An EGSnrc investigation of the PTP correction factor for ion chambers in kilovoltage x rays

As part of the standard practice for obtaining consistent ion chamber measurements with cavities open to the surrounding atmosphere, the raw measured response is corrected to the response at a reference temperature and pressure using the standard temperature‐pressure correction factor (PTP). In this...

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Bibliographic Details
Published in:Medical physics (Lancaster) 2006-12, Vol.33 (12), p.4590-4599
Main Authors: La Russa, Daniel J., Rogers, D. W. O.
Format: Article
Language:English
Subjects:
air pressure
Aluminium
Ancillary equipment
Bubble dynamics
Distribution theory and Monte Carlo studies
dosimetry
Electrodeposition
Electrodes
Electron beams
Graphite
ionisation chambers
Ionization chambers
Mammography
Monte Carlo
Monte Carlo methods
Photons
temperature‐pressure correction
X‐ray spectra
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Summary:As part of the standard practice for obtaining consistent ion chamber measurements with cavities open to the surrounding atmosphere, the raw measured response is corrected to the response at a reference temperature and pressure using the standard temperature‐pressure correction factor (PTP). In this study, the EGSnrc Monte Carlo code was used to investigate the validity of the PTP correction factor for kilovoltage x rays incident on various geometrically distinct ion chambers. The calculated PTP‐corrected chamber response deviated by over 2% relative to expected values for a 40kV spectrum incident on a graphite thimble chamber at an air density typical of Mexico City. The relative deviation from the expected response was much worse for a large spherical graphite chamber, exceeding 16% at an air density of 0.6kg∕m3 (≈0.5atm at 22°C) for the same beam energy. The breakdown of the PTP correction factor was also observed for a 26kV mammography spectrum incident on two mammography chambers. For Co60 beams, the PTP correction factor behaved as expected. For day‐to‐day variations in pressure, only a negligible of the PTP correction factor was observed with low x‐ray energies. Factors contributing to the breakdown of the PTP correction factor at low x‐ray energies and large pressure variations, such as the range of electrons, the material of the wall, the chamber dimensions and air‐photon interactions, are discussed in depth.
ISSN:0094-2405
2473-4209
DOI:10.1118/1.2392407