Cellular response of a rat brain tumor to a therapeutic carbon ion beam

Clonogenic cell survival of in situ 9L brain tumors was measured in air-breathing rats and rats killed with nitrogen. Animals were irradiated with BEVALAC accelerated carbon ions in the plateau and spread-out Bragg peak regions of the specific ionization curve. The relative biological effectiveness (RBE) for carbon ions in the plateau was 1.2 +/- 0.1 for oxic cells and 1.3 +/- 0.1 for anoxic cells; in the Bragg peak the RBE was 1.6 +/- 0.1 for oxic cells and 2.0 +/- 0.1 for anoxic cells. The oxygen enhancement ratio (OER) in the plateau was 2.0 +/- 0.1, not significantly different from the 2.1 +/- 0.2 found for x rays. The OER in the Bragg peak was reduced to 1.7 +/- 0.1. For cells having the same radiobiological sensitivity as in situ 9L cells, the neon ion beam may be therapeutically superior; however, this does not preclude selection of the carbon ion beam in many clinical situations.