Dose-response characteristics of an amorphous silicon EPID Peter Winkler a Division of Medical Radiation Physics, Department of Radiotherapy and Radiobiology, Medical University of Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austria Alfred Hefner Health Physics Division, ARC Seibersdorf Research GmbH, A-2444 Seibersdorf, Austria Dietmar Georg Division of Medical Radiation Physics, Department of Radiotherapy and Radiobiology, Medical University of Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austria Received 17 March 2005; revised 15 July 2005; accepted for publication 21 July 2005; published 21 September 2005 Electronic portal imaging devices EPIDs were originally developed for the purpose of patient setup verification. Nowadays, they are increasingly used as dosimeters e.g., for IMRT verification and linac-specific QA. A prerequisite for any clinical dosimetric application is a detailed under- standing of the detector’s dose-response behavior. The aim of this study is to investigate the dosimetric properties of an amorphous silicon EPID Elekta IVIEWGT with respect to three photon beam qualities: 6, 10, and 25 MV. The EPID showed an excellent temporal stability on short term as well as on long term scales. The stability throughout the day was strongly influenced by warming up, which took several hours and affected EPID response by 2.5%. Ghosting effects increased the sensitivity of the EPID. They became more pronounced with decreasing time intervals between two exposures as well as with increasing dose. Due to ghosting, changes in pixel sensitivity amounted up to 16% locally for the 25 MV photon beam. It was observed that the response characteristics of our EPID depended on dose as well as on dose rate. Doubling the dose rate increased the EPID sensitivity by 1.5%. This behavior was successfully attributed to a dose per frame effect, i.e., a nonlinear relationship between the EPID signal and the dose which was delivered to the panel between two successive readouts. The sensitivity was found to vary up to 10% in the range of 1 to 1000 monitor units. This variation was governed by two independent effects. For low doses, the EPID signal was reduced due to the linac’s changing dose rate during startup. Furthermore, the detector reading was influenced by intrabeam variations of EPID sensitivity, namely, an increase of detector response during uniform exposure. For the beam qualities which were used, the response characteristics of the EPID did not depend on energy. Differences in relative dose-response curves resulted from energy dependent temporal output characteristics of the accelerator. If ghosting is prevented from affecting the results and all dose-response effects are properly corrected for, the EPID signal becomes independent of dose rate, dose, and exposure time. © 2005 American Asso- ciation of Physicists in Medicine. DOI: 10.1118/1.2040711 I. INTRODUCTION Electronic portal imaging devices EPIDs were developed for the purpose of patient setup verification. The aim was to replace radiographic films, which were originally used to verify patient positioning during radiotherapy treatments. Detectors which are based on two different principles, liquid- filled matrix ionization chambers and camera-based fluoro- scopic EPIDs, became commercially available in the mid 1990’s. 1,2 Despite all the advantages of these devices e.g., feasibility of on-line patient setup correction, no film pro- cessing, their image quality was not satisfactory. The current generation of EPIDs is based on semiconductor materials, namely, amorphous selenium photoconductors 3 as well as amorphous silicon photodiodes. 4 These devices exhibit an improved image quality close to that of radiographic films. While amorphous selenium EPIDs are primarily used for di- agnostic applications, ion chamber based and fluoroscopic EPIDs used in radiotherapy tend to be replaced by amor- phous silicon devices. The dose distributions acquired with modern portal imag- ing systems can be used for a broad variety of applications. With respect to the verification of IMRT treatment plans, the measured distributions of primary fluence can be compared with the data predicted by the treatment planning system. 5,6 In combination with patient CT data, portal dose images PDIs can be used to reconstruct the dose distribution within the patient, i.e., for two-dimensional 2D or 3D in vivo dosimetry. 7,8 EPIDs also offer the possibility to verify the leaf position during dynamic beam delivery. 9,10 Recently, an amorphous silicon EPID was developed which allows in- phantom dosimetry. 11 Another possible application is linac specific quality assurance, e.g., monitoring leaf calibration 12 or beam flatness and symmetry measurements. Compared to former designs of EPIDs, amorphous silicon devices are expected to be superior for portal imaging and portal dosimetry. 13 An understanding of the relationship be- tween pixel value reading and dose or fluence is a prerequi- site for portal dosimetry. Two prototype amorphous silicon 3095 3095 Med. Phys. 32 „10…, October 2005 0094-2405/2005/32„10…/3095/11/$22.50 © 2005 Am. Assoc. Phys. Med.