Description of the Evidence
One very small randomized placebo-controlled study of a sunscreen (sun protection factor [SPF] 29) was conducted in 53 volunteers who had either clinical evidence of solar keratoses or NMSC. Only 37 of the participants returned for the planned 2-year follow-up (attrition rate of 30%). The rate of new solar keratoses was lower after 2 years in the sunscreen group than in the placebo (base-cream) group (estimated 36% reduction in annual rate, P = .001). Another study showed that regular sunscreen use helps reduce the incidence of solar keratoses and increase remission of existing lesions. In Australia, 588 persons aged 40 years and older who attended a free skin cancer screening clinic and had 1 to 30 solar keratoses were enrolled in a randomized controlled trial (RCT) assessing the effect of regular sunscreen (SPF 17) use on solar keratoses; 431 persons completed the study. Individuals in the sunscreen group developed fewer new lesions and had more remissions of existing lesions than those in the base-cream placebo group. There was an increase of 1.0 in the mean number of solar keratoses in the base-cream group versus a decrease of 0.6 in the sunscreen group (difference = 1.53; 95% confidence interval [CI], 0.81–2.25). The rate ratio of new lesions was 0.62 (95% CI, 0.54–0.71). Furthermore, in the sunscreen group, the development of new lesions and the remission of existing lesions were related to the amount of sunscreen used. Such a relationship was not observed in the base-cream group.
However, a different Australian randomized study (the Nambour Skin Cancer Prevention Trial) showed that, after 4.5 years of follow-up, there was no statistically significant difference in the incidence of BCCs or SCCs with regular SPF 16 sunscreen use. This study did not include a sunscreen placebo. Although a secondary subset analysis of the overall number of tumors showed a reduction in the frequency of SCCs on the sites of daily sunscreen application, the validity of the finding is questionable because of the possible effects of extensive multiple statistical testing. An 8-year post-trial observational follow-up demonstrated statistically significant reductions in both the frequency and the overall incidence of SCCs in the regular sunscreen-use arm, but the reliability of these findings is uncertain given their occurrence outside of the controlled-trial environment.