Repair characteristics and neutron RBE for early and late colo-rectal injury in the mouse

Dose response curves have been obtained for single doses and up to 10 fractions of /sup 137/Cs γ-rays and 3 MeV neutrons after localised irradiation of 22 mm of descending colon and rectum. Early assays included weight loss and lethality. Late assays (6-15 months) included faecal deformity and lethality due to stenosis. The data have been analysed in terms of repair capacity after γ-rays and neutrons, and the RBE. A large sparing effect was seen with γ-rays; up to 11 Gy was recovered if 2 large fractions were used, and at lower doses 50-65% of each dose was recovered. The data were analysed using a linear quadratic model; the α/β ratio was 9-28 for early injury and only 3-5 for late endpoints. No sparing was seen with fractionated neutrons. Because of the difference in the repair characteristics of γ-rays and neutrons the RBE was markedly dependent on dose per fraction. Over the observed dose range the RBE was similar for early and late assays, ranging from about 2 at high doses to 4-6 at low doses. A prediction of the limiting RBE at very low doses can be made from the ratio of α's in the linear quadratic equations. An RBE of 7.14 - 12.7 was predicted for the late endpoints, which is higher than the values of 5.7 - 8.5 for death and early weight loss although there is some overlap