VI. The RBE of Fast Neutrons in Producing Intestinal and Skin Injury in Rats

Three sets of investigations were carried out, using (1) the leakage of 131I-HSA (labelled human serum albumin) through the intestinal wall of rats as a measure of gut damage, (2) skin reactions in rats in order to compare with skin reactions in pigs, and (3) the slowing up of the passage of a barium meal through the stomach in rats, as an analogue to radiation sickness. Intestinal injury is known to be the main cause of death in animals given fast neutron doses of 150–300 rads. X rays, on the other hand, give rise to death by marrow injury, at doses in the LD50 range. In assessing the possible dangers of high neutron doses to various tissues, it is therefore important to include investigations on the RBE of intestinal injury. The LD50 in 30 days obviously cannot be used, as the mechanisms causing death by neutrons and X rays are different, although if the LD50 before eight days were used it would be possible to obtain an RBE for intestinal death. We have used a method which did not require lethal irradiations, and which gave a reasonably practical index of injury in the intact animal without extrapolation from simple systems. The mechanism of serious radiation injury is the depopulation of the intestinal epithelium at about three days after irradiation. The lining of the gut then becomes less effective as a selectively absorbing membrane for electrolytes and proteins and as a bacterial barrier.