Relative Biological Effectiveness of Carbon Ions for Local Tumor Control of a Radioresistant Prostate Carcinoma in the Rat

Purpose To study the relative biological effectiveness (RBE) of carbon ion beams relative to X-rays for local tumor control in a syngeneic rat prostate tumor (Dunning subline R3327-AT1). Methods and Materials A total of 198 animals with tumors in the distal thigh were treated with increasing single and split doses of either12 C ions or photons using a 20-mm spread-out Bragg peak. Endpoints of the study were local control (no tumor recurrence within 300 days) and volumetric changes after irradiation. The resulting values for D50 (dose at 50% tumor control probability) were used to determine RBE values. Results The D50 values for single doses were 32.9 ± 0.9 Gy for12 C ions and 75.7 ± 1.6 Gy for photons. The respective values for split doses were 38.0 ± 2.3 Gy and 90.6 ± 2.3 Gy. The corresponding RBE values were 2.30 ± 0.08 for single and 2.38 ± 0.16 for split doses. The most prominent side effects were dry and moist desquamation of the skin, which disappeared within weeks. Conclusion The study confirmed the effectiveness of carbon ion therapy for severely radioresistant tumors. For 1- and 2-fraction photon and12 C ion radiation, we have established individual D50 values for local tumor control as well as related RBE values.