Radiation Fractionation as Applied to Murine Colony-Forming Cells in Differing Proliferative States

DOSE INCREMENT and patterns of dose fractionation are often modified in clinical radiotherapy, depending upon the response and tolerance of the patient. Small changes in daily dose rate may result in obvious clinical differences. This is most apparent in the region of the head and neck, where the response of the mucosal tissues is easily observed. The purpose of this study is to quantitatively explore some current fractionation schemes and their modifications as applied to an in vivo system during differing proliferative states. A preliminary description of some of these data has been reported previously (3). An in vivo assay system developed by Till and McCulloch (9) was used to compare different clinical dose fractionation schemes. Mice were subjected to whole-body radiation of 500 to 900 R, and macro-scopic spleen colonies (endocolonies) were noted nine days later. Similar colonies can be formed by injecting syngeneic bone marrow intravenously into animals which have previously been irradiated sufficie...