Prospective Study of Ultraviolet Radiation Exposure and Thyroid Cancer Risk in the United States.

Background: Thyroid cancer incidence has tripled in the past three decades, yet relatively few risk factors have been identified. Some studies have suggested that ultraviolet radiation (UVR) may affect thyroid cancer risk.
Methods: We conducted a prospective analysis of 44,039 participants in the United States Radiologic Technologists Study (153 thyroid cancer cases) from all 50 states. We examined the association between risk of thyroid cancer and exposure to UVR, estimated by ambient UVR, time outdoors, and a combined variable. Participants reported location of residence and time outdoors during five age periods starting in childhood. Ambient UVR was estimated by linking satellite-based UVR measurements to geocoded residences. We assessed the association of UVR by age and average lifetime UVR with thyroid cancer risk using Cox proportional hazards models, starting at the time of the baseline questionnaire (2003-2005) through 2012-2013.
Results: Combined UVR from the latest age period (age 40+) was associated with a decreased risk of thyroid cancer (HR for 4th vs. 1st quartile = 0.56; 95% CI, 0.31-1.02, Ptrend = 0.04). This was limited to participants with benign thyroid disease and to those with darker complexions, although we found no evidence of effect modification. Thyroid cancer risk was unrelated to all metrics of UVR in earlier age periods and for average lifetime exposure.Conclusions: Recent UVR exposure was associated with a decreased risk of thyroid cancer. This association appeared to be modified by benign thyroid disease and skin complexion.Impact: UVR exposure may be associated with a decreased risk of thyroid cancer. Cancer Epidemiol Biomarkers Prev; 26(5); 684-91. ©2016 AACR. ©2016 American Association for Cancer Research.