Life Shortening in RFM and BALB/c Mice as a Function of Radiation Quality, Dose, and Dose Rate

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Abstract
A population of nearly 30,000 RFM and 11,000 BALB/c mice were exposed to graded radiation doses of .gamma.-rays at a high (40-45 rad/min) or intermediate (8.3 rad/day) dose rate, or to fission neutrons at a high (5-25 rad/min) or low (1 rad/day) dose rate. Effects on life shortening are reported. The dose-response curve for .gamma.-rays in RFM female mice at the high rate showed multiple changes in slope. In the region of 0-50 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 .apprx. 4 rad. For RFM male mice after irradiation at the high dose rate, and for both 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 [RBE] of neutrons in the 0-50 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. .gamma.-Rays were less effective in shortening life when delivered at an intermediate dose rate than at a high dose rate. With neutrons, the low dose rate was more effective than the 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.