TEMPORAL CHANGES IN THE INCIDENCE OF MALIGNANT-MELANOMA - EXPLANATIONFROM 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 ot her 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 th e site distribution of nonmelanoma skin cancer (relatively many more m elanomas occur on areas of the body not chronically exposed to sunligh t, such as the back of the trunk in males, and the legs in females). T his localization of melanoma, together with its epidemiology, suggest that a change in lifestyle has contributed to the fast-rising incidenc e in many countries. There is no convenient mammalian animal model for malignant melanoma. However, certain inter- and intra-specific hybrid s of fish of the genus Xiphophorus are very sensitive to light-induced melanomas; we have used them to determine the wavelengths effective i n melanoma induction. The action spectrum has a relatively very large component in the UVA region (320-400 nm) compared to human erythema. H ence, if the human and fish spectra were similar, the use of sunscreen s that minimize erythema would have little effect in preventing the in duction of melanoma, and if people using sunscreens expose themselves to sunlight for longer periods, they will be increasing dramatically t heir exposure to these melanoma-inducing wavelengths. Such considerati ons are sufficient to explain the rising incidence of malignant melano ma and its distribution on the body.