Dependence of RBE on fraction size for negative pi-meson induced renal injury

A method is described for quantitation of murine late renal radiation response using histopathologic grading. Effective dose-50 percent (ED 50) is defined as the radiation dose at which 50% of animals showed renal tubular alterations of a specific histologic grade. The designated histologic grade was selected on the basis of its correlation with renal failure manifested by elevated blood urea nitrogen levels, weight loss and mortality of the experimental animals. Using this system, the ED 50 was determined for single, two, five, and fifteen (daily) fraction exposures with 300 kVp X rays and negative pi-meson (pion) beams generated at the Los Alamos Meson Physics Facility. The correlation between total radiation dose and the number of fractions was linear for both x ray and pion exposures when plotted on logarith mic coordinates, i.e. this relationship followed a power function. The relative biological effectiveness (RBE) for pions increased from 1.12 for a single dose of 1130 red, to 2.18 for 15 daily fractions of 145 rad. This latter fraction size approaches that being used in clinical trials, and provides an estimate of the RBE to be expected for kidney injury under conventional treatment protocols.