Association of cutaneous malignant melanoma with intermittent exposure to ultraviolet radiation: results of a case-control study in Ontario, Canada.

BACKGROUND Although solar radiation is well established as a risk factor for melanoma, it is less clear how the pattern and timing of exposure to ultraviolet (UV) radiation might be important. The particular objective of this study was to evaluate the association of melanoma risk with various measures of intermittent and chronic exposures to UV radiation, and to assess how these exposures interact with other risk factors such as skin type. METHODS Data were analysed from a large case-control study (583 cases, 608 controls) of malignant melanoma, carried out in southern Ontario, Canada. RESULTS Significant risk increases were identified with several measures of intermittent exposure, including beach vacations in adolescence and in the past 5 years, previous sunburn, and use of sunbeds and sunlamps. Chronic exposure, indicated by days of outdoor activity during adolescence and by occupation in recent adult life, was associated with significantly reduced risk. Subgroup analyses showed: no major risk differences by body site of melanoma; stronger association of lentigo maligna melanoma with intermittent exposure; more pronounced effects of beach vacations and sunburn in younger subjects; and consistently higher risks for intermittent exposures among subjects with skin more susceptible to burning. CONCLUSIONS The data lend limited support to the hypothesis of increased risk associated with intermittent UV exposure. The findings suggest that future studies should take age at diagnosis, host susceptibility and histological subtype into account.