THE CELLULAR DIFFERENCES BETWEEN ACUTE AND CHRONIC NEUTRON AND GAMMA RAY IRRADIATION IN MICE

Abstract

Data from many laboratories are summarized that indicate that for mice the RBE of large single doses of gamma rays or neutrons is about the same for shortening of the life span as it is for acute lethality. However, for chronic irradiation the RBE is quite high for shortening of the life span, due to the fact that very small doses of neutrons are proportionately just as effective as large doses, while for gamma rays, small doses are proportionately only about 1/ 4 as effective as large doses. The reason for this difference was investigated by scoring the chromosome abberations in regenerating liver cells in mice subjected to both chronic and acute neutron and gamma irradiations. The results show that for gamma rays, very small doses are proportionately only about 25% as effective in producing chromosome aberrations as are large doses. However, for neutrons, large and small doses are proportionately equally effective. This result adequately explains the differences in RBE for life shortening and at the same time gives strong support for the somatic mutation theory of aging. Results indicate that for relatively large doses, equal rad doses of neutrons produce about twice the chromosome damage as do gamma rays. This would indicate that the RBE for acute effects can also be explained on the basis of chromosome damage. Following single doses of gamma rays, the percentages of abnormal chromosomes falls slowly over a period of many months, but following a single dose of neutrons the aberration frequency may be as high as 90% and stay this high for at least a year. There is no ready explanation for this difference in reaction to these two radiations. (auth)