Dose–response curves for MRI-detected radiation-induced temporal lobe reactions in patients after proton and carbon ion therapy: Does the same RBE-weighted dose lead to the same biological effect?

Abstract

Purpose: To derive the dose–response curve for temporal lobe reactions (TLRs) after proton therapy and to compare the resulting relative biological effectiveness (RBE)-weighted tolerance doses based on an RBE of 1.1 with published values for carbon ions, which were calculated by the two versions of the local effect model (LEM I or IV). Methods and materials: 62 patients treated with protons for skull base tumors were analyzed for TLRs using magnetic resonance imaging. Within the mean follow-up time of 38 months, TLRs were observed in six patients. Dose–response curves based on the RBE-weighted maximum dose, excluding the 1 cm3-volume with the highest dose, were derived and compared to previously published dose–response curves for carbon ions, which were obtained using LEM I or IV, respectively. Results: The dose–response curves for protons and LEM I were found to be almost identical while the curve of LEM IV was shifted toward higher doses. The resulting tolerance doses at the 5% effect level were 68.2+2.7-5.6, 68.6+3.0-3.9 and 78.3+3.8-5.0 Gy (RBE), respectively. Conclusions: The RBE-weighted dose prescription for protons leads to the same RBE-weighted dose–response curve for TLR as the one for LEM I-based carbon ions, while LEM IV predicts clinically significant higher tolerance doses.