Study on reactions in track for charged particle radiation therapy and radiation breeding

We have been studying ion-beam induced mutation of the budding yeast, S288c (RAD+), as a model of the eukaryote cell. In the case of ion-beam irradiations, several features of deep interest can be observed, contrasting with the low-LET radiations such as gamma-rays. The yeast cells were irradiated with carbon ions (12C5+; 220-290 MeV) with the dose 50 to 200 Gy. Carbon ion beams were generated from the AVF cyclotron in JAEA or synchrotron in HIMAC. When we analyzed the survival rate with different radiations, the survival rate was reduced along with the LET. While the mutation frequencies were enhanced along with the LET. The mutation frequency increased consistently with LET. This result indicates the high LET ion beam is more mutagenic than the low LET ion beam. Moreover, the remarkable feature of yeast mutations induced by carbon ions was that the mutation sites were localized near the linker regions of nucleosomes, whereas mutations induced by gamma-ray irradiation were located uniformly throughout the gene. In particular, in the low LET carbon ion beam (LET:13 keV/μm), the mutational spectrum exhibited a predominance towards small deletions or insertion. In addition, the DNA expression level analysis revealed a characteristic of the DNA damage by the ion beam irradiation. (author)