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 40 kV 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 0.6 kg/m3 (≈0.5 atm at 22°C) for the same beam energy. The breakdown of the PTP correction factor was also observed for a 26 kV mammography spectrum incident on two mammography chambers. For 60Co 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.

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Medical Physics
Department of Physics

La Russa, D.J. (Daniel J.), & Rogers, D.W.O. (2006). An EGSnrc investigation of the PTP correction factor for ion chambers in kilovoltage x rays. Medical Physics, 33(12), 4590–4599. doi:10.1118/1.2392407