title: Technical Note: Experimental determination of the effective point of measurement of two cylindrical ionization chambers in a clinical proton beam.
abstract: OBJECTIVE
  IAEA TRS-398 notes that cylindrical ionization chambers are preferred for reference proton dosimetry. If a cylindrical ionization chamber is used in a phantom to measure the dose as a function of depth, the effective point of measurement (EPOM) must be taken into account. IAEA TRS-398 recommends a displacement of 0.75 times the inner cavity radius (0.75R) for heavy ion beams. Theoretical models by Palmans and by Bhullar and Watchman confirmed this value. However, the experimental results vary from author to author. The purpose of this study is to accurately measure the displacement and explain the past experimental discrepancies.
  
  
  METHODS
  In this work, we measured the EPOM of cylindrical ionization chambers with high accuracy by comparing the Bragg-peak position obtained with cylindrical ionization chambers (PTW 30013, PTW 31016) to that obtained using a plane-parallel ionization chamber (PTW 34045).
  
  
  RESULTS
  The EPOMs of PTW 30013 and 31016 were shifted by 0.92 ± 0.07 R with R = 3.05 mm and 0.90 ± 0.14 R with R = 1.45 mm, respectively, from the reference point toward the source.
  
  
  CONCLUSIONS
  The EPOMs obtained were greater than the value of 0.75R proposed by the IAEA TRS-398 and the analytical results.
authors.affiliation: Proton Therapy Center, Aizawa Hospital, Nagano 390-0821, Japan and Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Yamanashi 409-3898, Japan.
authors.forename: Yuya
authors.initials: Y
authors.lastname: Sugama
authors.affiliation: Institute of Biomedical and Health Sciences, Hiroshima University, Hiroshima 734-8551, Japan.
authors.forename: Teiji
authors.initials: T
authors.lastname: Nishio
authors.affiliation: Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Yamanashi 409-3898, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Onishi
mesh_terms: D008962:Models, Theoretical; D019047:Phantoms, Imaging; D020959:Polyethylene; D061766:Proton Therapy / Q000295:instrumentation*; D011522:Protons; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D014867:Water
chemical_list: D011522:Protons; D014867:Water; D020959:Polyethylene