PURPOSE: Modeling relative biological effectiveness (RBE) is central to carbon ion radiotherapy treatment planning. The modified Microdosimetric Kinetic Model (mMKM) is a clinically established RBE framework that has guided treatment protocols at many existing carbon centers, while the Mayo Clinic Florida Microdosimetric Kinetic Model (MCF MKM) is a recently developed alternative. This work aims to implement the MCF MKM in the open-source treatment planning system matRad and to quantitatively compare its RBE-weighted dose predictions with those of the clinically established mMKM using identical physical dose distributions across multiple disease sites. These findings will help assess their dosimetric equivalence and inform protocol development for carbon ion radiotherapy at MCF. METHODS: Monte Carlo simulations of the MCF carbon beamline were performed to generate physical (IDD, LET, lateral spread) and biological base data for integration of each RBE model into matRad. Treatment plans were generated for six patients, each corresponding to a different disease site, using clinical beam configurations with carbon-reference dose prescriptions, and plans were optimized using the MCF MKM. To isolate differences attributable solely to the RBE model, the resulting physical dose distributions were held fixed and RBE-weighted doses were recalculated using the mMKM. Dose volume histogram (DVH) metrics and spatial dose-difference maps were used to compare target coverage and organ-at-risk doses between the two models. RESULTS: Across patient cases, RBE-weighted dose distributions from MCF MKM and mMKM showed strong agreement. Differences in target coverage were small, with CTV D95% differing by less than 1.6% across all disease sites and maximum target dose differences not exceeding 0.88%. Organ-at-risk dose deviations were limited, with differences of 3.0% or less across evaluated DVH metrics. Spatial dose-difference maps showed that the largest discrepancies occurred in regions of steep dose gradients near target to organ-at-risk interfaces, while overall dose conformity and plan quality remained comparable between the two models. CONCLUSIONS: This study served as the first systematic model comparison of the MCF MKM and mMKM within a treatment planning environment. These findings suggest that the MCF MKM and mMKM produce dosimetrically consistent RBE-weighted dose predictions under realistic planning conditions using carbon-reference parameters. Accordingly, the fractionation schemes developed from years of clinical experience with mMKM implementations may serve as a practical foundation for protocol development at MCF.
Hartzell et al. (Fri,) studied this question.