Study of biological effects of varying mixtures of Cf-252 and gamma radiation on the acute radiation syndromes: Relevance to clinical radiotherapy of radioresistant cancer

Abstract

Purpose : Data for the 30 day bone marrow syndrome (BM-50) and the 6–10 day gastrointestinal (GI-50) syndrome for a one and two fraction schedule and acute and low dose rate irradiation using pure and mixed Cf-252 and photon radiation are presented. Methods and Materials : The radiations of Cf-252 is a mixture of neutrons and gamma rays. We total body irradiated Balb/c mice of both sexes with acute Co-60, low dose rate Cs-137 and Cf-252 using a 1 sx and 2 × schedule. For low linear energy transfer radiations of Co-60 or Cs-137 there was expected to be an increase in the dose to produce the gastrointestinal and bone marrow syndromes with minimal change for Cf-252 neutrons. However, the radiations from Cf-252 are ∼ 65% neutrons and ∼ 35% photons and hence some repair may be expected. We further altered the proportion of photons in the Cf-252 radiation field by mixing Cs-137 with the Cf-252 sources and total body irradiated the mice to determine the effects on the syndromes. We determined the effects of mixing Cf-252 neutrons with different proportions of photons on the radiation syndromes. Results : There was increase in BM-50 and GI-50 doses with fractionated or low dose rate photon irradiations and the dose modifying factors were 1.3-1.4 for the GI syndrome and 1.2 for the bone marrow syndrome. For Cf-252 there was minimal fractionation effect for the GI-50 syndrome, which increased by a 1.1 for × 1 vs. × 2 fractions; for the BM-50 syndrome it rose by a 1.1 factor. For LDR Cs-137 the dose for the GI-50 syndrome rose by a 2.2 fold. For mixed neutron-photon radiation of 0%, 15%, 35%, and 65% η/γ mixtures, the dose to produce the BM-50 and GI-50 endpoints dropped sharply from 0 to 35% neutrons and remained flat thereafter. Conclusion : For major tissues such as the bone marrow and GI tract, Cf-252 behaved as high linear energy transfer for mixtures of neutrons and gamma rays of ∼ 35% neutrons when the radiation were delivered simultaneously at the low dose rates studied. There was little or no additional contribution to the effectiveness of the mixed radiations if neutrons contributed 35% or more of the dose.