BackgroundThe study objective was to validate the relative biological effectiveness (RBE) calculated by the modified microdosimetric kinetic model in RayStation (Ray-MKM) for active-energy scanning carbon-ion radiotherapy.MethodsThe Ray-MKM was benchmarked using a spread-out Bragg-peak (SOBP) plan, which was suggested in literature from the National Institute of Radiobiological Science (NIRS) in Japan. The residual RBE differences from the MKM at NIRS (NIRS-MKM) were derived using several SOBP plans with different ranges, SOBP widths, and prescriptions. To investigate the origins of the differences, we compared the saturation-corrected dose-mean specific energy\documentclass[12pt]{minimal}
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				\begin{document}$$Z_{1D}^{*}$$\end{document}Z1D∗of the aforementioned SOBPs. Furthermore, we converted the RBE-weighted doses with the Ray-MKM to those with local effect model I (LEM doses). The purpose was to investigate whether the Ray-MKM could reproduce the RBE-weighted conversion study.ResultsThe benchmark determined the value of the clinical dose scaling factor,\documentclass[12pt]{minimal}
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				\begin{document}$$F_{clin}$$\end{document}Fclin, as 2.40. The target mean RBE deviations between the Ray-MKM and NIRS-MKM were median: 0.6 (minimum: 0.0 to maximum: 1.69) %. The\documentclass[12pt]{minimal}
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				\begin{document}$$Z_{1D}^{*}$$\end{document}Z1D∗difference in-depth led to the RBE difference in-depth and was remarkable at the distal end. The converted LEM doses from the Ray-MKM doses were comparable (the deviation being − 1.8–0.7%) to existing literature.ConclusionThis study validated the Ray-MKM based on our active-energy scanning carbon-ion beam via phantom studies. The Ray-MKM could generate similar RBEs as the NIRS-MKM after benchmarking. Analysis based on\documentclass[12pt]{minimal}
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				\begin{document}$$Z_{1D}^{*}$$\end{document}Z1D∗indicated that the different beam qualities and fragment spectra caused the RBE differences. Since the absolute dose differences at the distal end were small, we neglected them. Furthermore, each centre may determine its centre-specific\documentclass[12pt]{minimal}
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				\begin{document}$$F_{clin}$$\end{document}Fclinbased on this approach.