Pixel 3 portal 2 image11/7/2022 ![]() Similarly to the linac QA applications, PSM calibration has demonstrated utility in EPID‐based patient‐specific QA applications. The PSM used in the Barnes publication is based upon a simplified version of the Greer Used PSM‐corrected EPID imaging as an absolute measure of wide‐field beam symmetry as a means of photon beam angle steering. Both the Yaddanapudi and Cai studies were based upon an adaptation of the Boriano et al. The focus of that study was to demonstrate that PSM‐corrected EPID imaging could provide consistent profiles for matched linacs. The study of Cai et al.Īlso looked at QA applications of PSM‐corrected EPID images. This study also suggested that the method could be used for beam symmetry evaluation, although no assessment of symmetry was presented. The study used changes in the flatness of the beam profile as measured using PSM‐corrected EPID imaging as a measure of beam energy. Utilized PSM‐corrected EPID images for linac acceptance testing purposes. ![]() Three studies have been published which examined the utilization of PSM‐corrected EPID images for linac QA purposes. The PSM is analogous to the array type calibrations used in commercial 2D‐array detectors to correct for response differences between individual detectors. Who named this new calibration procedure the pixel‐sensitivity‐map (PSM). Such a calibration procedure was first attempted by Greer This would potentially be a game‐changer for the use of EPID for quality assurance applications including linac testing and commissioning, ongoing quality assurance as well as pretreatment and in vivo dosimetry.īecause of the problems associated with the EPID flood field calibration a number of authors have attempted to develop alternate EPID calibration procedures where the non‐uniformity of the imager response is corrected without disturbing the non‐uniformity of the incident beam. Conversion of EPID to dose‐to‐water including beam profile variations would provide the convenience and high spatial resolution of EPID imaging while generating data in the general dose‐to‐water format similarly to the commercial array type devices. While several EPID dosimetry systems exist, the majority use non‐water equivalent dosimetry (i.e., EPID signal predictions) or if they estimate dose‐to‐water planes, the true beam profile is not reproduced, which means the EPID methods are not comparable to other detector systems with regards to their utility. This is particularly relevant for measurements of breast treatment fields. For patient QA the flood field correction is also problematic in that the EPID panel must not be translated and hence the extremities of asymmetric or off‐axis fields can miss the panel and not be recorded. The EPID cannot currently be used for evaluation of the absolute beam profile as this is removed by flood field calibration, and thus the user must apply different and less‐efficient QA methods. Has generally been a constancy check relative to a baseline that was set once the system had been calibrated using an alternate device. As such, the flood field correction represents a major current limiting factor to the use of EPID for dosimetry and QA applications.īecause of the flood field correction, the EPID‐based profile QA present in the literature However, correcting out the non‐uniformities introduced by the beam is problematic for many linac and patient QA applications because the beam non‐uniformities (i.e., profile shape) is the information required for investigation. This is standardly achieved via the flood field calibration procedure. ![]() ![]() As such, correcting out the non‐uniformities introduced by the beam and those introduced by the EPID panel itself are required. For such applications, the image non‐uniformities caused by the patient anatomy in the beam are of interest. ![]() The amorphous silicon electronic portal imaging device (EPID) was designed for patient positioning applications. The required routine linear accelerator (linac) quality assurance (QA) tests and their tolerances and frequencies are stipulated in best practice documents such as the American Association of Physicists in Medicine Task Group 142 reportĪnd Medical Physics Practice Guideline 8.a.īoth documents require routine QA testing of the beam dose profile shape and the photon beam quality for which a number of publications have suggested that metrics such as flatness and off‐axis‐ratio can be used as an effective measure. ![]()
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