Characterizing a Geant4 Monte Carlo model of a multileaf collimator for a TrueBeam™ linear accelerator

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Accurate modeling of linear accelerators (linacs) is crucial for radiation therapy.
• The TrueBeam™ linac with its multileaf collimator (MLC) is widely used in clinical practice.

Purpose of the Study:

• To develop and validate a Geant4 Monte Carlo (MC) model of a TrueBeam™ linac with a 120-leaf Millennium™ MLC.
• To assess the dosimetric characteristics of the modeled MLC.

Main Methods:

• A Geant4 application was created to simulate the TrueBeam™ linac geometry and materials.
• The Varian 120-leaf Millennium™ MLC was modeled, incorporating features like tongue-groove design.
• Simulated dose profiles and depth-doses were compared against experimental data for validation.

Main Results:

• Depth dose curves showed excellent agreement (within 1-2%) with experimental data across various field sizes.
• Cross-plane dose profiles demonstrated good agreement (<2% up to 10 cm depth, <4% beyond).
• MLC dosimetric characterization, including leakage and penumbra, aligned well with film measurements.

Conclusions:

• A validated Geant4 MC model of the TrueBeam™ linac with a 120-leaf MLC was successfully developed.
• The model's accuracy was confirmed through comparison with experimental measurements.
• This validated model can be used for independent dose verification and quality assurance (QA) in intensity-modulated radiation therapy (IMRT).