Relative biological effectiveness of low- and high-LET radiotherapy beams for jejunal crypt cell survival at low doses per fraction.

In order to determine the relative biological effectiveness (RBE) of various low- and high-LET radiotherapy beams of low coses, LAF1 mice were exposed to one dose and 10 equally fractionated doses of 60Co gamma rays, 137Cs gamma rays, 4-MeV x rays, 300-kVp x rays, helium ions in the plateau and Bragg-peak region, and 15-MeV neutrons; survival of jejunal crypt cells with the microcolony assay of Withers and Elkind was determined using 60Co as the standard. RBE values for the survival of 10 cells/circumference for 10 fractionated exposures (2.4 Gy [240 rad] per fraction of 60Co) were: 1.07 for 137Cs, 1.06 for 4-MeV x rays, 1.16 for 300-kVp x rays, 1.10 for helium ions in the plateau, 1.29 for helium ions in the peak, and 3.02 for 15-MeV neutrons. As LET increased, RBE increased with decrease of dose/fraction.