The single-hit multitarget cellular lethality model requires four to six parameters in order to simulate a complete course of fractionated radiation therapy and derive an estimate of the cellular-surviving fraction for a given treatment scheme. These parameters are the mean cellular lethal dose, the extrapolation number, the ratio of sublethal to irreparable events, the regeneration rate, and in some instances the repopulation limit (cell cycles), and a field-size or tumor-volume factor. If a number of different fractionation schemes yield similar reactions, the surviving fractions are presumed to be about equal in each instance. Under these circumstances, an equivalent number of simultaneous equations can be set up and the unknown parameters then derived by iterative numerical methods. A computer program was designed for this purpose and tested on available clinical and experimental data. Parameters were derived for skin tolerance and cure of epidermoid cancer in man, for radiation reactions in pig skin, rat spinal cord, various mouse tissues, and for control of mouse mammary carcinoma irradiated in situ. In some systems analogous parameters could be obtained for both X rays and neutrons, thus providing the coefficients required to calculate neutron RBE's for these tissues.

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