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Our objective was to test the hypothesis of a short-term late-promoting effect of ultraviolet (UV) exposure on the development of cutaneous melanoma as an explanation for the summer peak in melanoma incidence. Therefore, we studied seasonal variation in melanoma incidence in relation to recent UV levels by direct UV measurements. Data from the Belgian Cancer Registry on invasive cutaneous melanoma diagnosed during 2006-2011 were used for analysis. Daily data on UV measurements in Belgium were obtained from the Royal Meteorological Institute. Simple and multiple negative binomial regression models were used to investigate the influence of recent UV levels on melanoma incidence. The sum of the mean UV doses in the 2 months before diagnosis was used as a proxy for recent UV exposure in the population. To include variable sunburn risks during the year, the categorical variable 'semester' was created. The incidence of melanoma in Belgium shows a distinct seasonal variation, with peaks in June or July. We found that part of this variation could be explained by the variation in dermatologic activity and, therefore, used this as an offset in our models. We found a linear relationship between melanoma incidence and UV dose in the 2 months preceding the diagnosis. UV levels had more impact in the first semester. The effect of UV levels was not modified by sex nor age. The interaction between anatomical site and UV levels was significant (P=0.002) and showed a higher effect on the upper and lower limbs compared with the head and neck and trunk.

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