Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams

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DOIResolve DOI: http://doi.org/10.1118/1.3687864
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TypeArticle
Journal titleMedical Physics
ISSN0094-2405
Volume39
Issue3
Pages16181631; # of pages: 14
Subjectarticle; equipment; Monte Carlo method; photon; radiometry; time; uncertainty; Monte Carlo Method; Photons; Radiometry; Time Factors; Uncertainty
AbstractPurpose: To investigate the behavior of plane-parallel ion chambers in high-energy photon beams through measurements and Monte Carlo simulations. Methods: Ten plane-parallel ion chamber types were obtained from the major ion chamber manufacturers. Absorbed dose-to-water calibration coefficients are measured for these chambers and k Q factors are determined. In the process, the behaviors of the chambers are characterized through measurements of leakage currents, chamber settling in cobalt-60, polarity and ion recombination behavior, and long-term stability. Monte Carlo calculations of the absorbed dose to the air in the ion chamber and absorbed dose to water are obtained to calculate k Q factors. Systematic uncertainties in Monte Carlo calculated k Q factors are investigated by varying material properties and chamber dimensions. Results: Chamber behavior was variable in MV photon beams, especially with regard to chamber leakage and ion recombination. The plane-parallel chambers did not perform as well as cylindrical chambers. Significant differences up to 1.5 were observed in calibration coefficients after a period of eight months although k Q factors were consistent on average within 0.17. Chamber-to-chamber variations in k Q factors for chambers of the same type were at the 0.2 level. Systematic uncertainties in Monte Carlo calculated k Q factors ranged between 0.34 and 0.50 depending on the chamber type. Average percent differences between measured and calculated k Q factors were - 0.02, 0.18, and - 0.16 for 6, 10, and 25 MV beams, respectively. Conclusions: Excellent agreement is observed on average at the 0.2 level between measured and Monte Carlo calculated k Q factors. Measurements indicate that the behavior of these chambers is not adequate for their use for reference dosimetry of high-energy photon beams without a more extensive QA program than currently used for cylindrical reference-class ion chambers. © 2012 American Association of Physicists in Medicine.
Publication date
LanguageEnglish
AffiliationNational Research Council Canada (NRC-CNRC); NRC Institute for National Measurement Standards (INMS-IENM)
Peer reviewedYes
NPARC number21269390
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Record identifiera16b754e-c906-4d69-b0c3-2b77e46e23aa
Record created2013-12-12
Record modified2016-05-09
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