Challenges in assessing the sunscreen-melanoma association

Whether sunscreen use affects melanoma risk has been widely studied with contradictory results. To answer this question we performed a systematic review of all published studies, accounting for sources of heterogeneity and bias. We searched for original articles investigating the sunscreen-melanoma association in humans to February 28, 2018. We then used random-effects meta-analysis to combine estimates of the association, stratified by study design. Stratified meta-analysis and meta-regression were used to identify sources of heterogeneity. We included 21,069 melanoma cases from 28 studies published 1979-2018: 23 case-control (11 hospital-based, 12 population-based), 1 ecological, 3 cohort and 1 randomised controlled trial (RCT). There was marked heterogeneity across study designs and among case-control studies but adjustment for confounding by sun exposure, sunburns and phenotype systematically moved estimates toward decreased melanoma risk among sunscreen users. Ever- vs. never-use of sunscreen was inversely associated with melanoma in hospital-based case-control studies (adjusted odds ratio (OR) = 0.57, 95%confidence interval (CI) 0.37-0.87, p(heterogeneity) < 0.001), the ecological study (rate ratio = 0.48, 95%CI 0.35-0.66), and the RCT (hazard ratio (HR) = 0.49, 95%CI 0.24-1.01). It was not associated in population-based case-control studies (OR = 1.17, 95%CI 0.90-1.51, p(heterogeneity) < 0.001) and was positively associated in the cohort studies (HR = 1.27, 95%CI 1.07-1.51, p(heterogeneity) = 0.236). The association differed by latitude (p(interaction) = 0.042), region (p(interaction) = 0.008), adjustment for naevi/freckling (p(interaction) = 0.035), and proportion of never-sunscreen-users (p(interaction) = 0012). Evidence from observational studies on sunscreen use and melanoma risk was weak and heterogeneous, consistent with the challenges of controlling for innate confounding by indication. The only RCT showed a protective effect of sunscreen. What's new? Effectiveness of sunscreen in reducing UV-induced skin damage has been proven in experimental studies, but effectiveness in reducing melanoma in humans remains inconclusive. This is the first meta-analysis to analyze data from four study designs, stratify hospital- and population-based case-control studies, and include as many as five prospective studies. Evidence from observational studies on the sunscreen-melanoma association was heterogeneous, consistent with the challenges of controlling for innate confounding by indication. The only randomized controlled trial showed a protective effect. Public health recommendations should place greater emphasis on the proper use of sunscreen in conjunction with other means of sun protection.