Experimental determination of the effective point of measurement of cylindrical ionization chambers for high-energy photon and electron beams |
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| Authors: | Yanxiao Huang Christian Willomitzer Golam Abu Zakaria Guenther H Hartmann |
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| Institution: | 1. Department of Electrical, Mechanical and Industrial Engineering, Anhalt University of Applied Sciences, Koethen, Germany;2. Department of Radiotherapy, Staetisches Klinikum Dessau, Dessau, Germany;3. Department of Medical Radiation Physics, Kreiskrankenhaus Gummersbach, Academic Teaching Hospital of the University of Cologne, Gummersbach, Germany;4. Department of Medical Physics in Radiotherapy, German Cancer Research Center, Heidelberg, Germany;1. National Centre for Nuclear Research (NCBJ), 05-400 Otwock, Poland;2. Association Euratom/IST, Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal;1. Physics Department, Hakim Sabzevari University, Shohada-e Hastei Blvd, P.O. 9617976487, Sabzevar, Iran;2. Department of Medical Physics and Radiological Sciences, Sabzevar University of Medical Sciences, Shohada-e Hastei Blvd, Sabzevar University of Medical Sciences Campus, P.O. 9617913112, Sabzevar, Iran;3. Vasei Radiotherapy & Oncology Center, Vasei Hospital, P.O. 9617913113, Sabzevar, Iran;4. Department of Medical Physics, Iran University of Medical Sciences, Hemmat Exp. Way, Faculty of Medicine, P.O. 14496141525, Tehran, Iran;5. Department of Radiation Medicine, Shahid Beheshti University, Daneshjoo Street, Velenjak, P.O. 1983963113, Tehran, Iran;1. Centre for Proton Therapy, Paul Scherrer Institute, Villigen PSI, Switzerland;2. Department of Physics, Swiss Federal Institute of Technology Zurich, Zurich, Switzerland;3. INFN–Dipartimento di Ingegneria Industriale, Università di Roma ‘Tor Vergata’, Roma, Italy;4. PTW-Freiburg, Freiburg, Germany;1. Physics Service of Radiation Oncology Dept., Humanitas Clinical and Research Hospital, Milan-Rozzano, Italy;2. Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, ENEA-INMRI C R Casaccia, Roma, Italy;3. Radiotherapy and Radiosurgery Department, Humanitas Clinical and Research Hospital, Milan-Rozzano, Italy;4. Department of Biomedical Sciences, Humanitas University, Rozzano, Milano, Italy;1. Department of Oncology, Unit of Radiotherapy, Tampere University Hospital, P.O. Box 2000, FI-33521 Tampere, Finland;2. Department of Medical Physics, Medical Imaging Center, Tampere University Hospital, P.O. Box 2000, FI-33521 Tampere, Finland;1. Center for Nuclear Technologies, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, Denmark;2. Department of Oncology, Division of Radiotherapy (52AA), Herlev Hospital, University of Copenhagen, Herlev Ringvej 75, DK-2730 Herlev, Denmark |
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| Abstract: | Measurements of depth-dose curves in water phantom using a cylindrical ionization chamber require that its effective point of measurement is located at the measuring depth. Recommendations for the position of the effective point of measurement with respect to the central axis valid for high-energy electron and photon beams are given in dosimetry protocols. According to these protocols, the use of a constant shift Peff is currently recommended. However, this is still based on a very limited set of experimental results. It is therefore expected that an improved knowledge of the exact position of the effective point of measurement will further improve the accuracy of dosimetry. Recent publications have revealed that the position of the effective point of measurement is indeed varying with beam energy, field size and also with chamber geometry. The aim of this study is to investigate whether the shift of Peff can be taken to be constant and independent from the beam energy. An experimental determination of the effective point of measurement is presented based on a comparison between cylindrical chambers and a plane-parallel chamber using conventional dosimetry equipment. For electron beams, the determination is based on the comparison of halfvalue depth R50 between the cylindrical chamber of interest and a well guarded plane-parallel Roos chamber. For photon beams, the depth of dose maximum, dmax, the depth of 80% dose, d80, and the dose parameter PDD(10) were used. It was again found that the effective point of measurement for both, electron and photon beams Dosimetry, depends on the beam energy. The deviation from a constant value remains very small for photons, whereas significant deviations were found for electrons. It is therefore concluded that use of a single upstream shift value from the centre of the cylindrical chamber as recommended in current dosimetry protocols is adequate for photons, however inadequate for accurate electron beam dosimetry. |
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