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

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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 [relative biological effectiveness] values determined relative to 225-kVp X-rays for the killing of oxygenated tumors 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 vs. LET.infin. [linear energy transfer] 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 [oxygen enhancement ratio] values measured at the 1 and 10% survival levels for silicon ions were 1.2 and 1.3, respectively, compared to 1.7 for neon ions and 1.9 for carbon ions. The significant reduction in oxygen effect for accelerated silicon ions relative to the lighter elements suggests that this beam may be efficacious for the treatment of tumors whose probability of cure is limited by a large fraction of hypoxic cells.