The RBE of neutrons in vivo

The potential for neutrons in radiation therapy depends on a differential in RBE for tumor versus normal tissues. To study this we compared the responses of normal and tumor tissues to 15-MeV and 252Cf neutrons with responses to 137Cs gamma or 300-kVp x rays in the mouse in vivo. Using 1% survival as a biological end point, the RBEs of 15-MeV and 252Cf neutrons were 2.3 and 5.1 (137Cs at 4.5 rads/min) respectively for the EMT6 tumor and 1.5 and 2.5 (137Cs at 4.5 rads/min) respectively for bone marrow CFUs. For pulmonary and esophageal lethality, the RBE values of 15-MeV neutrons were 2.2 and 4.1. With 15-MeV neutrons, the RBE for the tumor was greater than the bone marrow CFU. The RBE for pulmonary lethality was less than that for esophageal lethality. With 252Cf, the RBE for the EMT6 tumor, although greater than the bone marrow CFU, was not significantly different from reported in vivo RBE values of normal tissues at comparable gamma dose rates.