Monitor unit verification for Varian TrueBeam VMAT plans using Monte Carlo calculations and phase space data

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Accurate verification of treatment planning system (TPS) calculations is crucial for patient safety in radiation therapy.
• Varian TrueBeam linear accelerators deliver Volumetric Modulated Arc Therapy (VMAT) plans, requiring robust monitor unit (MU) verification methods.
• Existing commercial software may have limitations in accuracy for complex VMAT plans.

Purpose of the Study:

• To evaluate the efficacy of open-source Monte Carlo (MC) software for TPS monitor unit (MU) verification of VMAT plans delivered by a Varian TrueBeam linear accelerator.
• To compare the accuracy and efficiency of MC calculations against a commercial software product (MUCheck) and TPS (Eclipse) using AAPM Task Group 219 guidelines.
• To establish a reliable MC framework for clinical implementation in VMAT plan verification.

Main Methods:

• Modeling of the Varian TrueBeam linear accelerator in EGSnrc using Varian-provided phase-space files.
• Simulation of thirteen VMAT TrueBeam treatment plans across diverse anatomical regions (37 arcs total) using a computing cluster.
• Comparison of point dose differences at five reference points per arc between Eclipse, MC, and MUCheck, with MC simulations using 5 × 10⁷ particle histories per arc.

Main Results:

• MC simulations with 5 × 10⁷ histories demonstrated good agreement with Eclipse, achieving an average absolute dose difference of 3.0% with only 22% of points exceeding the 5% action limit.
• The commercial software MUCheck showed a higher average absolute difference of 8.4%, with 60% of points exceeding the 5% action limit, particularly struggling with lung plans (approx. 16% difference).
• MC calculations provided reasonable average plan calculation times (9-18 minutes per full plan) and proved significantly better than MUCheck for MU verification.

Conclusions:

• An EGSnrc-based MC framework is effective for MU verification of VMAT plans on the Varian TrueBeam.
• The MC phase-space approach is adaptable to other treatment devices with appropriate phase-space files.
• The MC method, using 5 × 10⁷ histories, consistently met clinical action limits and offers a superior alternative to commercial MU verification software, with potential for widespread clinical adoption.