Assessment of flatness and symmetry of megavoltage x-ray beam with an electronic portal imaging device (EPID)
- 1 June 2002
- journal article
- research article
- Published by Springer Science and Business Media LLC in Australasian Physics & Engineering Sciences in Medicine
- Vol. 25 (2), 58-66
- https://doi.org/10.1007/bf03178467
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
This publication has 15 references indexed in Scilit:
- An electronic portal imaging device as a physics toolMedical Dosimetry, 1997
- The dose response relationship of a liquid-filled electronic portal imaging deviceMedical Physics, 1996
- Modeling dose distributions from portal dose images using the convolution/superposition methodMedical Physics, 1996
- High-precision prostate cancer irradiation by clinical application of an offline patient setup verification procedure, using portal imagingInternational Journal of Radiation Oncology*Biology*Physics, 1996
- Evaluation of electronic portal imaging device for missing tissue compensator design and verificationMedical Physics, 1995
- The use of an electronic portal imaging device for exit dosimetry and quality control measurementsInternational Journal of Radiation Oncology*Biology*Physics, 1995
- The use of on-line image verification to estimate the variation in radiation therapy dose deliveryInternational Journal of Radiation Oncology*Biology*Physics, 1993
- Fast evaluation of patient set-up during radiotherapy by aligning features in portal and simulator imagesPhysics in Medicine & Biology, 1991
- First clinical experience with a newly developed electronic portal imaging deviceInternational Journal of Radiation Oncology*Biology*Physics, 1990
- A matrix ionisation chamber imaging device for on-line patient setup verification during radiotherapyRadiotherapy and Oncology, 1988