Effect of dose fractionation on early and late skin responses to γ-rays and neutrons

Areas of pigs' skin were exposed to fractionated doses of γ-rays or E_d = 50 MV (B_e) neutrons and their responses measured in the acute phase in terms of desquammation and after several months as degree of contraction. The purpose of the experiments was 2-fold-to measure the biological effectiveness of these neutrons relative to γ-rays for acute and late effects and to determine the relative changes in acute and late responses when the dose fractionation pattern of a 6.5 week treatment was changed from five times to twice weekly. The relative biological effectiveness (RBE _{n gamma}) was 3.2-3.4 for late effects and less than that for acute desquammation. Dividing neutron doses of 200 rad or less into smaller fractions did not affect the dose-response relationship for late contraction. With γ-rays, however, there was a marked effect of dose fractionation on late contraction-when the 5 days per week fractionation pattern was changed to a twice weekly regimen, with an increase in fractional doses from 180-260 rad to 380-460 rad, there was a marked increase in the severity of late responses. This increase in late response was not predictable from acute responses which were the same or slightly reduced in the animals treated twice weekly. The two most important observations were, therefore, that increasing the size of dose fractions in conventional γ-ray therapy may be associated with severe late complications not predicted by increase in the acute response, and that the RBE _{n gamma} for late effects is higher than for acute effects in skin.