Sun and solarium exposure and melanoma risk: effects of age, pigmentary characteristics, and nevi.

BACKGROUND Few prospective studies have analyzed solar and artificial (solarium) UV exposure and melanoma risk. We investigated these associations in a Norwegian-Swedish cohort study and addressed effect modification by age, pigmentary characteristics, and nevi. METHODS The cohort included women ages 30 to 50 years at enrollment from 1991 to 1992. Host factors and exposure to sun and solariums in life decades were collected by questionnaire at enrollment. Relative risks (RR) with 95% confidence intervals (CI) were estimated by Poisson regression. RESULTS Among 106,366 women with complete follow-up through 2005, 412 melanoma cases were diagnosed. Hair color and large, asymmetric nevi on the legs were strongly associated with melanoma risk (P(trend) < 0.001), and the RR for > or =2 nevi increased from brown/black to blond/yellow to red-haired women (RRs, 1.72, 3.30, and 4.95, respectively; P(interaction) = 0.18). Melanoma risk increased significantly with the number of sunburns and bathing vacations in the first three age decades (P(trend) < or = 0.04) and solarium use at ages 30 to 39 and 40 to 49 years [RRs for solarium use > or =1 time/mo 1.49 (95% CI, 1.11-2.00) and 1.61 (95% CI 1.10-2.35), respectively; P(trend) < or = 0.02]. Risk of melanoma associated with sunburns, bathing vacations, and solarium use increased with accumulating exposure across additional decades of life. CONCLUSIONS Melanoma risk seems to continue to increase with accumulating intermittent sun exposure and solarium use in early adulthood. Apparently, super-multiplicative joint effects of nevi and hair color identify people with red hair and multiple nevi as a very high risk group and suggest important gene-gene interactions involving MC1R in melanoma etiology.