Elsevier

Medical Dosimetry

Volume 29, Issue 1, Spring 2004, Pages 11-17
Medical Dosimetry

Regular paper
Quality assurance measurements of a-si epid performance

https://doi.org/10.1016/j.meddos.2003.09.002Get rights and content

Abstract

The performance stability of a Varian aS500 amorphous silicon (a-Si) electronic portal imaging device (EPID) was monitored over an 18-month period using a variety of standard quality assurance (QA) tests. The tests were selected to provide ongoing information about image quality and dose response from the time of EPID acceptance into clinical service. To evaluate imaging performance, we made spatial resolution and contrast measurements using both PortalVision and QC-3V phantoms for 6- and 15-MV photon beams at repetition rates of 100, 300, and 400 MU/min in standard scanning mode. To assess operational stability for dosimetry applications, we measured central axis radiation response and beam pulse variability for the same image acquisition modes. Using the QC-3V phantom, values for the critical frequency of 0.435 ± 0.005 lp/mm for 6 MV and 0.382 ± 0.003 lp/mm for 15 MV were obtained. The contrast-to-noise ratio was found to be ∼20% higher for the lower photon energy. Beam pulse variability remained within the tolerance of 3% set by the manufacturer. The central axis pixel response of the EPID remained constant within ±1% over a 5-month period for the 6-MV beam, but fell ∼4% over the same period for the 15-MV beam. The Varian aS500 EPID studied exhibited consistent image quality and a stable radiation response. These characteristics render it suitable for quantitative applications such as clinical dose measurement.

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

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