"Low Level
Radiation Health Effects: Compiling  the Data"

Revision 2
March 19, 1999

by Radiation, Science, and Health, Inc.,
Edited by J. Muckerheide

1.3
Animal & Plant
Biology

1.3.1
Mammals

References

"Preirradiation with 0.05 Gy of X-rays 2 months before a second exposure to a mid-lethal dose significantly enhanced the survival rate in both female and male ICR strain mice. The radioresistance was observed between 2-2.5 months after exposure to 0.05 Gy. It did not appear within 1.5 months, and disappeared after 3 months. This radioresistance was induced only by whole-body preirradiation (not by partial irradiation of the head or the trunk). On the other hand, preirradiation with 0.30 Gy as well as 0.50 Gy resulted in radioresistance 2 weeks later, but not 2 months later. The radioresistance was induced by whole-body preirradiation or partial preirradiation of the trunk. No radioresistance was evident after exposure of intermediate preirradiation doses of 0.15 and 0.20 Gy administered before 2 months and 2-5 weeks, respectively. The present and previous results show that the biological effects of ionizing radiation may be distinguished with the following four radiation dose ranges; (l) below 0.025 Gy: no radioresistance after 2 months; (2) 0.05-0.10 Gy: significant radioresistance after 2-2.5 months; (3) 0.20 Gy: no radioresistance after 2-5 weeks; and (4) 0.30-0.50 Gy or more: significant radioresistance after 2 weeks. These results conflict with previous findings of the biological effects of ionizing radiation in which the radiation hazard increases in relation to increasing accumulated doses. Some stimulation, in addition to adaptation, by low dose irradiation may have occurred."

"Fig. 2 shows the survival curves after the second exposure to 8.0 Gy, which was carried out 2 months after partial-body preirradiation with 0.05 Gy. The survival rates of the sham-irradiated control group, the whole-body preirradiated group, the head preirradiated group and trunk preirradiated groups, on day 30 after the 2nd exposure, were 30.0% (15/50), 71.9% (41/57), 46.0% (23/50) and 49.0% (24/49), respectively. Only the whole-body preirradiated group showed a significant (p < 0.001) increase in the survival rate. Preirradiation of the head or the trunk was not sufficient to induce radioresistance. On the other hand, in the case of preirradiation with 0.50 Gy, the 30-day survival rates after the second irradiation with 7.35 Gy of the trunk preirradiated group, whole-body preirradiated group, head-preirradiated group and sham-irradiated control group were 67.5% (27/40), 57.5% (23/40), 42.5% (17/40) and 32.0% (16/50), respectively (Fig. 3). Mice acquired the radioresistance by an exposure to the trunk (p < 0.01) more effectively than to the whole-body (p < 0.05)."

       Figure 2       Figure 3

"Our observations conflict with the hypotheses of biological effects on ionizing radiation in which the radiation hazard increases with the accumulated doses. The mice seem to receive the 4 dose ranges of X rays, mentioned above, as different biological stimuli."