THE EFFECT OF DOSE FRACTIONATION ON RADIATION-INJURY IN THE RAT STOMACH

Localized 300 kV X-irradiation of the rat stomach with single and frac tionated (two and five fractions given daily and in 4 weeks) doses was performed. In the second and third week after irradiation acute radia tion gastritis developed which resulted in a dose-dependent drop in bo dy weight. The alpha/beta value for a 10% drop in body weight was calc ulated to be 9.6 Gy (95% confidence interval 2.1-25.9 Gy). Between 4 a nd 40 weeks after irradiation subchronic radiation damage was observed which presented itself as atonic dilatation of the stomach. For this effect, the alpha/beta value is 4.8 Gy (graphical estimation) and 5.3 Gy (95% confidence interval 0.4-13.1 Gy; direct Poisson model). In the five-fraction (5F) experiment a significant increase in tolerance amo unting to 0.8 Gy/day for the acute effect and 0.4 Gy/day for the subch ronic effect was observed when intervals were increased from 1 day to 1 week. In a separate experiment animals were pre-irradiated with subt oxic and toxic radiation doses. One hundred days later acute tolerance to a second irradiation was completely restored. However, no recovery of subchronic damage was observed.