Temporal changes in the incidence of malignant melanoma: explanation from action spectra.

The incidence of malignant cutaneous melanoma has been increasing for more than 50 years, and is now rising more rapidly than that of any other cancer. This increase is not explicable by changes in the physical environment, particularly by any observed increase in UVB radiation (290-320 nm). The distribution of melanomas on the body differs from the site distribution of nonmelanoma skin cancer (relatively many more melanomas occur on areas of the body not chronically exposed to sunlight, such as the back of the trunk in males, and the legs in females). This localization of melanoma, together with its epidemiology, suggest that a change in lifestyle has contributed to the fast-rising incidence in many countries. There is no convenient mammalian animal model for malignant melanoma. However, certain inter- and intra-specific hybrids of fish of the genus Xiphophorus are very sensitive to light-induced melanomas; we have used them to determine the wavelengths effective in melanoma induction. The action spectrum has a relatively very large component in the UVA region (320-400 nm) compared to human erythema. Hence, if the human and fish spectra were similar, the use of sunscreens that minimize erythema would have little effect in preventing the induction of melanoma, and if people using sunscreens expose themselves to sunlight for longer periods, they will be increasing dramatically their exposure to these melanoma-inducing wavelengths. Such considerations are sufficient to explain the rising incidence of malignant melanoma and its distribution on the body.