Does Radioadaptive Response also Apply to the Case of Heavy-ion Irradiations in Fetal and Adult Mice? Part IV. A Summary on the Data Obtained in Fetal Mice.

Radiation-induced adaptive response (AR) is the phenomenon that a priming low dose induced resistance against the subsequent challenging irradiation at higher doses. Study on AR is of great concern as it is expected to provide important scientific basis for risk estimates, offer significant insight into the biological defense mechanisms, and lead to possible novel radiotherapy for practical application. In a series of investigation, induction of AR with both low LET X-rays and high LET accelerated heavy ions was attempted in fetal and adult mice. The data have been successively reported in the last three JRRS annual meetings. In the present presentation, all the work on fetal mice was summarized. A priming dose from X-rays at either 0.05 Gy or 0.30 Gy on the 11th day of gestation (E11) could induce an AR (judged as the suppression of prenatal death and malformation) against a challenging high dose of X-rays at 3.50 Gy on E12. Using this AR model, 1) priming low doses of X-rays against high doses of high LET irradiations from accelerated heavy ions, and 2) priming low doses of high LET irradiation from accelerated heavy ions against the high dose of X-rays, were tested to verify if an AR could be observed. Accelerated heavy ion particles from mono beams of carbon, neon, silicon and iron generated by HIMAC, with the LET values of about 15, 30, 55, and 200 keV/micrometer respectively, were examined. Results showed that priming low dose of X-rays could induce an AR against the high challenging doses from carbon, neon and silicon ion irradiations while no AR was observed when the challenging dose was from iron ion irradiations. On the other hand, no AR was observed in the case that the priming low doses were from the high LET heavy ions and the challenging high dose was from the low LET X-rays. These findings indicate that under certain conditions the low doses of low LET X-rays could induce AR against the high doses of high LET heavy ion irradiations, and suggest that the successful AR induction by X-rays is dependent on the LET value and/or the particle species of challenging irradiation.