (991 cases) through follow-up (2011). We tested for interaction with established endometrial cancer risk factors. RESULTS: We observed no association for endometrial cancer overall (HRQ1vsQ5 0.92; 95% CI 0.78-1.08; p trend = 0.67) or endometrioid adenocarcinoma (HRQ1vsQ5 1.01; 95% CI 0.82-1.24; p trend = 0.36). Although body mass index and menopause hormone therapy were both associated with risk, there was no evidence of interaction with LAN (p interactions = 0.52 and 0.50, respectively). CONCLUSION: Our study did not find an association between outdoor LAN and endometrial cancer risk, but was limited by the inability to account for individual-level exposure determinants. Future studies should consider approaches to improve characterization of personal exposures to light.