DOI: https://doi.org/10.14710/jpa.v2i2.7771

Electron contamination for 6 MV photon beams from an Elekta linac: Monte Carlo simulation

*Choirul Anam orcid scopus  -  Department of Physics, Faculty of Sciences and Mathematics, Diponegoro University, Indonesia
Djarwani S Soejoko  -  Department of Physics, Faculty of Mathematics and Natural Sciences, University of Indonesia, Indonesia
Freddy Haryanto  -  Department of Physics, Faculty of Mathematics and Natural Sciences, Bandung Institute of Technology, Indonesia
Sitti Yani  -  Department of Physics, Faculty of Mathematics and Natural Sciences, Insitut Pertanian Bogor, Indonesia
Geoff Dougherty  -  Department of Applied Physics and Medical Imaging, California State University Channel Islands, United States
Received: 2 May 2020; Revised: 25 May 2020; Accepted: 28 May 2020; Available online: 10 Jun 2020; Published: 10 Jun 2020.

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Abstract
In external beam radiotherapy, the photons from a linear accelerator (linac) machine undergo multiple interactions, not only in the patient but also in the linac head and the air column between the linac head and the patient. Electrons are released from these interactions and contaminate the beams. The current study evaluates electron contamination for 6 MV photon beams from an Elekta linac using Monte Carlo simulation. The linac head was simulated by the BEAMnrc code and the absorbed dose in a phantom was calculated using the DOSXYZnrc code. The parameters of the initial electron beams on the target, such as mean energy and radial intensity distribution, were determined by matching the calculated dose distributions with the measured dose (at 10 x 10 cm2 field size and 90 cm source-skin distance). The central axis depth-dose curves of electron contamination were calculated for various field sizes from 5 x 5 cm2 to 40 x 40 cm2. We investigated the components that generated the electron contamination for a field size of 10 x 10 cm2. The optimal initial electron beam energy was 6.3 MeV with a full-width half maximum (FWHM) of the radial intensity distribution of 1.0 mm. These parameters were found to be in good agreement with the measured data. Electron contamination increased as the field size increased. At a depth of 1.0 mm and field sizes of 5 x 5, 10 x 10, 20 x 20, 30 x 30, and 40 x 40 cm2, the doses from electron contamination were 3.71, 5.19, 14.39, 18.97 and 20.89 %, respectively. Electron contamination decreased with increased depth. At a depth of 15 mm, the electron contamination was about 1 %. It was mainly generated in the air column between the linac head and the phantom (3.65 %), the mirror (0.99 %), and the flattening filter (0.59 %) (for the depth of 1.0 mm and the field size of 10 x 10 cm2).
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Keywords: Monte Carlo simulation; Elekta linear accelerator; Electron contamination
Funding: Diponegoro University

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Section: Articles
Language : EN
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