Incidence of radiation-induced skin tumours in mice and variations with dose rate

Mice were exposed to weakly penetrating beta-particles from an external source, using 12 different surface doses ranging from 5.4 to 260 Gy and given at four different dose rates from 200 to 1.7 cGy/min. As in previous investigations, both epidermal and dermal tumours occurred with the latter predominating. The lowest surface dose to produce a statistically significant increase in skin tumours was 21.7 Gy, no effect being detected with doses of 5.4-16.3 Gy. The dose-response curves rose steeply when obvious increases occurred. Consideration of these findings and the fact that radiation-induced skin tumours can have an exceptionally long latent period leads to the suggestion that there is some relatively radioresistant factor which normally restrains potential radiation-induced cancer cells in the skin from becoming tumours until the skin is subjected to high local doses. Tumour-induction was unaffected by reducing the highest dose rate by a factor of 10 and the dose-response curves were almost identical. Further reductions of dose rate, encompassing a further factor of 10, in general resulted in fewer tumours.