An electronic portal imaging device as a physics tool

doi:10.1016/S0958-3947(97)00002-2

Medical Dosimetry

Volume 22, Issue 2, Summer 1997, Pages 101-105

https://doi.org/10.1016/S0958-3947(97)00002-2 Get rights and content

Abstract

An electronic portal imaging device (EPID) can be used not only to acquire megavoltage patient images but also to measure certain radiation beam parameters of the linear accelerator. EPID images can be used to verify field junctions, center of collimator rotation, or radiation vs. light field coincidence. If the EPID images are calibrated in terms of dose rate, an EPID can be applied to beam penumbra measurement, collimator transmission determination, or compensator verification. Beam parameters measured with EPIDs are in close agreement with those measured with film or ionization chamber, making EPIDs reliable physics tools for quality control of various beam parameters in radiotherapy.

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Cited by (21)

Positions of radiation isocenter and the couch rotation center established by Winston-Lutz and optical measurements
2022, Technical Innovations and Patient Support in Radiation Oncology
Citation Excerpt :
More than a decade ago [10,11,12,13] the usefulness of EPIDs as a physics tool was recognized and they were used for measurements of beam penumbra, radiation versus light field coincidence, and center of collimator rotation. EPIDs have also been used instead of film for mechanical alignment [14] and radiation isocenter assessment [11,15,16,17] of linear accelerators. In the present study an EPID is used to determine the radiation isocenter defined by gantry rotation and the couch rotation center with the Winston-Lutz approach.
To compare x-ray and optical imaging methods for measuring the relative position of radiation isocenter and couch rotation center. To show the impact of radiation isocenter size and target motion on the margins for target contours.
Winston-Lutz measurements are made using EPID images. Image analysis was done with public domain software, ImageJ, and spreadsheets written in Microsoft Excel. A comparison between the center of a high density test object and center of the MLC collimated beam is used to judge the relative position of the radiation isocenter in space for gantry and couch rotation. Additionally, motion of the target with couch rotation is determined with an optical imaging system. Five different accelerators, two TrueBeams, a Trilogy, and two VersaHDs, were assessed by Winston-Lutz and optical methods.
The shift in the radiation isocenter with gantry rotation is found to be a tri-axial ellipsoid. Shifts in the target position with respect to radiation isocenter with couch rotation were between 0.4 and 0.6 mm. The Winston-Lutz and optical method determination of couch rotation center agreed within measurement uncertainty.
Image analysis yields precise data on linear accelerator radiation isocenter and rotation centers of the couch. The Winston-Lutz and optical methods agreed within measurement uncertainty.
A simple approach for EPID dosimetric calibration to overcome the effect of image-lag and ghosting
2012, Applied Radiation and Isotopes
Citation Excerpt :
The ability of EPID in general, and the a-Si based detectors in particular, to quantify dose has made them the subject of many dosimetry studies (Herman et al., 2001; Juste et al., 2009a, 2009b; van Elmpt et al., 2008). The use of EPID was investigated for routine physics quality assurance (Curtin-Savard and Podgorsak, 1997), multi-leaf collimators (MLC) quality assurance (Baker et al., 2005; Sonke et al., 2004), pre-treatment verification (Talamonti et al., 2006; van Zijtveld et al., 2007), exit dose and mid-plane dose measurements (Chen et al., 2006; van Elmpt et al., 2008; Vieira et al., 2003; Wendling et al., 2006), and for in vivo dosimetry (Essers and Mijnheer, 1999; Fidanzio et al., 2008; Juste et al., 2009a, 2009b; Lanson et al., 1999; McDermott et al., 2008; Piermattei et al., 2006; van Elmpt et al., 2008, 2009. However, a-Si EPID suffer from the so-called image-lag and ghosting effects (Mail et al., 2007; McDermott et al., 2004), which results in the delayed release of the radiation induced total charge, or a measurable residual signal (SR), after the irradiation has been ceased.
