Estimation of Radiation Dose-Reduction Factor for β-Mercaptoethylamine by Endogenous Spleen Colony Counts

Abstract

As recovery progresses in irradiated mice, colonies of hemopoietic cells become grossly visible on the spleen, the number of colonies being inversely related to radiation dose (1). In studying characteristics of the colony-forming units, Till, McCulloch, and others most frequently employ donor cells injected into lethally irradiated recipients (for example, 2, 3). For some purposes, however, it is appropriate to study colonies arising endogenously. It seemed to us that the endogenous colony counts might be used to advantage in estimating dose-reduction factors (DRF) for radioprotective agents. Mortality as a basis for DRF estimation has the obvious disadvantage that death is a remote effect of irradiation, subject to conditions which may be difficult or impossible to control. M\oreover, the period of approximately 30 days needed for completion of a test is considerably longer than the time required for protection to become apparent. Although granulocyte and lymphocyte counts in peripheral blood may be used for DRF estimation as early as 3 or 4 days after irradiation (4), these also are subject to extraneous influences, both physiological and technical. Reduction in the number of colony-forming units (CFU) would appear to be a more direct effect of irradiation than either death of the animal or leucopenia, and hence a preferable end point. Meaningful counts in untreated mice can be made over a dose range approximately equal in extent to the range yielding 1 % to 99 % mortality and have the advantage of being completed 10 days after irradiation. The distribution of endogenous colony counts is quite skewed, however, and a transformation is needed if the counts are to be used for DRF estimation.