RBE-weighted dose in carbon ion therapy for ACC patients: Impact of the RBE model translation on treatment outcomes

Purpose/objective: The purpose of this study is to assess the impact of the conversion scheme for relative biological effectiveness (RBE)-weighted dose (D_RBE), implemented at our center, on treatment outcomes of adenoid cystic carcinoma (ACC) patients. Material/methods: Treatment plans of 78 ACC patients, optimized with the Local Effect Model (LEM), were recalculated with the modified Microdosimetric Kinetic Model (mMKM). D_RBE to 95%, 50% and 2% (D_V%) of the clinical target volume (CTV), were selected as relevant parameters to compare LEM and mMKM D_RBE. The pattern of failure of ACC treatments was analyzed in relation to uncertainties involved in the D_RBE translation methodology. Results: mMKM recalculations of LEM plans, optimized to a prescription dose of 68.8 Gy(RBE), showed a D_50% 8% higher, on average, than the expected value (60.8 Gy(RBE)), closer to the most frequently used mMKM prescription D_RBE (64 Gy(RBE)). D_95% and D_2% deviations, with respect to the optimization goals in the two RBE systems, increased of 0.5% and 14.2%, respectively, due to the steeper mMKM RBE variation along the beam path. Local recurrences were mainly (63%) reported in areas where CTV coverage was not satisfactory in the original LEM plan and the mMKM analysis showed that OARs constraints were too conservative. Conclusion: No case of local recurrence could be explained by inadequate mMKM target coverage that was not already present in the LEM plan. New constraints have been defined for optic pathways and brainstem to improve target coverage with no expected increase in tissue complications.