Response of mouse small intestine to fractionated doses of pions

The effect of single or fractionated doses of stopping pions or 200 kV X-rays on the mouse jejunal crypt cells was used to determine in vivo RBE values. For single fraction, the pion/X-ray RBE was 1.27 and it increased to about 1.31 when two fractions were applied at 3 or 24 h interval. When four fractions were given at 3 h intervals, the RBE was 1.46. This is because the fraction of "dose repaired" was always higher for X-rays than for pions and this difference was enhanced when more dose fractions were used. The data presented is, in general, consistent with the biological effects of pions reported for other in vivo end points.