RBE values and repair characteristics for colo-rectal injury after caesium 137 gamma-ray and neutron irradiation. II. Fractionation up to ten doses

Abstract

Early and late colo-rectal damage in mice was assessed after 137Cs .gamma.-irradiation and 3 MeV neutrons given as 1, 2, 5 or 10 fractions. Damage was measured by early changes in body weight, the late production of short fecal pellets and the pattern of lethality after irradiation. The data were analyzed in terms of the time course of expression of damage, fractionation effects and the RBE [relative biological effectiveness] for neutrons over a wide range of doses/fraction (0.5-12.5 Gy [gray] neutrons, 3.5-33.5 Gy .gamma.-rays). An initial epithelial denudation led to an early loss of weight, maximal at 11-17 days after irradiation. A dose-dependent weight reduction persisted over the animals'' lifetime. Deaths after localized pelvic .gamma.-irradiation were progressive with no sharp demarcation between early or late phases of injury. The time course for lethality was qualitatively similar after neutrons. Beyond 6 mo. the rectum became constricted by fibrosis; a higher proportion of small fecal pellets was observed. At 6-15 mo. relatively shallow dose-response curves were obtained for this change. The sparing effect of fractionation was marked for the .gamma.-irradiated mice and almost absent after neutrons. A very high repair increment (11 Gy) was seen with 2 .gamma.-ray fractions of 20 Gy. At lower doses/fraction the proportion of each .gamma.-ray fraction recovered was 50-69% for all assays, i.e., similar to that for other normal tissues. There was a slight enhancement in the sparing effect for the late compared with the early assays over the lower dose range. The RBE was strongly dependent on dose/fraction because of the lack of reparable damage after neutrons. The RBE for both early and late effects was 5.0 at a neutron dose/fraction of 1 Gy. Extrapolation of the RBE data to lower doses, using the linear quadratic model, predicts a higher RBE for late (7.4-12.7) than for early damage (5.7-8.5) if .gamma.-ray doses below 5 Gy are used.