Portal dose measurement in radiotherapy using an electronic portal imaging device (EPID)

Abstract

Physical characteristics of a commercially available electronic portal imaging device (EPID), relevant to dosimetric applications in high-energy photon beams, have been investigated. The EPID basically consists of a fluorescent screen, mirrors and a CCD camera. Image acquisition for portal dose measurement has been performed with a special procedure, written in the command language that comes with the system. The observed day-to-day variation in local EPID responses, i.e. measured grey scale value (EPID signal) per unit of delivered portal dose, is 0.4% (1 SD); day-to-day variation in relative EPID responses (e.g. normalized to the on-axis response) are within 0.2% (1 SD). Measured grey scale values are linearly proportional to transmitted portal doses with a proportionality constant which is independent of the thickness of a flat, water-equivalent absorber in the beam, but which does significantly depend on the size of the applied x-ray beam. It is shown that the observed increased in EPID response with increasing field size is mainly due to contributions to the EPID signals from scattered light: visible photons produced by the x-ray beam in a point of the fluorescent screen not only generate a grey scale value in the corresponding point of the EPID image, but also lead (due to scatter from components of the EPID structure onto the CCD chip) to an increased grey scale value at all other points of the image. A point spread function, derived from measured data and describing the increase in EPID response at the beam axis due to off-axis irradiation of the fluorescent screen, has been successfully applied to connect portal doses with grey scale values measured with the EPID.

On flattening filter‐free portal dosimetry

“…Most modern portal systems are diode‐based (amorphous silicon). That means (even with the compensator Cu layer) that low energy photons are over‐detected 9 , 10 , 11 , 12 , 13 , 14 …”

On Radiotherapy Dose Verification With a Flat-Panel Imager

“…33,84 Other studies converted EPID images to dose images at the detector plane, in some cases behind a phantom. 24,42,48,60,73,87,95,129 Alternatively, EPID images were used to reconstruct the dose in a plane within the patient or phantom 2,11,45,65,94,115,131 . Of these studies, some have explored the possibility of using an a-Si EPID for dose verification of IMRT fields.…”

“…This approach has been applied using an algorithm to back-project the PDI to the mid-plane 11,14,65 or to reconstruct the patient dose based on convolution / superposition methods, back-projecting the fluence measured with the EPID. 79 Applications of portal dosimetry have been reported mainly for the type of EPIDs based on a liquid-filled (Li-Fi) ionisation chamber matrix 11,14,36,59,65,123 59,122 the CCD camera-based EPID 28,48,60,96,114,130 and a solid-state based EPID. 45 These studies compared the PDI with various combinations of film and ionisation chamber measurements.…”

A feasible method for clinical delivery verification and dose reconstruction in tomotherapy

“…In fact, these processes belong to a larger class of verification techniques developed for radiotherapy using detectors located in an exit plane behind the patient. 21 These include techniques using film and/or diodes, [22][23][24][25][26][27][28][29][30] scintillator-camera based electronic portal imaging devices ͑EPIDs͒, [31][32][33][34][35][36] and liquid-filled ion chamber based EPIDs. [37][38][39][40][41][42] In comparison with these approaches, there are two main advantages of the transfer matrix method.…”