Indoor tanning is associated with increased risk of melanoma, but most evidence comes from case-control studies. Using data from the Norwegian Women and Cancer Study, a large prospective cohort study, we investigated the associations of age at initiation of indoor tanning, duration of tanning-device use, and dose response with melanoma risk and examined the role of indoor tanning in age at melanoma diagnosis. We used Poisson regression to estimate relative risks and 95% confidence intervals for the relationship of indoor tanning to melanoma risk and linear regression to examine age of indoor tanning initiation in relation to age at diagnosis. During follow-up of 141,045 women (1991-2012; mean duration follow-up = 13.7 years), 861 women were diagnosed with melanoma. Melanoma risk increased with increasing cumulative number of tanning sessions (for highest tertile of use vs. never use, adjusted relative risk = 1.32, 95% confidence interval (CI): 1.08, 1.63); P-trend = 0.006. Age at initiation <30 years was associated with a higher risk in comparison with never use (adjusted relative risk = 1.31, 95% CI: 1.07, 1.59). Moreover, women who started indoor tanning prior to 30 years of age were 2.2 years (95% CI: 0.9, 3.4) younger at diagnosis, on average, than never users. This cohort study provides strong evidence of a dose-response association between indoor tanning and risk of melanoma and supports the hypothesis that vulnerability to the harmful effects of indoor tanning is greater at a younger age.
Keywords: Norway; cohort studies; indoor tanning; melanoma; prospective studies; tanning beds; ultraviolet radiation.
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