Background Compelling evidence shows the association between the relative biological effectiveness (RBE) of carbon‐ion radiotherapy (CIRT) and the dose averaged linear energy transfer ( LETd ). However, the ability to calculate the LETd in commercially available treatment planning systems (TPS) is lacking. Purpose This study aims to develop a method of calculating the LETd of CIRT plans that could be robustly carried out in RayStation (V10B, Raysearch, Sweden). Methods The calculation used the fragment spectra in RayStation for the CIRT treatment planning. The dose‐weighted averaging procedure was supported by the microdosimetric kinetic model (MKM). The MKM‐based pencil beam dose engine (PBA, v4.2) for calculating RBE‐weighted doses was reformulated to become a LET ‐weighted calculating engine. A separate module was then configured to inversely calculate the LETd from the absorbed dose of a plan and the associated fragment spectra. In this study, the ion and energy‐specific LET table in the LETd module was further matched with the values decoded from the baseline data of the Syngo TPS (V13C, Siemens, Germany). The LETd distributions of several monoenergetic and modulated beams were calculated and validated against the values derived from the Syngo TPS and the published data. Results The differences in LETds of the monoenergetic beams between the new method and the traditional method were within 3% in the entrance and Bragg‐peak regions. However, a larger difference was observed in the distal region. The results of the modulated beams were in good agreement with the works from the published literature. Conclusions The method presented herein reformulates the MKM dose engine in the RayStation TPS to inversely calculate LETds . The robustness and accuracy were demonstrated.
Background The 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. Methods The 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 $$Z_{1D}^{*}$$ Z 1 D ∗ 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. Results The benchmark determined the value of the clinical dose scaling factor, $$F_{clin}$$ F clin , 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 $$Z_{1D}^{*}$$ Z 1 D ∗ 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. Conclusion This 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 $$Z_{1D}^{*}$$ Z 1 D ∗ 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 $$F_{clin}$$ F clin based on this approach.
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