Life shortening in RFM and BALB/c mice as a function of radiation quality, dose, and dose rate

A population of nearly 30,000 RFM and 11,000 BALB/c mice were exposed to graded radiation doses of γ-rays at a high (40 to 45 rad/min) or intermediate (8.3 rad/day) dose rate, or to fission neutrons at a high (5 to 25 rad/min) or low (1 rad/day) dose rate. Effects on life shortening are reported. The dose-response curve for γ rays in RFM female mice at the high rate showed multiple changes in slope. In the region of 0 to 50 rad, the relationship between life shortening and dose could be described by a dose squared or a linear-dose-squared model with the dose-squared component predominating above ~4 rad. For RFM and BALB/c female mice irradiated at the intermediate dose rate, the curve was linear over the entire range of doses tested. Linear dose-response relationships were also observed with neutrons at both dose rates over the lower end of the dose range. The relative biological effectiveness of neutrons in the 0- to 50-rad range for RFM female mice irradiated at high dose rates was proportional to the -0.5 power of neutron dose, i.e., the RBE increased with decreasing total dose. γ rays were less effective in shortening life when delivered at an intermediate dose rate than at a high dose rate at high total doses for both strains. For the RFM, the low-dose rate neutron irradiation at low total doses was less effective than the high dose rate, while no dose-rate effect was observed for BALB/c mice in the low dose range.