Pergamon Int. J. Radiation Oncology Biol. Phys., Vol. 33. No. 5, pp. 1265- 1272. 1995 Copyright 0 1995 Elsevier Science Inc. Printed in the USA. All rights reserved 0360.3016/95 $9.50 + .OO 0360-3016(95)00108-5 l Treatment Verification DOSIMETRIC CHARACTERISTICS OF A LIQUID-FILLED ELECTRONIC PORTAL IMAGING DEVICE MARION ESSERS, MSc., BART R. HOOGERVORST, M.Sc., MARCEL VAN HERK, PH.D., HUGO LANSON AND BEN J. MIJNHEER, PH.D. Department of Radiotherapy, The Netherlands CancerInstitute, Antoni van Leeuwenhoek Huis, Plesmanlaan 121, 1066CX Amsterdam, The Netherlands Purpose: To determine the characteristics of a commercial electronic portal imaging device (EPID), based on a two-dimensional matrix of liquid-filled ionization chambers, for transmission dose measurements during patient treatment. Methods and Materials: Electronic portal imaging device measurements were performed in a cobalt-60 beam and two accelerator x-ray beams, and compared with measurements performed with a Farmer-type ionization chamber in air in a miniphantom and in an extended water phantom. Results: The warming up time of the EPID is about 1 h. The long-term stability of the detector is better than 1% under reference conditions for a period of about 3 months. The signal of the ionization chambers follows approximately the square root of the dose rate, although the relation becomes more linear for larger (> 1 Gy/min) dose rates. The signal can be transformed to dose rate with an accuracy of 0.6% (1 SD). The short-term influence of integrated dose on the sensitivity of the ionization chambers is small. The sensitivity increases about 0.5% for all ionization chambers after an absorbed dose of 8 Gy and returns to its original value in less than 5 min after stopping the irradiation. This small increase in sensitivity can be ascribed to the electrode distance of the ionization chambers in commercial EPIDs, which is 0.8 + 0.1 mm. The sensitivity increase depends on the electrode distance and is 4% for a 1.4 mm electrode distance. The scattering properties of the EPID ionization chambers were between those of an ionization chamber in a miniphantom and in a water phantom. Conclusion: The matrix ionization chamber EPID has characteristics that make it very suitable for dose rate measurements. It is therefore a very promising device for in vivo dosimetry purposes, Radiotherapy, Portal imaging, Dosimetry, Ionization chamber, Quality assurance, In vivo dosimetry, Trans- mission dosimetry. INTRODUCTION introduced in the clinic to overcome some drawbacks Various electronic portal imaging devices (EPIDs) are of conventional portal films, such as the short exposure currently available for clinical applications. Some of these time, which necessitates an interruption in the patient systems are based on a scintillating screen/video camera irradiation, and the amount of time needed to develop combination, while others are based on scanning tech- and read a film (3). Electronic portal imaging devices niques that either use a moving array of photodiodes or are extremely useful for on-line geometrical patient a matrix of liquid-filled ionization chambers (3). In our setup measurements (2, 8, 15). They can, in addition, institution we have developed an EPID that consists of a be used for other purposes such as the determination matrix of 256 X 256 liquid-filled ionization chambers of the diode position during in viva dosimetry (5). An (12, 13). Because the liquid density is high compared EPID might, however, also be suitable for simultaneous with gassesand signal integration occurs in the liquid patient set-up measurement and transmission dosimetry (12), a good signal-to-noise ratio can be obtained with in a large number of points; for example, during high small ionization chambers. dose-high precision treatments. Electronic portal imaging devices were originally The possibilities of using conventional portal films and Reprint requests to: Marion Essers, M.Sc. E-mail address: with the measurements performedwith the second commercial mess@nki.nl EPID at the “Zeeuws Radiotherapie Instituut” in Vlissingen, Acknowledgements-This work was financially supported by and Ronald Boellaard (M.Sc.) and JoosLebesque (Ph.D.) for the Netherlands Cancer Foundation(NKB Grant NKI 92-40). many useful discussions. We would like to thank Nice van Bree (M.Sc.) for helping us Acceptedfor publication 24 February 1995. 1265