EPID dosimetry has known drawbacks. The main issue is that a measurable residual signal is observed after the end of irradiation for prolonged periods of time, thus making measurement difficult. We present a detailed analysis of EPID response and suggest a simple, yet accurate approach for calibration that avoids the complexity of incorporating ghosting and image-lag using the maximum integrated signal instead of the total integrated signal. This approach is linear with dose and independent of dose rate.
Verification of dose delivery for a prostate sIMRT treatment using a SLIC-EPID
2008, Applied Radiation and Isotopes
The current work focuses on the verification of transmitted dose maps, measured using a scanning liquid ionization chamber–electronic portal imaging device (SLIC–EPID) for a typical step-and-shoot prostate IMRT treatment using an anthropomorphic phantom at anterior–posterior (A–P), and several non-zero gantry angles. The dose distributions measured using the SLIC–EPID were then compared with those calculated in the modelled EPID for each segment/subfield and also for the corresponding total fields using a gamma function algorithm with a distance to agreement and dose difference criteria of 2.54 mm and 3%, respectively.
A literature review of electronic portal imaging for radiotherapy dosimetry
2008, Radiotherapy and Oncology
Citation Excerpt :
Non-transmission images can be useful for performing quality control of treatment parameters independent of the patient, related to dosimetric and geometric characteristics of the linear accelerator. Test fields have been designed to check the field flatness or symmetry of the beam, absolute output of the linear accelerator and radiation-light field coincidence [52,53,84–86]. Furthermore, a large number of papers have been published on the use of EPIDs for the geometrical verification of MLC leaf positions or leaf trajectories during dynamic multileaf collimation [52,53,84,86–97].
Electronic portal imaging devices (EPIDs) have been the preferred tools for verification of patient positioning for radiotherapy in recent decades. Since EPID images contain dose information, many groups have investigated their use for radiotherapy dose measurement. With the introduction of the amorphous-silicon EPIDs, the interest in EPID dosimetry has been accelerated because of the favourable characteristics such as fast image acquisition, high resolution, digital format, and potential for in vivo measurements and 3D dose verification. As a result, the number of publications dealing with EPID dosimetry has increased considerably over the past ∼15 years. The purpose of this paper was to review the information provided in these publications. Information available in the literature included dosimetric characteristics and calibration procedures of various types of EPIDs, strategies to use EPIDs for dose verification, clinical approaches to EPID dosimetry, ranging from point dose to full 3D dose distribution verification, and current clinical experience. Quality control of a linear accelerator, pre-treatment dose verification and in vivo dosimetry using EPIDs are now routinely used in a growing number of clinics. The use of EPIDs for dosimetry purposes has matured and is now a reliable and accurate dose verification method that can be used in a large number of situations. Methods to integrate 3D in vivo dosimetry and image-guided radiotherapy (IGRT) procedures, such as the use of kV or MV cone-beam CT, are under development. It has been shown that EPID dosimetry can play an integral role in the total chain of verification procedures that are implemented in a radiotherapy department. It provides a safety net for simple to advanced treatments, as well as a full account of the dose delivered. Despite these favourable characteristics and the vast range of publications on the subject, there is still a lack of commercially available solutions for EPID dosimetry. As strategies evolve and commercial products become available, EPID dosimetry has the potential to become an accurate and efficient means of large-scale patient-specific IMRT dose verification for any radiotherapy department.
Daily monitoring of linear accelerator beam parameters using an amorphous silicon EPID
2007, Physics in Medicine and Biology
Dosimetric validation of digital megavolt imager for flattening filter free beams in the pre-treatment quality assurance of stereotactic body radiation therapy for liver metastases
2020, Asian Pacific Journal of Cancer Prevention

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Original contributionAn electronic portal imaging device as a physics tool

Abstract

Int. J. Radiat. Oncol. Biol. Phys.

A review of electronic portal imaging devices

Med. Phys.

Original contribution
An electronic portal imaging device as a physics tool