The effects of multifraction irradiation with X rays, neutrons, and pions on the rat cervical and lumbar spinal cord, mouse lung, and rat rectum have been investigated. The linear-quadratic model was used to analyze the effectiveness per unit dose for various tissue responses. It is concluded that the dependence of tolerance doses on fraction size is considerably reduced for both intermediate (pions) and high-LET (neutrons) radiations, as shown by the observed α/β ratios in the range of 20-50. With accurately defined α/β values for various tissues and types of radiation, the same tolerance formalisms can be used as proposed for low-LET radiation. The effectiveness of pion irradiation shows a significant dependence on dose rate when treatment times are long and repair of subeffective damage occurs during the irradiations. For late effects in spinal cord, lung, and rectum, RBE values of pions are 1.5 or less at doses per fraction in the range of 1.2-4.5 Gy.

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