Solar ultraviolet radiation and colorectal cancer risk in Canada
Background: Colorectal cancer (CRC) is the third most common cancer in Canada with an estimated 26,000 new cases in 2020. Novel modifiable risk factors such as solar ultraviolet radiation (UVR) warrant research to promote prevention of avoidable cases. Previous studies examining the relationship between solar UVR and CRC risk show inconsistent results. This thesis examined this potential relationship using nationally collected data to provide a unique Canadian perspective. Differences in effect between colon and rectal as cancer sites are examined in addition to interactions with other variables of interest. A protective U-shaped relationship is hypothesized with the lowest risk at moderate exposure levels. Methods: Self-reported questionnaire data from three cohorts within the Canadian Partnership for Tomorrow project provided sun exposure and covariate data. Cohorts were linked to cancer registries or administrative data to identify cancer cases. Cox proportional hazard models estimated the hazard ratios for time spent in sun as well as ambient UVR in relation to CRC risk. Hazard ratios were also estimated separately for colon and rectal cancers. Interactions by ambient UVR, sex, vitamin D intake, and skin tone were explored. Results: Neither time spent in sun nor ambient UVR had a statistically significant association with CRC risk. No association was found with rectal or colon cancer when examined individually. No significant interactions between time spent in sun and sex, ambient UVR, and vitamin D were observed. Conclusion: Overall, results do not support a relationship between exposure to UVR in Canada and risk for CRC. This project adds to the small body of literature by examining this relationship in a Canadian setting and with two measures of solar UVR exposure. Investigation into interactions also provide insight on differing effects. Future research should expand upon this study with greater variation in ambient UVR and a more comprehensive timeline of UVR exposure to understand potential induction periods.