The relative biological effectiveness of mixed fission-neutron-gamma radiation on the hematopoietic syndrome in the canine: effect of therapy on survival.

Acute lethality syndromes produced by the accidental exposure of humans to mixed neutron and gamma radiation from external sources can be related to acute lethality from photon irradiation using the relative biological effectiveness (RBE) for common end points. We used the canine as a model to study injury following exposure to mixed neutron and gamma radiation from the AFRRI TRIGA reactor. Exposures from the reactor were steady-state mode (40 cGy/min, bilateral) with an average neutron energy of 0.85 MeV; tissue-air ratio = 0.59 at midline abdominal. Healthy male and female canines were irradiated free-in-air behind a 6-in. lead wall; the neutron-gamma ratio was 5.4:1 at the entrance skin surface; exposures are reported as midline tissue doses. Bilateral exposure resulted in an LD50/30 of 153 cGy without therapeutic clinical support. Addition of clinical support consisting of fluids, antibiotics, and fresh irradiated platelets/whole blood increased the bilateral LD50/30 to 185 cGy, a dose modifying factor (DMF) of 1.21. This corresponds to respective LD50/30 values for bilateral 60Co gamma exposures of 260 and 338 cGy for nonsupported and clinically supported animals, and a DMF of 1.30. The RBE based on the values determined at midline tissue is approximately 1.69. Clinical support after bilateral irradiation produced a similar DMF to those of mixed fission neutrons and gamma rays and 60Co gamma rays alone. The RBE of 1.69 for midline tissue bilateral exposures is higher than 1, an RBE often cited for large animals. Therapeutic support administered to lethally irradiated canines significantly improved survival and increased the LD50/30 independent of radiation quality.