Dose prescription in carbon ion radiotherapy: How to compare two different RBE-weighted dose calculation systems

Background and purpose: In carbon ion radiotherapy (CIRT), the use of different relative biological effectiveness (RBE) models in the RBE-weighted dose $(D_{RBE})$ calculation can lead to deviations in the physical dose $(D_{phy})$ delivered to the patient. Our aim is to reduce target $D_{phy}$ deviations by converting prescription dose values. Material and methods: Planning data of patients treated at the National Institute of Radiological Sciences (NIRS) were collected, with prescribed doses per fraction ranging from 3.6 Gy (RBE) to 4.6 Gy (RBE), according to the Japanese semi-empirical model. The $D_{phy}$ was Monte Carlo (MC) re-calculated simulating the NIRS beamline. The local effect model (LEM)_I was then applied to estimate $D_{RBE}$. Target median $D_{RBE}$ ratios between MC + LEM_I and NIRS plans determined correction factors for the conversion of prescription doses. Plans were re-optimized in a LEM_I-based commercial system, prescribing the NIRS uncorrected and corrected $D_{RBE}$. Results: The MC + LEM_I target median $D_{RBE}$ was respectively 15% and 5% higher than the NIRS reference, for the lowest and highest dose levels. Uncorrected $D_{RBE}$ prescription resulted in significantly lower target $D_{phy}$ in re-optimized plans, with respect to NIRS plans. Conclusions: Prescription dose conversion factors could minimize target physical dose variations due to the use of different radiobiological models in the calculation of CIRT RBE-weighted dose.