Author: B. Gino Fallone, Alireza Gazor, Andrei D. Ghila, Gawon Han, Patricia A. K. Oliver, Michael W. Reynolds, Keith D. Wachowicz, Tania Rosalia Wood, Shima Y. Tari, Eugene Yip 👨🔬
Affiliation: Medical Physics Division, Department of Oncology, University of Alberta, Nova Scotia Health, Dept. of Medical Physics and Dalhousie University, Dept. of Physics and Atmospheric Science, Dept. of Radiation Oncology, Dept. of Medical Physics, Cross Cancer Institute and Dept. of Oncology, University of Alberta; MagnetTx Oncology Solutions, www.magnetTX.com, Department of Medical Physics, Arthur J. E. Child Comprehensive Cancer Centre, Dept. of Medical Physics, Cross Cancer Institute and Dept. of Oncology, University of Alberta, Department of Medical Physics, BC Cancer, Medical Physics Division, Department of Oncology, University of Alberta and Department of Medical Physics, Cross Cancer Institute 🌍
Purpose: To develop and validate a high-fidelity Monte-Carlo (MC) model of a 0.5T bi-planar Linac-MR in TOPAS, focusing on accurate Multileaf Collimator (MLC) modelling and positioning for open apertures.
Methods: The MC model replicates the Linac-MR and accounts for the system's complex dynamics. The MLC leaves are imported into the simulation via stereolithography mesh, and their positioning is controlled by an empirical equation developed in this work. The MLC positioning is validated with water tank (PTW-BEAMSCAN MR) PDDs/profiles (CAX), and profiles (off-axis) in MLC-defined fields. The incident electron energy and spatial spread are optimized to match measured photon-beam PDDs/profiles. The model accuracy is evaluated in a heterogeneous (polystyrene-lung-polystyrene) phantom using a parallel-plate chamber (IBA-PPC05) measurement and MC simulation with main magnetic field on and off.
Results: The best-fit value for electron energy is 5.5MeV with 1.3mm Gaussian spatial spread. The 1%|2mm gamma evaluation for photon-beam profiles at various depths yields a >95% pass for field-sizes from 3x3--25x25cm2. PDDs for the same field-sizes achieve 100% gamma pass-rate for 1%|1mm. Off-axis field (3.92x3.92cm2--9.8x9.8cm2) profiles at 10cm depth pass >95% for 1%|2mm. The MC simulated PDDs in the heterogenous phantom for an 8x8cm2 field yields >98% gamma pass-rate for 1%|1mm. Additionally, surface doses (8x8cm2 field-size) are 73.6%,74.9% (measured/MC, field on) and 62.8%, 61.8% (measured/MC, field off).
Conclusion: The MC model of the Linac-MR demonstrates high accuracy for dose verification in magnetic fields. This MLC positioning method models open apertures and off-axis fields, achieving >95% pass-rate for 1%|2mm gamma-criteria for photon-beam profiles compared to data. Simulated PDDs in a polystyrene-lung-polystyrene phantom match measured data and pass gamma 1%|1mm criterion with >98%. Additionally, MC surface doses are within 1.3%, 1% (field on/off) of the measurement. This validated model provides a reliable framework for dose simulations in a magnetic field, supporting treatment verification and clinical applications.