This study aims to investigate a star shot analysis using a threedimensional (3D) gel dosimeter for the imaging and radiation isocenter verification of a magnetic resonance linear accelerator (MR-Linac). Methods: A mixture of methacrylic acid, gelatin, and tetrakis (hydroxymethyl) phosphonium chloride, called MAGAT gel, was fabricated. One MAGAT gel for each Linac and MR-Linac was irradiated under six gantry angles. A 6 MV photon beam of Linac and a 6 MV flattening filter free beam of MR-Linac were delivered to two MAGAT gels and EBT3 films. MR images were acquired by MR-Linac with a clinical sequence (i.e., TrueFISP). The 3D star shot analysis for seven consecutive slices of the MR images with TrueFISP was performed. The 2D star shot analysis for the central plane of the gel was compared to the results from the EBT3 films. The radius of isocircle (IC r ) and the distance between the center of the circle and the center marked on the image (IC d ) were evaluated. Results: For MR-Linac with MAGAT gel measurements, IC d at the central plane was 0.46 mm for TrueFISP. Compared to EBT3 film measurements, the differences in IC d and IC r for both Linac and MR-Linac were within 0.11 and 0.13 mm, respectively. For the 3D analysis, seven consecutive slices of TrueFISP images were analyzed and the maximum radii of isocircles (IC r_max ) were 0.18 mm for Linac and 0.73 mm for MR-Linac. The tilting angles of radiation axis were 0.31 • for Linac and 0.10 • for MR-Linac. Conclusion:The accuracy of 3D star shot analysis using MAGAT gel was comparable to that of EBT3 film, having a capability for integrated analysis for imaging isocenter and radiation isocenter.3D star shot analysis using MAGAT gel can provide 3D information of radiation isocenter, suggesting a quantitative extent of gantry-tilting.
BackgroundTreatment planning is essential for in silico particle therapy studies. matRad is an open‐source research treatment planning system (TPS) based on the local effect model, which is a type of relative biological effectiveness (RBE) model.PurposeThis study aims to implement a microdosimetric kinetic model (MKM) in matRad and develop an automation algorithm for Monte Carlo (MC) dose recalculation using the TOPAS code. In addition, we provide the developed MKM extension as open‐source tool for users.MethodsCarbon beam data were generated using TOPAS MC pencil beam irradiation. We parameterized the TOPAS MC beam data with a double‐Gaussian fit and modeled the integral depth doses and lateral spot profiles in the range of 100–430 MeV/u. To implement the MKM, the specific energy data table for Z = 1–6 and integrated depth‐specific energy data were acquired based on the Kiefer–Chatterjee track structure and TOPAS MC simulation, respectively. Generic data were integrated into matRad, and treatment planning was performed based on these data. The optimized plan parameters were automatically converted into MC simulation input. Finally, the matRad TPS and TOPAS MC simulations were compared using the RBE‐weighted dose calculation results. A comparison was made for three geometries: homogeneous water phantom, inhomogeneous phantom, and patient.ResultsThe RBE‐weighted dose (DRBE) distribution agreed with TOPAS MC within 1.8% for all target sizes for the homogeneous phantom. For the inhomogeneous phantom, the relative difference in the range of 80% of the prescription dose in the distal fall‐off region (R80) between the matRad TPS and TOPAS MC was 0.6% (1.1 mm). DRBE between the TPS and the MC was within 4.0%. In the patient case, the difference in the dose–volume histogram parameters for the target volume between the TPS and the MC was less than 2.7%. The relative difference in R80 was 0.7% (1.2 mm).ConclusionsThe MKM was successfully implemented in matRad TPS, and the RBE‐weighted dose was comparable to that of TOPAS MC. The MKM‐implemented matRad was released as an open‐source tool. Further investigations with MC simulations can be conducted using this tool, providing a good option for carbon ion research.
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