Radiological Physics and Technology

, Volume 2, Issue 1, pp 97–103 | Cite as

Polarity effect in commercial ionization chambers used in photon beams with small fields

  • Tetsunori Shimono
  • Kichiro Koshida
  • Hidekazu Nambu
  • Kosuke Matsubara
  • Hiroshi Takahashi
  • Hiroshi Okuda
Article

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.

Keywords

Polarity effect Ionization chamber Small field 

References

  1. 1.
    Andreo P. On the beam quality specification of high-energy photons for radiotherapy dosimetry. Med Phys. 2000;27:434–44.CrossRefPubMedGoogle Scholar
  2. 2.
    Francescon P, Cora S, Cavedon C. Use of a new type of radiochromic firm, a new parallel-plate micro-chambers, MOSFETs and TLD 800 microcubes in the dosimetry of small beams. Med Phys. 1998;25:503–11.CrossRefPubMedGoogle Scholar
  3. 3.
    Martens C, Watger DC, Neve DW. The value of the PinPoint ion chamber for characterization of small field segments use in intensity-modulated radiotherapy. Phys Med Biol. 2000;45:2519–30.CrossRefPubMedGoogle Scholar
  4. 4.
    Low DA, Gerber RL, Mutic S, Purdy JA. Phantoms for IMRT dose distribution measurement and treatment verification. Int J Radiat Oncol Biol Phys. 1998;40:1231–5.CrossRefPubMedGoogle Scholar
  5. 5.
    Low DA, Chao KS, Mutic S, Gerber RL, Perez CA, Purdy JA. Quality assurance of serial tomotherapy for head and neck patient treatment. Int J Radiat Oncol Biol Phys. 1998;42:681–92.CrossRefPubMedGoogle Scholar
  6. 6.
    Ezzel X, et al. Guidance document on delivery, treatment planning, and clinical implementation of IMRT Report of the IMRT subcommittee of the AAPM radiation therapy committee. Med Phys. 2003;30:2089–115.CrossRefGoogle Scholar
  7. 7.
    Aget H, Rosenwald JC. Polarity effect for various ionization chambers with multiple irradiation conditions in electron beam. Med Phys. 1991;18:67–72.CrossRefPubMedGoogle Scholar
  8. 8.
    Gerbi BJ, Khan FM. The polarity effect for commercially available plane-parallel ionization chambers. Med Phys. 1987;14:210–5.CrossRefPubMedGoogle Scholar
  9. 9.
    Reft CS, Kuchnir F. Measurement of the replacement correction factor for parallel-plate chambers in electron fields. Med Phys. 1991;18:1237–43.CrossRefPubMedGoogle Scholar
  10. 10.
    Mattsson LO, Johansson KA, Svensson H. Calibration and use of plane parallel ionization chambers for the determination of absorbed dose is electron beams. Acta Radiol Oncol. 1981;20:385–99.CrossRefPubMedGoogle Scholar
  11. 11.
    Williams JA, Agarwal SK. Energy-dependent polarity correction factors for four commercial ionization chambers used in electron dosimetry. Med Phys. 1997;24:785–90.CrossRefPubMedGoogle Scholar
  12. 12.
    Wickman G, Holmstrom T. Polarity effect in plane-parallel ionization chambers using air or a dielectric liquid as ionization medium. Med Phys. 1992;19:637–40.CrossRefPubMedGoogle Scholar
  13. 13.
    Kubo H. Evaluations of two solid water parallel-plate chambers in high-energy photon and electron beams. Med Phys. 1993;20:341–5.CrossRefPubMedGoogle Scholar
  14. 14.
    Gerbi BJ. The response characteristics of a newly designed plane-parallel ionization chamber in high-energy photon and electron beams. Med Phys. 1993;20:1411–5.CrossRefPubMedGoogle Scholar
  15. 15.
    AAPM, American Association of Physicists in Medicine, Task Group No. 25, Radiation Therapy Committee. Clinical electron beam dosimetry: report of AAPM radiation therapy committee task group No. 25. Med Phys. 1991;18:73–109.Google Scholar
  16. 16.
    McCaffrey JP, Downton B, Shen H, Niven D, McEwen M. Pre-irradiation effects on ionization chambers used in radiation therapy. Phys Med Biol. 2005;50:121–33.CrossRefGoogle Scholar
  17. 17.
    Boag JW. Distortion of the electric field in an ionization chamber due to a difference in potential between guard electrode and collector. Phys Med Biol. 1963;9:25–32.CrossRefGoogle Scholar
  18. 18.
    Novkovic D, Milosevic Z, Subotic K, Manic-Kudra S. Numerical solutions of differential equations of a cylindrical ionization chamber. Phys Med Biol. 1996;41:725–41.CrossRefPubMedGoogle Scholar
  19. 19.
    Araki F. Monte Carlo study of a Cyberknife stereotactic radiosurgery system. Med Phys. 2006;33:2955–63.CrossRefPubMedGoogle Scholar
  20. 20.
    Burch SE, Kearfott KJ, Trueblood JH, Sheils WC, Yeo JI, Wang CKC. A new approach to firm dosimetry for high energy photon beams: lateral scatter filtering. Med Phys. 1997;24:775–83.CrossRefPubMedGoogle Scholar
  21. 21.
    Sykes JR, James HV, Williams PC. Letter to the editor: how much does firm sensitivity increase at depth for larger field sizes? Med Phys. 1999;26:329–30.CrossRefPubMedGoogle Scholar

Copyright information

© Japanese Society of Radiological Technology and Japan Society of Medical Physics 2008

Authors and Affiliations

  • Tetsunori Shimono
    • 1
  • Kichiro Koshida
    • 2
  • Hidekazu Nambu
    • 3
  • Kosuke Matsubara
    • 2
  • Hiroshi Takahashi
    • 1
  • Hiroshi Okuda
    • 1
  1. 1.Department of RadiologyHoshigaoka Koseinenkin HospitalHirakataJapan
  2. 2.Department of Quantum Medical TechnologyKanazawa University Graduate School of Medical SciencesKanazawaJapan
  3. 3.Department of RadiologyNara Hospital Kinki University Medical SchoolIkoma, NaraJapan

Personalised recommendations