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. 2023 Oct;24(10):e14063.
doi: 10.1002/acm2.14063. Epub 2023 Jul 19.

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

Ankit Pant  1   2 , Narges Miri  1 , Stephen Bhagroo  3 , Joshua A Mathews  4 , Daryl P Nazareth  1   2
Affiliations

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

Ankit Pant et al. J Appl Clin Med Phys. 2023 Oct.

Abstract

To use the open-source Monte Carlo (MC) software calculations for TPS monitor unit verification of VMAT plans, delivered with the Varian TrueBeam linear accelerator, and compare the results with a commercial software product, following the guidelines set in AAPM Task Group 219. The TrueBeam is modeled in EGSnrc using the Varian-provided phase-space files. Thirteen VMAT TrueBeam treatment plans representing various anatomical regions were evaluated, comprising 37 treatment arcs. VMAT plans simulations were performed on a computing cluster, using 107 -109 particle histories per arc. Point dose differences at five reference points per arc were compared between Eclipse, MC, and the commercial software, MUCheck. MC simulation with 5 × 107 histories per arc offered good agreement with Eclipse and a reasonable average calculation time of 9-18 min per full plan. The average absolute difference was 3.0%, with only 22% of all points exceeding the 5% action limit. In contrast, the MUCheck average absolute difference was 8.4%, with 60% of points exceeding the 5% dose difference. Lung plans were particularly problematic for MUCheck, with an average absolute difference of approximately 16%. Our EGSnrc-based MC framework can be used for the MU verification of VMAT plans calculated for the Varian TrueBeam; furthermore, our phase space approach can be adapted to other treatment devices by using appropriate phase space files. The use of 5 × 107 histories consistently satisfied the 5% action limit across all plan types for the majority of points, performing significantly better than a commercial MU verification system, MUCheck. As faster processors and cloud computing facilities become even more widely available, this approach can be readily implemented in clinical settings.

Keywords: Monte Carlo calculation; RapidArc; VMAT; independent calculation verification; monitor units; phase space; secondary check.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

FIGURE 1
FIGURE 1
An illustration of accelerator components showing the location of phase space source plane along with components.
FIGURE 2
FIGURE 2
A flowchart showing the workflow of MC‐MUV process. Sub‐processes are shown in dashed borders. Software used throughout the steps are shown in italicized.
FIGURE 3
FIGURE 3
Percentage of reference point dose differences greater than 5% for the various cases and methodologies. MUCheck differences tend to be larger, except for head & neck cases.
FIGURE 4
FIGURE 4
Reference point dose differences between TPS and MC histories and MUCheck methodologies across various sites. Boxes show 25th (Q1), 50th (Q2), and 75th (Q3) percentiles; whiskers show maxima and minima within 1.5 interquartile range (Q3‐Q1); + symbols show outliers. Average and standard deviation of reference point dose differences are shown as blue diamond and line respectively.
FIGURE 5
FIGURE 5
Average absolute reference point dose difference across various cases. MUCheck differences are particularly large for lung cases.
FIGURE 6
FIGURE 6
Total average time required for MC calculations, including overhead, for each plan type across various histories.
FIGURE 7
FIGURE 7
Average MC simulation time for each plan type across various histories.
FIGURE 8
FIGURE 8
Overhead time required for MC calculations as size of the dose matrix varies. Head & Neck and Lung cases have the largest sizes.
See this image and copyright information in PMC

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