Biological Effect of Carbon Beams on Cultured Human Cells

: This study was performed to determine the biological effect of carbon beams on 13 human tumor cells, in comparison with 200 KVp X-rays. Carbon beams were generated by the Riken Ring Cyclotron. The RBE (relative biological effectiveness) values were distributed from 1.46 to 2.20 for LET of 20 ke V/micron, and 2.29-3.54 for 80 keV/micron. The RBEs were increased in all cell lines as the LET of carbon beams was increased from 20 to 80 keV/micron. There was no significant difference in radiosensitivity between cells from adenocarcinoma and those from squamous cell carcinoma. The relationship between the radiosensitivity of cells to X-rays and RBE was analyzed, but no significant correlation was suggested. Several survival curves of 20-40 keV/micron carbon beam irradiation showed the initial shoulders and the recovery ratios between two split doses were determined. Recovery was observed for LET of 20 keV/micron but not for that of 40 keV/micron. Furthermore, recovery ratios were 1.0-1.8, smaller than those for X-rays (1.5-2.4).