Sunscreen and melanoma prevention: evidence and expectations

Sunscreen has long been recommended by cancer societies and dermatologists as a skin cancer prevention tool.^1–3 However, for melanoma there are few pieces of high-quality evidence to support these recommendations. The only randomized controlled trial (RCT) of sunscreen for melanoma prevention, the Nambour Trial,⁴ was designed to evaluate basal and squamous cell carcinomas and their precursors, with melanoma as a secondary end point. It showed a nonstatistically significant protective effect for all melanomas (11 melanomas in the sunscreen group vs. 22 in the control group) and a statistically significant protective effect for invasive melanoma (three invasive melanomas in the sunscreen group vs. 11 in the control group). This study is the best available evidence but has limitations, such as the small number of melanomas diagnosed during the study. Although a high-quality body of evidence for sunscreen and melanoma prevention is not fully established, it is important that we evaluate the expected results of increased sunscreen use based on the available information if we are to recommend sunscreen on a population level.

In the accompanying article by Olsen et al., the authors evaluated the difference between the number of melanomas predicted with current levels of sunscreen use and the number that would be expected under several scenarios of increased sunscreen use over a 20-year period.⁵ These models included data on current levels of sunscreen use in Australia and the U.S.A., projected melanoma incidences for these countries, and the protective effect for all melanomas found in the Nambour Trial. The main sunscreen scenario the authors evaluated was a 5% increase in sunscreen users per year over a 10-year period, and they found an approximately 10% reduction in melanomas in the U.S.A. (230 000 fewer melanomas) and Australia (28 000 fewer melanomas) after 20 years. A sensitivity analysis was performed using the results from a large prospective cohort study,⁶ which had shown a weaker protective effect of sunscreen than in the Nambour Trial. The results from this sensitivity analysis were more modest (7% and 6% fewer melanomas in the U.S.A. and Australia, respectively). Another sunscreen scenario the authors included was immediate sunscreen use by 100% of the population (unrealistic but the theoretical maximum), in which they found 38% melanoma reduction in the U.S.A. and 34% in Australia.

The strengths of the study by Olsen et al. hinge on the clear, data-driven inputs used in the sunscreen scenarios, namely effect sizes based on the highest level of evidence available, an RCT, as well as current, published levels of sunscreen use and melanoma incidences. The authors also present a conservative sensitivity analysis using an effect size from the next best level of evidence available, which had found a smaller protective effect than the Nambour Trial. In addition, the authors have presented a theoretical maximum scenario – this highlights that not all melanomas are preventable and that sunscreen is not the only intervention available. It is key to present the best-case scenario for sunscreen use to be able to evaluate and prioritize different prevention options for melanoma.

Any modelling study such as this one is limited by the quality of data available to use in constructing the model. Unfortunately, high-quality evidence on sunscreen use in melanoma prevention is sparse. Additionally, the protective effect the authors use from the Nambour Trial was not statistically significant, making the interpretation of the predictive models less clear. These non-statistically significant results in the Nambour Trial do not show that sunscreen does not have a protective effect, but we currently do not have level-1 evidence supporting a protective effect for all melanomas (both in situ and invasive). Another limitation is that the study by Olsen et al. does not tell us how many people would need to apply sunscreen regularly for how many years to prevent critical patient outcomes like metastatic disease or death from melanoma. In the future, this will be fundamental in fully understanding the importance of sunscreen interventions.

The study by Olsen et al. is an essential one. Dermatologists and public health officials alike need to be able to understand what we might expect to accomplish if we increase the number of regular sunscreen users in a population. Prior to implementing an intervention, we want to know that the intervention is worthwhile. Resources are finite and individuals have many health messages aimed at them already (e.g. maintain a healthy weight, stay away from tobacco). If sunscreen interventions are to be taken seriously and implemented, studies such as this one must be available. Olsen et al. have shown that, given the data we currently have, we could potentially see a substantial (10%) decrease in melanoma after 20 years if we could accomplish a 5% yearly increase in regular sunscreen users over a 10-year period.

This work should galvanize us to continue to develop and, importantly, provide evidence for interventions that can prevent melanoma. There is much work still to be done – there was only one RCT available to use in this study, which was not primarily designed to evaluate melanoma outcomes. There are no RCTs available for protective clothing or shade-seeking behaviour and melanoma risk. We need to continue to improve the evidence for sunscreen, protective clothing and shade as measures to prevent melanoma, as well as work towards increasing the number of people utilizing these potentially preventative measures in our populations.