The Role of Ultraviolet Radiation in the Development of Skin Cancer and Non-Hodgkin Lymphoma in Canada
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Background: Exposure to ultraviolet radiation (UVR) is an established cause of melanoma and non-melanoma skin cancer (NMSC) and moderate exposure is hypothesized to be protective against some non-cutaneous cancers, including non-Hodgkin lymphoma (NHL). The first aim of this thesis was to estimate the current and future burden of melanoma and NMSCs associated with UVR and modifiable UVR behaviours in Canada. The second aim was to explore the relationship of time spent in the sun and the risk of NHL in a Canadian setting. Methods: The burden of skin cancer associated with UVR in 2015 was estimated by comparing 2015 rates of melanoma and NMSCs with estimated rates of a 1920 birth cohort for melanoma and with rates of less exposed body sites (trunk and legs) for NMSCs. Skin cancer attributable to modifiable UVR behaviours (indoor tanning, sunburn, and sunbathing) was estimated using relative risks relevant to Canada by conducting meta-analyses and prevalence of exposures from a nationally representative survey. A prospective cohort study examining the risk of NHL was conducted using self-reported questionnaire data from three Canadian cohorts that were linked to cancer incidence data. Multivariable Cox proportional hazards models, using age as the time-scale was employed to investigate this relationship with control for relevant confounders. Results: For 2015, we estimated that 62.3% of melanomas, 80.5% of basal cell carcinomas (BCC), and 83.0% of squamous cell carcinomas (SCC) were attributable to UVR, while 29.7% of melanomas, 46.0% of BCCs, and 17.4% of SCCs were attributable to modifiable UVR behaviours. Compared to individuals spending < 30 min/day in the sun the risk of developing NHL was 0.84 (95% CI: 0.55-1.28) for 30-59 minutes, 0.63 (95% CI: 0.40-0.98) for 1-2 hours, and 0.91 (95% CI: 0.61-1.36) for > 2 hours. Conclusions: Prevention efforts aimed at modifiable UVR behaviours are crucial for reducing the burden of skin cancer in Canada, while moderate exposure to the sun may reduce the risk of NHL. Further research is required to clarify the optimal UVR exposure patterns in terms of preventing the development of both cutaneous and non-cutaneous cancers, including NHL.