title: Experimental determination of the effective point of measurement for cylindrical ionization chambers in 60Co gamma radiation.
abstract: The displacement effect of cylindrical ionization chambers is taken into account either by an effective point of measurement (EPOM) or, alternatively, by using a displacement perturbation factor. The dependence of these effects in water was examined as a function of the cavity radius using cylindrical chambers with different radii and a plane-parallel chamber, whose EPOM is well known. Depth-dose curves were measured in terms of absolute absorbed dose in water and evaluated according to the international protocol IAEA TRS-398 as well as the German protocol DIN 6800-2. As expected, evaluation of absorbed dose under reference conditions following both protocols agreed well within a standard uncertainty of 0.1%. However, values of absorbed dose at depths beyond the dose maximum showed deviations up to 0.3% and 0.5% for IAEA TRS-398 and DIN 6800-2, respectively. Values in the build-up and maximum region did not agree very well. Deviations of more than 1% were found for both protocols. It was concluded that the corrections recommended in both protocols are not fully appropriate. A procedure is suggested to measure the absorbed depth-dose distribution including the build-up region with an improved accuracy by means of cylindrical chambers.
authors.affiliation: Department of Medical Physics in Radiation Oncology, German Cancer Research Center, Heidelberg, Germany. c.legrand@dkfz-heidelberg.de
authors.forename: C
authors.initials: C
authors.lastname: Legrand
authors.forename: G H
authors.initials: GH
authors.lastname: Hartmann
authors.forename: C P
authors.initials: CP
authors.lastname: Karger
mesh_terms: D002138:Calibration; D003037:Cobalt Radioisotopes; D005720:Gamma Rays; D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation*; D014867:Water
chemical_list: D003037:Cobalt Radioisotopes; D014867:Water