Effect of varying dose of UV radiation on mammalian skin: simulation of decreasing stratospheric ozone.

To better understand the dependence of the incidence of squamous cell carcinoma on changes in solar spectral distribution and dose regimen, we exposed SK-1 hairless mice to solar-simulating radiation (290-400 nm). Selective UV filtration was accomplished by passing this radiation through Schott WG-320 cutoff filters of 0, 0.5, 1.0, 2.0, and 3.0 mm thickness. Minimal erythema doses (MED) were determined for each filter combination. Starting with 0.5 and with 0.9 MED, groups of 20 mice were irradiated 5 days per week; this was increased by 20% increments (of the original dose) every 6th day for 40 days ("0.5 MED" and "0.9 MED" experimental groups, respectively). Other groups of mice were irradiated with the same incremental increases, starting at 6.5 J/cm2 ("equal dose" regimen). The salient results were: (1) shorter wavelength components appear to preferentially produce tumors; (2) resultant observable dose-response behavior for each regimen is a complicated function of concurrent "light" and "dark" reactions; (3) time-dose reciprocity is absent; and (4) there are no straightforward relationships among tumor efficiency, dose fractionation, and spectral distribution of excitation radiation. These results indicate that photocarcinogenesis is a dynamic process, in which events that result in tumor growth compete with those that cause tumor regression.