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The Influence of Brass Compensator Thickness and Field Size on Neutron Contamination Spectrum in 18Mv Elekta Sl 75/25 Medical Linear Accelerator With and Without Flattening Filter: A Monte Carlo Study Publisher



Talebi AS1 ; Maleki M2 ; Hejazi P1 ; Jadidi M1 ; Ghorbani R3
Authors
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Authors Affiliations
  1. 1. Department of Medical Physics, Semnan University of Medical Sciences, Semnan, Iran
  2. 2. Department of Medical Physics, Isfahan University of Medical Sciences, Isfahan, Iran
  3. 3. Head of Social Determinants of Health Research Center, Semnan University of Medical Sciences, Semnan, Iran

Source: Journal of Biomedical Physics and Engineering Published:2018


Abstract

Background: One of the most significant Intensity Modulated Radiation Therapy treatment benefits is a high target to normal tissue dose ratio. To improve this advantage, an additional accessory such as a compensator is used to deliver doses. Compensator-based IMRT treatment is usually operated with an energy higher than 10 MV. Photoneutrons, which have high linear energy transfer and radiobiological effectiveness, are produced by colliding high-energy photon beams with linear accelerator structures, then they deliver the unwanted doses to patients and staff. Therefore, the neutron energy spectra should be determined in order to calculate and reduce the photoneutron risk. Objective: We have conducted a comprehensive and precise study on the influence of brass compensator thickness and field size on neutron contamination spectrum in an Elekta SL 75/25 medical linear accelerator with and without the flattening filter by Monte Carlo method. Material and Methods: MCNPX MC Code version 2.6.0 was utilized to simulate the detailed geometry of Elekta SL 75/25 head components based on Linac’s manual. This code includes an important feature to simulate the photo-neutron interactions. Photoneutrons spectrum was calculated after the Linac output benchmarking based on tuning the primary electron beam. Results and Conclusion: Based on the Friedman and Wilcoxon nonpara-metric tests results (P<0.05), photoneutron fluence directly depends on the field size and compensator thickness. Moreover, the unflattened beam provides lower photoneutron fluence than the flattened beam. Photoneutrons fluence is not negligible in compensator-based IMRT treatment. However, in order to optimize treatment plans, this additional and unwanted dose must be accounted for patientss. © 2018, Shiraz University of Medical Sciences. All rights reserved.
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