High Versus Low Sun Protection Factor Sunscreens and Cutaneous Squamous Cell Carcinoma Risk: A Population-Based Cohort Study.

Evidence on sunscreen use and cutaneous squamous cell carcinoma (cSCC) risk is limited. Most studies did not take sun protection factor (SPF) into consideration, and used nonusers of sunscreen as the reference group. Nonusers are likely a priori at lower cSCC risk than users. No study has investigated the effect of high versus low SPF sunscreens on cSCC appropriately adjusting for time-varying confounding. Using data from the Norwegian Women and Cancer study (1991-2016), we investigated whether use of SPF≥15+ versus SPF<15+ sunscreens reduces cSCC risk. We used marginal structural Cox proportional hazards model with inverse probability of treatment and censoring weights to estimate hazard ratios (HRs) and 95% confidence intervals (CIs). During follow-up of 148,781 women (1991-2016, mean 14.3 years), 653 women were diagnosed with cSCC. The effect on cSCC risk of SPF≥15+ versus SPF<15+ sunscreens was close to the null when used at any latitudes (HR=1.02, 95% CI: 0.82, 1.27) and when used in lower latitude settings (HR=1.05, 95% CI: 0.84, 1.32). In conclusion, we found no indication that SPF≥15+ sunscreens reduced Norwegian women's cSCC risk more than SPF<15+ sunscreens, suggesting that either there is no difference in their effects long-term, or the difference is diluted by incorrect application.