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Type: Journal article
Title: Assessment of flatness and symmetry of megavoltage x-ray beam with an electronic portal imaging device (EPID)
Author: Liu, G.
Van Doorn, T.
Bezak, E.
Citation: Australasian Physical and Engineering Sciences in Medicine, 2002; 25(2):58-66
Publisher: Australasian College of Physical Scientists and Engineers in Medicine
Issue Date: 2002
ISSN: 0158-9938
Statement of
G. Liu, T. van Doorn and E. Bezak
Abstract: The input/output characteristics of the Wellhofer BIS 710 electronic portal imaging device (EPID) have been investigated to establish its efficacy for periodic quality assurance (QA) applications. Calibration curves have been determined for the energy fluence incident on the detector versus the pixel values. The effect of the charge coupled device (CCD) camera sampling time and beam parameters (such as beam field size, dose rate, photon energy) on the calibration have been investigated for a region of interest (ROI) around the central beam axis. The results demonstrate that the pixel output is a linear function of the incident exposure, as expected for a video-based electronic portal imaging system. The field size effects of the BIS 710 are similar to that of an ion chamber for smaller field sizes up to 10 x 10 cm2. However, for larger field sizes the pixel value increases more rapidly. Furthermore, the system is slightly sensitive to dose rate and is also energy dependent. The BIS 710 has been used in the current study to develop a QA procedure for measurements of flatness and symmetry of a linac x-ray beam. As a two-dimensional image of the radiation field is obtained from a single exposure of the BIS 710, a technique has been developed to calculate flatness and symmetry from a defined radiation area. The flatness and symmetry values obtained are different from those calculated conventionally from major axes only (inplane, crossplane). This demonstrates that the technique can pick up the "cold" and "hot" spots in the analysed area, providing thus more information about the radiation beam. When calibrated against the water tank measurements, the BIS 710 can be used as a secondary device to monitor the x-ray beam flatness and symmetry.
Keywords: Humans
Radiotherapy Planning, Computer-Assisted
Models, Statistical
Sensitivity and Specificity
Reproducibility of Results
Equipment Failure Analysis
Scattering, Radiation
Quality Control
Image Processing, Computer-Assisted
Description: Copyright © 2002 ACPSEM. All rights reserved. The document attached has been archived with permission from the publisher.
DOI: 10.1007/BF03178467
Appears in Collections:Aurora harvest 7
Physics publications

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