Residential Radon Levels and Ovarian Cancer Among Postmenopausal Women.

Importance: Few environmental risk factors for ovarian cancer have been discovered. Women exposed to ionizing radiation from the atomic bomb during World War II experienced an increased risk of ovarian cancer. Today, the largest source of ionizing radiation is radon gas in the home, but whether ionizing radiation is associated with increased risk of ovarian cancer more broadly is unknown. Objective: To evaluate whether higher home radon levels are associated with increased risk of ovarian cancer. Design, Setting, and Participants: This prospective cohort study included 127 547 women from the Women's Health Initiative, including 40 clinical centers across the US, with outcomes followed up for 31 years (1993-2024). Postmenopausal women aged 50 to 79 years were enrolled in an observational study or 1 or more randomized clinical trials. All cases of ovarian cancer were physician adjudicated. Exposure: Radon measurements from the 1993 US Geological Survey, classified into low (<2 pCi/L), medium (2-4 pCi/L), and high zones (>4 pCi/L), were linked with the geocoded home addresses of participants at baseline (1993-1998). Main Outcomes and Measures: Hazard ratios (HRs) for ovarian cancer, adjusted for covariates with 95% CIs. Results: Among the 127 547 women (mean [SD] age, 63.1 [7.2] years) with available radon zone values, 1645 incident ovarian cancers and 1048 ovarian cancer deaths were observed over a mean (SD) follow-up of 17.7 (8.4) years. After adjustment for covariates, the HR for all ovarian cancers for women living in the medium radon zone compared with women living in the low radon zone was not significantly higher (HR, 1.13 [95% CI, 1.00-1.29]). However, the HR was significantly higher for women living in the high radon zone compared with those living in the low radon zone (HR, 1.31 [95% CI, 1.11-1.54]). Similar findings were observed for the most common histologic type, serous ovarian cancer, for which the HR in the medium zone was 1.06 (95% CI, 0.88-1.27) and the HR in the high zone was 1.38 (95% CI, 1.09-1.74). Ovarian cancer mortality also was significantly higher in the high radon zone compared with the low radon zone (HR, 1.31 [95% CI, 1.07-1.60]). Sensitivity analyses using 3 alternate radon measures produced similar results. Conclusions and Relevance: In this large, prospective cohort of postmenopausal women, the risks of ovarian cancer incidence and mortality were significantly higher for women living in homes in the high radon zone. Residential radon is a ubiquitous and modifiable risk factor. This is the first epidemiologic study of radon and ovarian cancer among postmenopausal women to date, and its findings suggest a potential target for mitigating cancer risk.