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Investigation of Monte Carlo User Codes for the Simulation of the Xoft Axxent Electronic Brachytherapy Source: A Comparative Study

A Walter*, L DeWerd, Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, WI


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To investigate the ability of Monte Carlo user codes to simulate the Xoft Axxent electronic brachytherapy source through evaluation of basic electron and photon transport and simulation of air kerma.

Methods: Electron backscatter, transmission, and absorption in a 1 µm thick tungsten foil was simulated in vacuum for monoenergetic electron beams of 40 keV, 45 keV, and 50 keV to represent the energy range of the Axxent source. The backscattered and transmitted energy fractions were calculated on the front and back surfaces of the foil, respectively and were used to determined absorbed energy fraction. Next, a monoenergetic 50 keV electron beam was incident upon a 1 µm tungsten foil and the bremsstrahlung spectrum was tallied in vacuum at a distance of 1 mm away from the back surface of the foil. The S7600 model of the Xoft Axxent source was then modeled and air kerma at a distance of 50 cm from the source was calculated. Results from TOPAS were compared to simulations performed in EGSnrc.

Results: The TOPAS simulated electron transport results agreed within 0.631% for the three energy fractions with respect to EGSnrc. Similar spectra were observed with the two codes, with the largest discrepancies due to lower intensity peaks generated in TOPAS when compared to EGSnrc. Similar levels of filtration and general spectra shape were observed and deemed TOPAS suitable for Axxent source simulations. The EGSnrc air kerma results results and measurements made with the Attix Free Air Chamber agreed within 2%. Larger discrepancies were observed between TOPAS and EGSnrc for these simulations.

Conclusion: results indicate TOPAS and EGSnrc perform similarly for basic electron and photon transport at low energies but show larger disagreement for simulation of full source models. Further investigation of variance reduction techniques will need to be performed for air kerma simulations.

Funding Support, Disclosures, and Conflict of Interest: This work is funded in part by Xoft, a subsidiary of iCad, Inc.


Monte Carlo, Brachytherapy, X-ray Production


TH- Brachytherapy: Computational dosimetry: Monte Carlo

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