For the application of fast neutrons in radiotherapy, it is important to evaluate the responses of various types of normal tissues in comparison with responses of experimental tumors after fast-neutron and x-irradiation. For rapidly proliferating cell-renewal systems such as the intestinal tissue, skin, and the immunohematopoietic system, extensive information is available on the RBE values as a function of dose or dose per fraction for various neutron beams. Only recently have quantitative studies been initiated on the response of slowly proliferating cell-renewal systems such as the vascular endothelium which are considered to be dose limiting in clinical radiotherapy. Results concerning the vascular proliferation in subcutaneous tissue after irradiations with collimated beams of 300-kV x-rays and 15-MeV neutrons are presented. For neutron doses between 250 and 500 rad, it has been observed that the RBE varies only slightly from 1.9 to 1.8. These studies indicate that the RBE values for effects of 15-MeV neutrons on the capillary endothelium are similar to the RBE values obtained for skin and intestinal epithelium and confirm that, in this dose range, the overall gain factor <tex-math>${\rm RBE}_{\text{tumor}}/{\rm RBE}_{\text{normal tissue}}$</tex-math> will be 1.2-1.3.

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