Some radiobiological aspects of neoplasm therapy by means of 252Cf

Variations in R-I rhabdomyosarcoma volume in the rat after its irradiation with sources of 252Cf in doses of 611, 994 rad (by neutrons) and sources of 60Co in doses of 4000, 6500 rad were studied. Dose rates for 252Cf 11.7-15.2 rad/h (by neutrons) and for 60Co-81-114 rad/h were used. The morphological features and the value of the label index of human tumours before and after therapy with the use of 252Cf were studied. No differences in the nature of a R-I tumour regression during irradiation with 252Cf and 60Co sources were revealed. There was noted a smaller range in tumour volumes in irradiation with definite doses of 252Cf than with 60Co in different animals. A comparison of isoeffective doses (causing a certain decrease in the volume of the irradiated tumour) made it possible to determine the value of RBE for this index which equalled 6.4+-0.1. Having used the data presented in this paper and their other works done in parallel to them on the same problem of investigation the authors calculated a therapeutic gain factor for neutron radiation of 252Cf which appeared to be equal to 1.1. The histological characteristics of radiation injuries and the value of the label index of biopsy materials of the patients in whom the minimum dose in the tumour volume was 955-1170 rad evidences a favourable effect of contact therapy with the use 252Cf for these doses, and the conformity of the clinically used doses to the value of RBE obtained by the authors