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

PurposeTo 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 materials62 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. ResultsThe 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. ConclusionsThe 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.