Organs at risk dose constraints in carbon ion radiotherapy at MedAustron: Translations between LEM and MKM RBE models and preliminary clinical results.

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Carbon ion radiotherapy (CIRT) treatment planning is based on relative biological effectiveness (RBE) weighted dose calculations. A large amount of clinical evidence for CIRT was collected in Japan with RBE estimated by the modified microdosimetric kinetic model (MKM) while all European centres apply the first version of the local effect model (LEM). Japanese schedules have been used in Europe with adapted prescription dose and organs at risk (OAR) dose constraints. Recently, less conservative adapted LEM constraints have been implemented in clinical practice. The aim of this study was to analyse the new set of LEM dose constraints for brain parenchyma, brainstem and optic system considering both RBE models and evaluating early clinical data. 31 patients receiving CIRT at MedAustron were analysed using the RayStation v9A planning system by recalculating clinical LEM-based plans in MKM. Dose statistics (D1cm3, D5cm3, D0.1cm3, D0.7cm3, D10%, D20%) were extracted for relevant critical OARs. Curve fitting for those values was performed, resulting in linear quadratic translation models. Clinical and radiological toxicity was evaluated. Based on derived fits, currently applied LEM constraints matched recommended MKM constraints with deviations between -8% and +3.9%. For particular cases, data did not follow the expected LEM vs MKM trends resulting in outliers. Radiological (asymptomatic) toxicity was detected in two outlier cases. Respecting LEM constraints does not automatically ensure that MKM constraints are met. Constraints for both RBE models need to be fulfilled for future CIRT patients at MedAustron. Careful selection of planning strategies is essential.