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Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator
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Ionization chamber gradient effects in nonstandard beam configurations.

Hugo Bouchard1, Jan Seuntjens, Jean-Francois Carrier

  • 1Département de Physique, Université de Montréal, Pavilion Roger-Gaudry (D-428), 2900 Boulevard Edouard-Montpetit, Montréal, Québec H3T 1J4, Canada. hugo.bouchard.chum@ssss.gouv.qc.ca

Medical Physics
|November 26, 2009
PubMed
Summary
This summary is machine-generated.

Investigating nonstandard beam reference dosimetry reveals that the current method of reporting absorbed dose at a single point leads to large correction factors. Alternative reporting methods are proposed to improve accuracy and simplify quality assurance.

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Area of Science:

  • Medical Physics
  • Radiation Dosimetry
  • Radiotherapy Physics

Background:

  • Accurate absorbed dose determination is critical for radiotherapy.
  • Nonstandard beam reference dosimetry, particularly for intensity-modulated radiation therapy (IMRT), presents unique challenges.
  • Current protocols rely on reporting dose at a specific point within the ionization chamber, which may not accurately represent the dose distribution in nonstandard beams.

Purpose of the Study:

  • To investigate the impact of the current definition of reporting absorbed dose at a point in water on nonstandard beam reference dosimetry.
  • To evaluate the role of the measurement point definition in conceptualizing improved dosimetry protocols.
  • To assess the magnitude of various perturbation correction factors under nonstandard beam conditions.

Main Methods:

  • Monte Carlo simulations using the EGSnrc-based user code EGS_chamber were employed.
  • Ionization chamber perturbation factors were calculated for two cylindrical chamber models (Exradin A12 and A14).
  • Simulations were performed for 14 IMRT beams known to produce significant dose gradients across the chamber volume.

Main Results:

  • The gradient perturbation correction factor was found to be the most significant, averaging 11% higher for the Exradin A12 and 5% higher for the Exradin A14 compared to reference conditions.
  • Other perturbation factors (Pwall, Pstem, Pcel) showed minimal variation (<0.8%) from reference conditions.
  • The current practice of defining the measurement point at the chamber centroid is identified as the primary reason for large reported correction factors in nonstandard beam dosimetry.

Conclusions:

  • Reporting absorbed dose as an average over the chamber's sensitive volume (option 1) or over the chamber itself (option 2) can reduce or eliminate perturbation correction factors.
  • Option 1, averaging dose to the sensitive volume, is suitable when clinical beam correction factors are negligible.
  • Option 2, averaging dose to the chamber, simplifies quality assurance when non-negligible correction factors require Monte Carlo calculations.