Abstract
The increased biological effectiveness associated with high LET radiation when compared to low LET X-rays is thought to be one of the important causes for the successful clinical outcome of heavy-ion radiotherapy. DNA double strand breaks (DSBs) are considered to be the most crucial lesion induced by ionizing radiation, and the repair of DSBs is much slower in cells irradiated with high LET heavy ions than that with X-rays. This was reflected at the level of chromosomes; for example, the remaining number of chromosome breaks after a certain post-irradiation period as measured by premature chromosome condensation (PCC) is significantly higher in cells irradiated with high LET radiation than cells with X-rays. The phosphorylation status of DNA-PKcs, a non-homologous end joining (NHEJ) protein, was markedly different in cells irradiated with high LET radiation when compared with that with X-rays. HiCEP method, a sensitive gene expression profiling technique developed at NIRS, indicated there are a few genes characteristic specifically for heavy ion irradiation. Using the synchronized CHO and its repair defective mutant cells we investigated the cell survival levels throughout the cell cycle with carbon ions, and the result was compared with that with X-rays. Our data indicated that much less variation in the cell survival level was observed when carbon ions were used to irradiate cells. Although this cell cycle effect has been known for many years, our new experiments seem to give novice insight in the field of heavy-ion radiobiology.
