Author: Odunola Grace Babawale, B. Gino Fallone, Alireza Gazor, Andrei D. Ghila, Patricia A. K. Oliver, Michael W. Reynolds, Keith D. Wachowicz, Tania Rosalia Wood, Shima Y. Tari, Eugene Yip, Jihyun Yun 👨🔬
Affiliation: 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 evaluate the dose to a moving target and its surroundings in a magnetic field using 4D-Monte-Carlo (MC) simulation.
Methods: The study utilizes a previously validated MC model of our linac-MR (LMR) - a hybrid system composed of a 6MV x-ray linac and 0.5T biplanar MRI on a rotating gantry. Simulations were performed using Tool for Particle Simulation (TOPAS) – a radiotherapy-specific MC platform built from Geant4. TOPAS’ 4D ability allows movement of model components during simulation and the outputting of time-dependent quantities. Our model comprises a realistic 3D 0.5T magnetic field oriented parallel to the direction of radiation, along with some basic linac components (target, 120-leaf multileaf collimator (MLC), jaws). A cubic lung phantom (side 30cm) embedded with a spherical tissue target (3cm diameter), was placed along the beam’s CAX (110cm SSD). The target is located at isocenter (120cm SAD).
Simulations were run for three scenarios. Scenario_1 was run with the target held static at CAX. For scenario_2, the target moved in the 2D-BEV plane, (modified-cosine pattern in the superior-inferior direction and sine pattern in lateral direction, imitating realistic lung tumour motion. Static MLCs were opened to include a beam margin, encompassing the extent of target’s motion). Scenario_3 has same target motion as scenario_2 but without beam margin (MLCs continuously move with target, shaping beam to it). Dose recorded with a scorer (10x10x4cm3) moving in unison with the target.
Results: Dose profiles for scenario_1 and scenario_3 show agreement within ±1.3mm at 50% dose and penumbra regions.
Conclusion: This is the first study simulating the real-time MLC tracked irradiation of a mobile target within our LMR, using 4D-MC simulation. It is a baseline for designing a physical experiment utilizing real-time imaging capability of the LMR to track moving target, shape the beam to it, and deliver conformal dose.