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Consider the following voltaic cell:

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The central electrode correction factor for high-Z electrodes in small ionization chambers.

B R Muir1, D W O Rogers

  • 1Ottawa Carleton Institute for Physics, Carleton University Campus, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada. bmuir@physics.carleton.ca

Medical Physics
|April 2, 2011
PubMed
Summary

High-Z electrode ion chambers show significant deviations in beam quality conversion factors (kQ). These issues stem from the central electrode correction factor (Pcel), necessitating careful consideration in dosimetry protocols.

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Area of Science:

  • Medical Physics
  • Radiation Dosimetry
  • Monte Carlo Simulations

Background:

  • Current dosimetry protocols (TG-51, TRS-398) may yield inaccurate beam quality conversion factors (kQ) for ion chambers with high-Z electrodes.
  • Monte Carlo calculations reveal discrepancies in kQ values compared to established protocols.

Purpose of the Study:

  • Investigate the central electrode correction factor (Pcel) for ion chambers utilizing high-Z electrodes.
  • Quantify the impact of high-Z electrodes on Pcel and its implications for kQ calculations.

Main Methods:

  • Modeled ionization chambers using EGSnrc user code (egs_chamber).
  • Calculated Pcel in photon and electron beams under reference conditions.
  • Assessed Pcel as a function of distance from an Ir-192 source and depth in water for various beam qualities (200 kVp X-rays, 6 MV).

Main Results:

  • Photon beams: Pcel differences up to 3% observed between high-Z and aluminum electrodes, influenced by beam filtration.
  • Electron beams: Pcel differences up to 0.45% between high-Z and aluminum electrodes.
  • Low-energy photon fields: Pcel showed large, distance-dependent variations for both high-Z and aluminum electrodes.

Conclusions:

  • Ion chambers with high-Z electrodes present significant challenges in various dosimetry applications.
  • Central electrode effects lead to kQ values that differ substantially from current protocol values.
  • Recommendations are made to avoid the use of high-Z electrodes in ion chamber design.