Studies on Dose Fractionation as Measured by Endogenous Spleen Colonies in the Mouse

Abstract

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 the manipulation of transplantation. They suffer from the disadvantage that they are more variable in size, hence are more difficult to count. Colony-forming cells normally exist in a steady state with a relatively low rate of turnover, yet they have the capacity to respond to a stimulus by greatly increasing their proliferative rate (4). This dividing mammalian tissue provides a useful means of quantitatively assessing differences in fractionation in a normal tissue able to vary its proliferative rate. With this system the effects of 200 R daily and 250 R daily are compared to each other and to the single dose survival curve. For dose increments of 250 R, the effect of one-and two-day rests in the treatment cycle is investigated, and the influence of the position of these breaks in the treatment course is assessed. Methods Male C3R mice3 were used in all experiments. Most animals were four to six months old although a few were older, and these were randomly allocated to the experimental groups. Whole-body radiation was given to groups of 10 animals in varying schedules. X-radiation factors were 250 kVp, 15 mA, h.v.l. 1.65 mm of copper, SSD 50 cm, and output approximately 114 R per minute. Dose rate was measured with a Victoreen chamber in a mouse “phantom.” Nine days following completion of radiation the surviving animals were sacrificed, and their spleens were removed and placed in Bouin's fixative. Subsequently, the surface macroscopic colonies were counted. Results were collected in two experiments. Since resultant data were similar, they were pooled. Results Single doses of radiation resulted in the data depicted in Figure 1. These can be plotted as an exponential function with a D0 of 115 R. Data at less than 500 R cannot be scored since the colonies are so numerous that they become confluent. Doses of greater than 900 R give no colonies.