Solar radiation and malignant melanoma of the skin.

Several observations suggest that a majority of cases of malignant melanoma of the skin are linked to sun exposure. Evidence includes higher occurrence of melanoma on anatomic areas heavily exposed during recreation, development of melanoma more frequently in lightly pigmented persons, and correlation of melanoma incidence and mortality with proximity to the equator. The role of the sun exposure in the pathogenesis of melanoma remains unclear, however. Many cases of melanoma may be related to heavy doses of solar radiation received during recreation. Chronic sun exposure is not so clearly linked to the development of melanoma (except in the uncommon lentigo maligna variety). Sunspot cycles have been associated with changes in melanoma incidence; an excess of melanoma cases has been observed every 9 to 12 years after peak sunspot activity. These excess cases may be caused by more intense exposure to solar ultraviolet radiation during sunspot maxima, perhaps related to changes in the stratospheric ozone layer. These epidemiologic and clinical clues suggest that many cases of melanoma are related to sun exposure triggering the appearance of clinically evident melanoma. In this regard, solar radiation behaves as a cocarcinogen or promoter, rather than a dose-dependent carcinogen. These observations also suggest that other factors may be involved in the pathogenesis of melanoma, e.g., nevi, heredity, or exposure to chemical carcinogens.