A high energy fast neutron beam potentially suitable for radiotherapy has been described in a companion paper. Its biologic effects have been studied in the following experimental systems: (1) clonal survival and mutation induction after irradiation in vitro in Chinese hamster cells and human diploid fibroblasts; (2) survival of reproductive capacity in vivo of murine hemopoietic colony-forming cells and murine intestinal crypts after irradiation in vivo; (3) survival of reproductive capacity in vivo after irradiation in vitro or in vivo of murine lymphocytic leukemia cells, (4) acute intestinal death following total body irradiation of mice and guinea pigs; and (5) hemopoietic death following total body irradiation of mice and guinea pigs. The relative biologic effectiveness of these high energy neutrons varied among the different biologic systems, and in several cases varied with the size of the radiation dose. The oxygen enhancement ratio was studied in murine lymphocytic leukemia cells irradiated under aerobic or hypoxic conditions in vitro and assayed for survival of reproductive capacity in vivo. Compared with x-rays, the potential therapeutic gain factor for these neutrons was about 1.5. This work represents a 'radiobiologic calibration' program which it is suggested should be undertaken before new and unknown fast neutron spectra are used for experimental radiotherapy. The results are compared with biologic studies carried out at high energy fast neutron generators in the United States.
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