Development and validation of a TOPAS Monte Carlo model for a 0.5 T bi-planar Linac-MR

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Modeling

Background:

• Magnetic Resonance (MR)-guided radiotherapy offers real-time imaging for adaptive treatment but poses challenges due to magnetic field effects on dose deposition.
• Accurate dose calculation in MR-guided radiotherapy necessitates detailed Monte Carlo (MC) modeling to account for magnetic field influences.

Purpose of the Study:

• To develop and validate a comprehensive MC model of a 0.5 T bi-planar Linac-MR system.
• To integrate and validate a custom-designed multileaf collimator (MLC) module within the TOPAS MC simulation platform.

Main Methods:

• Developed a detailed MC model in TOPAS for the 0.5 T bi-planar Linac-MR (Aurora-RT) with a 6 MV FFF beam.
• Incorporated a 3D magnetic field vector map and modeled linac head components, including MLCs from STL files.
• Validated the model by comparing simulated dose distributions (PDDs, profiles, output factors, MLC transmission, surface dose) with water tank measurements and film dosimetry.

Main Results:

• Optimized electron source parameters (5.5 MeV energy, 1.3 mm radial distribution) achieved excellent agreement between MC simulations and measurements.
• Simulated dose distributions (PDDs, profiles) met stringent gamma criteria (1%/1 mm and 2%/2 mm) compared to measurements across various field sizes.
• MLC transmission and positioning accuracy were validated, with simulated surface doses closely matching experimental data.

Conclusions:

• The developed TOPAS MC model accurately simulates dose deposition in the 0.5 T Linac-MR, providing a reliable tool for dose verification.
• The integrated MLC module and its positioning mechanism are validated for open aperture applications, enhancing treatment planning capabilities.
• This validated MC model serves as a robust framework for precise dose simulations in the presence of magnetic fields.