title: Wall correction factors for calibration of plane-parallel ionization chambers with high-energy photon beams.
abstract: Most dosimetry protocols recommend that calibration of plane-parallel ionization chambers be performed in an electron beam of sufficiently high energy by comparison with cylindrical chambers. For various plane-parallel chambers, the 1997 IAEA TRS-381 protocol includes an overall perturbation factor pQ for electron beams, a wall correction factor p(wall) for a 60Co beam and the product of two wall corrections k(att)k(m) for 60Co in-air calibration. The recommended values of p(wall) for plane-parallel chambers, however, are limited to certain phantom materials and a 60Co beam, and are not given for other phantom materials and x-ray beams. In this work, the p(wall) values of the commercially available NACP, PTW/Markus and PTW/Roos plane-parallel chambers in a solid water phantom have been determined with 60Co and 4 and 10 MV photon beams. The k(att)k(m) values for the NACP and PTW/Markus chambers have also been obtained. The wall correction factors p(wall) and k(att)k(m) have been determined by intercomparison with a calibrated Farmer chamber. The average value of p(wall) for these plane-parallel chambers was 1.005 +/- 0.1% (1 SD) for 60Co beams and 1.007 +/- 0.2% (1 SD) for both 4 MV and 10 MV photons. The k(att)k(m) values for the NACP and PTW/Markus chambers were about 1.5% lower than other published data.
authors.affiliation: Department of Radiological Technology, Kumamoto University College of Medical Science, Japan.
authors.forename: F
authors.initials: F
authors.lastname: Araki
authors.forename: R
authors.initials: R
authors.lastname: Ikeda
authors.forename: Y
authors.initials: Y
authors.lastname: Shirakawa
authors.forename: T
authors.initials: T
authors.lastname: Shimonobou
authors.forename: N
authors.initials: N
authors.lastname: Moribe
authors.forename: T
authors.initials: T
authors.lastname: Takada
authors.forename: M
authors.initials: M
authors.lastname: Takahashi
authors.forename: H
authors.initials: H
authors.lastname: Oura
authors.forename: M
authors.initials: M
authors.lastname: Matoba
mesh_terms: D002138:Calibration; D003037:Cobalt Radioisotopes; D004566:Electrodes; D004583:Electrons; D019047:Phantoms, Imaging; D017785:Photons; D019904:Polymethyl Methacrylate; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D014867:Water; D014965:X-Rays
chemical_list: D003037:Cobalt Radioisotopes; D014867:Water; D019904:Polymethyl Methacrylate