Relative Biological Effectiveness Measurements Using Murine Lethality and Survival of Intestinal and Hematopoietic Stem Cells after Fermilab Neutrons Compared to JANUS Reactor Neutrons and 60 Co Gamma Rays

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Abstract
The relative biological effectiveness (RBE) of the 25-MeV (average energy) neutron beam at the Fermi National Accelerator Laboratory was measured using murine bone marrow (LD50/30) and gut (LD50/6) lethality and killing of hematopoietic colony forming units (CFU-S) or intestinal clonogenic cells (ICC). The reference radiation was 60Co .gamma. rays. The LD50/30 and LD50/6 for mice exposed to the Fermilab neutron beam were 6.6 and 8.7 Gy [gray], respectively, intermediate between those of JANUS neutrons and 60Co .gamma. rays. The D0 [mean lethal dose] values for CFU-S and ICC were 47 cGy and 1.05 Gy, respectively, also intermediate between the lowest values found for JANUS neutrons and the highest values found after 60Co .gamma. rays. The split-dose survival ratios for CFU-S at intervals of 1-6 h between doses were essentially 1.0 for both neutron sources, while the corresponding split-dose survival ratio for 60Co .gamma. rays was consistantly above 1, reaching a maximum of 1.7 with a 1-h interval between doses. The 3 h split-dose survival ratios for IcC were 1.0 for JANUS neutrons, 1.85 for Fermilab neutrons, and 6.5 for 60Co .gamma. rays. The RBE estimates for LD50/30 were 1.5 and 2.3 for Fermilab and JANUS neutrons, respectively. Based on LD50/6, the RBES were 1.9 (Fermilab) and 3.0 (JANUS). The RBE for CFU-S D0 were 1.4 (Fermilab) and 1.9 (JANUS) and for jejunal microcolony D0 1.4 (Fermilab) and 2.8 (JANUS).