title: Effective point of measurement for parallel plate and cylindrical ion chambers in megavoltage electron beams.
abstract: The presence of an air filled ionization chamber in a surrounding medium introduces several fluence perturbations in high energy photon and electron beams which have to be accounted for. One of these perturbations, the displacement effect, may be corrected in two different ways: by a correction factor pdis or by the application of the concept of the effective point of measurement (EPOM). The latter means, that the volume averaged ionization within the chamber is not reported to the chambers reference point but to a point within the air filled cavity. Within this study the EPOM was determined for four different parallel plate and two cylindrical chambers in megavoltage electron beams using Monte Carlo simulations. The positioning of the chambers with this EPOM at the depth of measurement results in a largely depth independent residual perturbation correction, which is determined within this study for the first time. For the parallel plate chambers the EPOM is independent of the energy of the primary electrons. Whereas for the Advanced Markus chamber the position of the EPOM coincides with the chambers reference point, it is shifted for the other parallel plate chambers several tenths of millimeters downstream the beam direction into the air filled cavity. For the cylindrical chambers there is an increasing shift of the EPOM with increasing electron energy. This shift is in upstream direction, i.e. away from the chambers reference point toward the focus. For the highest electron energy the position of the calculated EPOM is in fairly good agreement with the recommendation given in common dosimetry protocols, for the smallest energy, the calculated EPOM positions deviate about 30% from this recommendation.
authors.affiliation: Institut für Medizinische Physik und Strahlenschutz - IMPS, Technische Hochschule Mittelhessen, University of Applied Sciences, Gießen, Germany. Electronic address: philip.von.voigts-rhetz@kmub.thm.de.
authors.forename: Philip
authors.initials: P
authors.lastname: von Voigts-Rhetz
authors.affiliation: Institut für Medizinische Physik und Strahlenschutz - IMPS, Technische Hochschule Mittelhessen, University of Applied Sciences, Gießen, Germany.
authors.forename: Damian
authors.initials: D
authors.lastname: Czarnecki
authors.affiliation: Institut für Medizinische Physik und Strahlenschutz - IMPS, Technische Hochschule Mittelhessen, University of Applied Sciences, Gießen, Germany; University Hospital Marburg, Department of Radiotherapy and Radiation Oncology, Philipps-University, Marburg, Germany.
authors.forename: Klemens
authors.initials: K
authors.lastname: Zink
mesh_terms: D000465:Algorithms; D016477:Artifacts; D003198:Computer Simulation; D017076:Computer-Aided Design; D004583:Electrons; D004867:Equipment Design; D019544:Equipment Failure Analysis; D015233:Models, Statistical; D009010:Monte Carlo Method; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods*; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy / Q000295:instrumentation*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: Elektronendosimetrie; Ionisationskammer; Monte Carlo simulations; Monte-Carlo-Simulation; effective point of measurement; effektiver Messort; electron dosimetry; ionization chambers