Seasonality of cutaneous melanoma diagnoses: a comprehensive comparison of results in Bavaria and Northern Ireland.

Ultraviolet radiation is an established skin carcinogen. By analysing the seasonality of melanoma diagnoses, the effect of this risk factor can be examined indirectly. However, previous studies yielded conflicting results, because of vastly differing analytical methods and diverse study designs. Therefore, to validate the findings by Chaillol and colleagues based on 3868 melanoma diagnoses from Northern Ireland, we used an identical methodology to examine the seasonal effects on monthly incidences of 11,901 cutaneous malignant melanoma cases registered from 2003 until 2008 in Bavaria, Germany. Multivariable regression models for count data were used to estimate the effect of seasons while taking into account sex, age, year of diagnosis, Breslow thickness and anatomical site. Point and interval estimates of the adjusted relative risks associated with seasons were compared with the findings of the Irish study. Both analyses demonstrated a promoting effect of intermittent ultraviolet radiation on cutaneous melanoma of a thickness of 2 mm or less evolving on the extremities, whereas no seasonality for melanomas on the trunk was evident. Differences were identified with regard to thick melanomas. In the present analysis, only melanomas with a Breslow thickness of more than 2 mm were also found to be associated with season, in particular, the melanomas on the limbs. Hence, in contrast to the data of, and to the interpretation by, Chaillol and colleagues, an effect of ultraviolet radiation on melanoma progression from thin to thick cannot be excluded.