Cell Survival in Rat Rhabdomyosarcoma Tumors Irradiated in Vivo with Extended-Peak Silicon Ions

Abstract

Cell survival curves were measured by the in vivo to in vitro assay procedure for rat rhabdomyosarcoma tumors irradiated in the distal 4-cm extended-peak region of a 670-MeV/u silicon-ion beam. RBE values determined relative to 225-kVp X rays for the killing of oxygenated tumor cells were 2.1 and 2.2 at the 1 and 10% survival levels, respectively. These RBE values are greater than those obtained previously for extended-peak carbon ions (1.7-1.8), but less than those obtained for extended-peak neon ions (2.5-2.7). This observation indicates that the RBE obtained in the distal-peak region of the 670 MeV/u silicon-ion beam lies beyond the maximum in the RBE versus ${\rm LET}_{\infty}$ curve for the killing of oxygenated tumor cells. The RBE value for the killing of hypoxic tumor cells in this beam was 3.6 at both the 1 and 10% survival levels. This RBE value exceeds the previously measured values for both extended-peak carbon ions (2.0) and extended-peak neon ions (3.1-3.3). OER values m...