[Carbon beam irradiation of monolayer cells]

This study was performed to determine the biological effect of carbon beams on mammalian cells (HeLa, RMUG), in comparison with 200 KVp X-rays. Carbon beams were generated by the Riken Ring Cyclotron. An increase in the relative biological effect (RBE) was observed in both cell lines as the LET of the carbon beam increased between 20 and 80 keV/microns. The RBE depended on the size of the irradiation dose at the higher LET region, and increasing the dose from 0.8 Gy to 3.2 Gy decreased the RBE from 2.9 to 2.7. The survival curves of cells irradiated with 20 keV/microns carbon beams showed an initial shoulder. When RMUG was irradiated at 4 Gy with 200 KVp X-rays, the recovery rate between two split doses (6 hour interval) was 1.7. RMUG irradiated with 20 keV/microns carbon beams (4 Gy) showed a recovery rate of 1.4. Recovery between split dose irradiation was observed with both X-ray and carbon beam irradiation; however, the recovery rate was lower with carbon beams. Cells irradiated with LET higher than 40 keV/microns did not show recovery between split doses.