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Validation of the TOPAS Monte Carlo Toolkit for Brachytherapy Simulations

F Berumen*1,2, Y Ma1,2, JR Mendez3, J Perl4, L Beaulieu1,2, (1) Universite Laval, Quebec, QC, CA, (2) CHU de Quebec - Universite Laval et CRCHU, Quebec, QC, CA, (3) University of California San Francisco, San Francisco, CA, (4) Stanford Linear Accelerator Center, Menlo Park, CA


(Monday, 7/13/2020) 3:30 PM - 5:30 PM [Eastern Time (GMT-4)]

Room: Track 3

Purpose: A validated Monte Carlo (MC) toolkit is essential for many aspects of brachytherapy dose calculations. Traditionally, MC codes are complex and necessitate coding knowledge that limit their application as a validation tool for many end users. The goal of this work is to validate the user-friendly MC toolkit TOPAS for brachytherapy applications.

Methods: The MBDCA-WG Ir-192 generic source was simulated. The energy spectrum was extracted and compared with the reference spectra (Ballester et. al. 2015). The air-kerma strength and the dose-rate constant were compared with the reference data. All four international brachytherapy working group test cases were simulated: case 1 (TG-43), case 2 (source centered in water), case 3 (source displaced), and case 4 (source centered in applicator). The dose was scored in a centered 20.1 cm side cube using 1 mm³ voxels. Dose maps were compared with the IROC Houston reference data (MCNP6) through histograms of the local (with respect each voxel of the reference volume) and global (with respect a unique relevant reference point) difference volumes.

Results: The energy spectrum exiting the source capsule was compatible with the reference data. The air-kerma strength was (9.772±0.001)x10?8 U Bq?¹ (within 0.3% of the reference value). The dose-rate constant was 1.1107±0.004 cGy h?¹ U?¹ (within 0.02% of the reference value). For test cases 1, 2, and 4, at least the 99.5% of voxels had a local difference within ±1%, test case 3 had 96.9%. For all cases, at least 99.9% of all voxels had a global difference within ±0.1%. TOPAS performed well extracting dosimetry source parameters and scoring in a voxelized geometry such as the test cases.

Conclusion: Overall, brachytherapy simulations made with TOPAS showed excellent agreement with the reference data. TOPAS is a state-of-the-art MC code for brachytherapy simulations with a much-simplified simulation and geometry description approach.


Brachytherapy, Monte Carlo


TH- Brachytherapy: Computational dosimetry: Monte Carlo

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