The relative effectiveness of super- and orthovoltage X rays for intestinal damage in man and mouse

The RBE for 8MV X rays relative to 250 kVp X rays was found to be 1 for damage to the mouse intestinal mucosa measured by jejunal crypt counts. This value is at variance with the RBE of about 0·85 observed for clinical tolerance of patients in irradiation of the abdomen. It is concluded that this difference in RBE observed in mouse and man is due to the overriding importance of changes in depth-dose distribution and of integral dose in the determination of clinical tolerance in man. Only when differences in dose distribution between a new radiation and the reference radiation are minimal can factors obtained from small rodents be applied to man.