Solar Ultraviolet Radiation and Breast Cancer Risk

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Hillier, Troy
breast cancer , epidemiology , meta-analysis , prospective , ultraviolet radiation
Background: A moderate amount of time spent in the sun has been hypothesized to be protective against several internal cancers, including breast cancer. This project summarized and synthesized existing literature on this topic and identified sources of heterogeneity (manuscript 1). Secondly, this project examined this relationship using data from Canadian prospective cohort studies (manuscript 2). Methods: First, a meta-analysis of case-control and cohort studies was conducted to investigate the relationships between time spent in the sun, and ambient UVR with breast cancer risk. Heterogeneity was quantified with I-squared statistics and potential sources were investigated through subgroup analyses. Second, a prospective cohort study was conducted using self-reported questionnaire data from the Alberta Tomorrow Project (ATP) and Quebec’s CARTaGENE (CaG) that was linked to cancer registry (ATP) and administrative (CaG) data. Cox proportional hazards models, using age as the time-scale were employed to investigate the association between solar UVR exposure and pre- and postmenopausal breast cancer with control for relevant confounders. Results: In a dose-response meta-analysis, spending 1-2 hours in the sun per day was protective against breast cancer, with no additional benefit for >2 hours per day. Few studies performed analyses stratified by menopausal status. In manuscript 2, a suggestive protective effect for greater than 2 hours spent in the sun per day during summer months was observed in postmenopausal women only. No significant interactions between time spent in the sun and ambient UVR, vitamin D intake, or skin tone were observed.
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