Bone Marrow Response of Beagles to Fractionated Doses of Mixed Gamma-neutron Radiation

Male beagles were bilaterally exposed to mixed gamma-neutron radiation from the AFRRI-TRIGA reactor. The dose rate was 20 rads/minute for all irradiations and midline tissue doses were 215 rads, 225 rads and 240 rads. At each dose approximately one-half of the animals received a single irradiation while the others were given one-fourth the dose each day for 4 consecutive days. The objective of this study was to determine if doses in the median lethal range delivered in four equal fractions at 24-hour intervals were more lethal and more damaging to survival-limiting hematopoietic cell renewal systems than similar, unfractionated doses. Serial rib marrow aspirates and peripheral blood samples were studied for 32 days after irradiation. Single irradiations were the more lethal, first evident at 225 rads and increasing markedly in effectiveness at 240 rads. Radiation-induced changes in bone marrow cellularity were similar after single and fractionated irradiations. Thus, data did not support an enhanced effect on key end points as would have been expected from mitotic cell cycle synchronization and increased marrow cell death from the carefully spaced dose