Response of mouse intestine to neutrons and gamma rays in relation to dose fractionation and division cycle

Using an assay for jejunal crypt stem cell survival, the biological effectiveness of two cyclotron-produced neutron beams relative to one another and to 60Co γ-radiation has been determined at doses of the order used per fraction in clinical trials of neutrons. Dose survival curves were determined using up to 5 doses of neutrons and up to 20 dose-fractions of γ rays. There is little sparing of jejunal mucosa as a result of repair of sublethal injury during intervals between neutron dose fractions, whereas such repair is considerable when γ-ray doses are fractionated. The response of jejunal crypt stem cells to both neutron and gamma radiation shows considerable variation throughout the division cycle, but the variation is less with neutrons.