STUDIES ON DOSE FRACTIONATION AS MEASURED BY ENDOGENOUS SPLEEN COLONIES IN THE MOUSE.

Dose fractionation is often modified in clinical radiotherapy, depending on the response and tolerance of the patient. Small changes in daily dose increment or brief interruption of daily treatment can make obvious clinical differences. It is the purpose of this study to explore quantitatively in an animal system some current fractionation schemes and their modifications. An in vivo assay system developed by Till and McCulloch (1) is used to compare different clinical dose-fractionation schemes. In mice subjected to 500–900 R whole-body irradiation, macroscopic spleen colonies are noted nine days later. These colonies appear to be derived from a single progenitor cell (2). By varying the dose, a survival curve for these endocolonies can be drawn and is similar to survival curves of irradiated bone marrow transplanted into lethally irradiated mice (exocolonies) (3). Endogenous spleen colonies have an advantage compared to exocolonies in that they afford a means of assessing the population in situ without t...