Producing a Beam Model of the Varian ProBeam Proton Therapy System using TOPAS Monte Carlo Toolkit

• Type of Material: Article
• Authors: Mahbubur Rahman, Dartmouth College; Petr Bruza, Dartmouth College; Yuting Lin, Emory University; David J Gladstone, Dartmouth College; Brian W Pogue, Dartmouth College; Rongxiao Zhang, Dartmouth College
• Language: English
• Date: 2020-10-08
• Publisher: Wiley
• Publication Version: Author Accepted Manuscript (After Peer Review)
• Copyright Statement: © 2025 American Association of Physicists in Medicine
• Final Published Version (URL): https://aapm.onlinelibrary.wiley.com/doi/10.1002/mp.14532
• Title of Journal or Parent Work: Medical Physics
• Volume: 47
• Issue: 12
• Start Page: 6500
• End Page: 6508
• Grant/Funding Information: This work has been supported by NIH grants R01 EB023909, 5P30 CA023108–41 and R44 CA199681. Facilities (irradiation sources) to conduct research were provided by Emory Proton Therapy Center, Atlanta, GA.
• Keywords: beam model; TOPAS; Monte Carlo; proton pencil beam; linear energy transfer
• Research Categories: Health Sciences, Radiology; Biology, General; Biology, Radiation

Abstract

Purpose: A Geant4-based TOPAS Monte Carlo toolkit was utilized to model a Varian ProBeam proton therapy system, with the aim of providing an independent computational platform for validating advanced dosimetric methods. Materials and Methods: The model was tested for accuracy of dose and LET prediction relative to the commissioning data, which included integral depth dose (IDD’s) in water and spot profiles in air measured at varying depths (for energies of 70 to 240MeV in increments of 10MeV, and 242MeV), and absolute dose calibration. Emittance was defined based on depth dependent spot profiles and Courant-Snyder’s particle transport theory, which provided spot size and angular divergence along the inline and crossline plane. Energy spectra were defined as Gaussian distributions that best matched the range and maximum dose of the IDD’s. The validity of the model was assessed based on measurements of range, dose to peak difference, mean point to point difference, spot sizes at different depths, spread out Bragg Peak (SOBP) IDD and was compared to the current treatment planning software (TPS). Results: Simulated and commissioned spot sizes agreed within 2.5%. The single spot IDD range, maximum dose, and mean point to point difference of each commissioned energy agreed with the simulated profiles generally within 0.07 mm, 0.4%, and 0.6% respectively. A simulated SOBP plan agreed with the measured dose within 2% for the plateau region. The protons/MU and absolute dose agreed with the current TPS to within 1.6% and exhibited the greatest discrepancy at higher energies. Conclusions: The TOPAS model agreed well with the commissioning data and included inline and crossline asymmetry of the beam profiles. The discrepancy between the measured and TOPAS simulated SOBP plan may be due beam modeling simplifications of the current TPS and the nuclear halo effect. The model can compute LET, and motivates future studies in understanding equivalent dose prediction in treatment planning, and investigating scintillation quenching.