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Beam quality conversion factors for parallel‐plate ionization chambers in MV photon beams
Author(s) -
Muir B. R.,
McEwen M. R.,
Rogers D. W. O.
Publication year - 2012
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3687864
Subject(s) - quality (philosophy) , photon , ionization , laser beam quality , beam (structure) , ionization chamber , optics , physics , dosimetry , materials science , atomic physics , laser beams , nuclear medicine , ion , medicine , quantum mechanics , laser
Purpose: 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 andk Qfactors 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 calculatek Qfactors. Systematic uncertainties in Monte Carlo calculatedk Qfactors 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 althoughk Qfactors were consistent on average within 0.17%. Chamber‐to‐chamber variations ink Qfactors for chambers of the same type were at the 0.2% level. Systematic uncertainties in Monte Carlo calculatedk Qfactors ranged between 0.34% and 0.50% depending on the chamber type. Average percent differences between measured and calculatedk Qfactors 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 calculatedk Qfactors. 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.