Regular paperQuality assurance measurements of a-si epid performance
Introduction
The traditional method of portal imaging using films is gradually being replaced with electronic imaging based on video, scanning liquid ion chamber (SLIC), and amorphous silicon (a-Si) electronic portal imaging devices (EPIDs).1 The advantages of EPIDs are that they make field verification faster and they deliver digital images that can be modified later. Routine quality assurance (QA) tests are performed periodically to verify the consistency of portal imager operation with time, and to ensure compliance with performance specification limits for key parameters (e.g., mechanical movement, image contrast) established during acceptance testing. Recently, the use of EPIDs has been extended for dosimetry purposes that warrant the need for stability in detector response.2 Because one of our goals is to employ an a-Si EPID as a dosimetric tool, we previously measured the radiation response of this type of EPID,3 which was found to be almost linear with a small, photon spectrum-dependent quadratic component. To study imaging and dose response stability, we have made use of a variety of established methods. This paper presents the results of monthly QA measurements performed for a Varian aS500 EPID over a period of 18 months, during which time the EPID was deployed on 2 Varian dual-energy linacs to accommodate changing clinical priorities.
Section snippets
Description and operation of EPID
The Varian aS500 EPID (PortalVision, Varian Medical Systems, Palo Alto, CA) with a retractable arm was attached initially for a period of 11 months to a Varian 21EX linac and then moved to a Clinac 2300 CD linac, both of which are dual-energy machines delivering 6- and 15-MV photon beams (Varian Medical Systems). The imager can be positioned at source to EPID distances (SEDs) ranging from 105 to 180 cm. The standard deployment distance, which was used for QA purposes, is 140 cm. The sensitive
Cassette calibration
After acceptance testing and at regular intervals thereafter, the detector cassette has to be calibrated prior to clinical use. EPID calibration is designed to remove background noise and provide a spatially uniform response for clinical imaging. The response of the EPID is dependent on the photon energy and the repetition rate of the radiation beam. Hence, calibration is performed separately for each acquisition mode. Calibration involves acquiring a dark-field image and a flood-field image.
A
QA tests
The QA tests consisted of measurements of (a) contrast resolution, (b) image quality, specifically modulation transfer and contrast-to-noise ratio (CNR), (c) DRS stability, and (d) central axis pixel response.
Conclusions
This paper reports the results of a longitudinal QA study performed on a Varian aS500 EPID. Measurements included the contrast resolution (qualitative), image quality (quantitative), DRS stability, and central axis pixel response. The contrast resolution studies were performed by observing the number of holes visible in a contrast-detail phantom. We did not observe any dramatic change in contrast resolution, which was superior to that reported earlier by our center for a SLIC EPID. The QC-3V
References (15)
- et al.
Commissioning and periodic quality assurance of a clinical electronic portal imaging device
Int. J. Radiat. Oncol. Biol. Phys.
(1996) Portal imaging
Br. J. Radiol.
(2001)Electronic portal imaging devices: A review and historical perspective of contemporary technologies and research
Phys. Med. Biol.
(2002)- et al.
Retracted compensator quality control using an amorphous silicon EPID
Med. Phys.
(2003) - IEC. IEC 1217: Radiotherapy equipment: Coordinates, movements and scales....
- PortalVision™ aS500 Rel.6. Reference Manual, Varian Medical Systems Inc., June...
- Thomason, C.L. Implementation and clinical use of portal imaging. In: Mittal, B.B.; Purdy, J.A.; Ang, K.K., editors....
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