The historical background for large-animal studies with neutrons of various energies

Studies of the biological effects of fast neutrons were started as soon as adequate sources became available, and the first of these was the cyclotron at Berkeley. Limitations on methods for physical dosimetry render most of the earliest studies nearly useless, and the first landmark studies with modern methodology were done at Berkeley in the 1950s. This report is an attempt to review and summarize all the work that took place from 1950 to 1965. The RBE for fast neutrons in the dog is generally in the range of 1.0 or less, depending upon reference radiation, the biological end point chosen, and the exposure geometry. Also the RBE seems to be relatively independent of the fast-neutron spectrum, at least for the range of values that has been studied. Mean after-survival times in irradiated dogs and goats indicate that the bone marrow syndrome usually predominates in large animals irradiated with fast neutrons. The gastrointestinal threshold for the dog is around 1200 cGy, and for neutrons the biological effectiveness for the GI syndrome still does not bring the GI threshold down to the point where it predominates over the bone marrow syndrome. As expected from biological first principles, for dose rates in excess of a few tens of centigrays per minute, dose rate seems to have no effect on the outcome with either neutron, gamma or X irradiations when the end point is lethality.