title: Polarity effect in commercial ionization chambers used in photon beams with small fields.
abstract: Ionization chambers are the instruments of choice for use in photon dosimetry. Ionization chambers together with radiographic films represent the best detectors for measurement of dose distribution for a quality assurance (QA) program in intensity-modulated radiotheraphy (IMRT). The polarity effect was investigated for seven different commercially available ionization chambers. This study concentrated on measuring the dependence of the polarity effect at various energies, and for various field size, ionization chamber, and electrometer combinations. Of the seven chambers, CC01, PTW23323, and PTW31006 had the largest polarity effect for small field sizes. The materials of the central electrode of these three chambers were steel or air-equivalent plastic C-552. The magnitude of the polarity effect was shown to be strongly dependent on the material of the collecting electrode. This polarity effect dependence was observed for the ionization chambers and small field sizes studied.
authors.affiliation: Department of Radiology, Hoshigaoka Koseinenkin Hospital, Hirakata, Osaka, Japan. shimono-hoshi@kcat.zaq.ne.jp
authors.forename: Tetsunori
authors.initials: T
authors.lastname: Shimono
authors.forename: Kichiro
authors.initials: K
authors.lastname: Koshida
authors.forename: Hidekazu
authors.initials: H
authors.lastname: Nambu
authors.forename: Kosuke
authors.initials: K
authors.lastname: Matsubara
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Takahashi
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Okuda
mesh_terms: D002138:Calibration; D004560:Electricity; D017785:Photons; D011874:Radiometry / Q000295:instrumentation*
references.citation: Med Phys. 2003 Aug;30(8):2089-115
references.pmid: 12945975
references.citation: Acta Radiol Oncol. 1981;20(6):385-99
references.pmid: 6278862
references.citation: Med Phys. 1991 Nov-Dec;18(6):1237-43
references.pmid: 1753910
references.citation: Phys Med Biol. 1996 Apr;41(4):725-41
references.pmid: 8730666
references.citation: Phys Med Biol. 1964 Jan;9:25-32
references.pmid: 14108129
references.citation: Med Phys. 1999 Feb;26(2):329-30
references.pmid: 10076992
references.citation: Phys Med Biol. 2000 Sep;45(9):2519-30
references.pmid: 11008953
references.citation: Med Phys. 1992 May-Jun;19(3):637-40
references.pmid: 1508102
references.citation: Phys Med Biol. 2005 Jul 7;50(13):N121-33
references.pmid: 15972973
references.citation: Med Phys. 1993 Sep-Oct;20(5):1411-5
references.pmid: 8289723
references.citation: Med Phys. 2000 Mar;27(3):434-40
references.pmid: 10757593
references.citation: Med Phys. 2006 Aug;33(8):2955-63
references.pmid: 16964874
references.citation: Med Phys. 1997 May;24(5):785-90
references.pmid: 9167172
references.citation: Med Phys. 1987 Mar-Apr;14(2):210-5
references.pmid: 3587140
references.citation: Med Phys. 1997 May;24(5):775-83
references.pmid: 9167171
references.citation: Med Phys. 1991 Jan-Feb;18(1):67-72
references.pmid: 2008173
references.citation: Int J Radiat Oncol Biol Phys. 1998 Oct 1;42(3):681-92
references.pmid: 9806530
references.citation: Int J Radiat Oncol Biol Phys. 1998 Mar 15;40(5):1231-5
references.pmid: 9539580
references.citation: Med Phys. 1998 Apr;25(4):503-11
references.pmid: 9571618
references.citation: Med Phys. 1991 Jan-Feb;18(1):73-109
references.pmid: 1901132
references.citation: Med Phys. 1993 Mar-Apr;20(2 Pt 1):341-5
references.pmid: 8497220