Lifetime Sunburn Trajectories and Associated Risks of Cutaneous Melanoma and Squamous Cell Carcinoma Among a Cohort of Norwegian Women

IMPORTANCE To our knowledge, no study has prospectively investigated sunburn patterns over age periods from childhood to adulthood and their associations with skin cancer risk. OBJECTIVE To identify lifetime trajectories of sunburns and compare the association between these trajectories and subsequent risk of cutaneous melanoma and squamous cell carcinoma (cSCC). DESIGN, SETTING, AND PARTICIPANTS This population-based cohort study included participants from the Norwegian Women and Cancer Study, established in 1991, with follow-up through 2018. Baseline questionnaires were issued from 1991 to 2007, with follow-up questionnaires every 5 to 7 years. Data analysis was performed from March 16, 2021, to December 4, 2021. EXPOSURES Participants reported pigmentation factors, sunbathing vacations, and indoor tanning. Annual frequencies of sunburns were reported for childhood, adolescence, and adulthood. MAIN OUTCOMES AND MEASURES Information on cancer diagnoses, emigration, and death were obtained through linkage to the Cancer Registry of Norway using the unique personal identification number of Norwegian citizens. RESULTS Of the 172 472 women (age range, 31-70 years) who returned questionnaires, 169 768 received questions about sunburns at study inclusion. Five classes (stable low, low-moderate-low, low to high, high to low, and stable high) of individual lifetime sunburn trajectories with similar shapes were estimated in 3 samples up to 39 years (n = 159 773), up to 49 years (n = 153 297), and up to 59 years (n = 119170). Mean follow-up ranged from 14.3 to 19.5 years in the 3 samples, during which 1252 to 1774 women were diagnosed with incident primary melanoma and 739 to 871 women with incident primary cSCC. With hazard ratios (HRs) and 95% Cls estimated using a Cox proportional hazards model, the stable high and high to low trajectories showed statistically significant increased melanoma and cSCC risks compared with the stable low trajectory across all samples (<= 39 years for stable high and high to low trajectories: melanoma: HR, 1.50 [95% CI, 1.28-1.75] and HR, 1.44 [95% CI, 1.20-1.73]; cSCC: HR, 1.51[95% CI, 1.22-1.87] and HR, 1.47 [95% CI, 1.14-1.91]). Other trajectories showed increased risk, though generally weaker and mainly estimates that were not statistically significant. There was no statistically significant heterogeneity between melanoma and cSCC estimates. CONCLUSION AND RELEVANCE This cohort study showed that high sunburn frequency throughout life was associated with increased melanoma and cSCC risk. Furthermore, sunburns in childhood are especially important for subsequent risk of these skin cancers. Avoiding sunburns throughout life, in particular in childhood, is therefore crucial.