An Egsnrc investigation of the P(TP) 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 (P(TP)). In this study, the EGSnrc Monte Carlo code was used to investigate the validity of the P(TP) correction factor for kilovoltage x rays incident on various geometrically distinct ion chambers. The calculated P(TP)-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 (approximately 0.5 atm at 22 degrees C) for the same beam energy. The breakdown of the P(TP) correction factor was also observed for a 26 kV mammography spectrum incident on two mammography chambers. For 60Co beams, the P(TP) correction factor behaved as expected. For day-to-day variations in pressure, only a negligible of the P(TP) correction factor was observed with low x-ray energies. Factors contributing to the breakdown of the P(TP) 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.
La Russa
Daniel J
DJ
Carleton Laboratory for Radiotherapy Physics, Ottawa-Carleton Institute for Physics, Carleton University, Ottawa, ON, KIS 5B6, Canada. dlarussa@physics.carleton.ca
Rogers
D W O
DW
D000388:Air; D000465:Algorithms; D000535:Aluminum / Q000737:chemistry; D003037:Cobalt Radioisotopes / Q000627:therapeutic use; D004566:Electrodes; D006108:Graphite / Q000737:chemistry; D006801:Humans; D007477:Ions; D008327:Mammography / Q000295:instrumentation* / Q000379:methods*; D015233:Models, Statistical; D009010:Monte Carlo Method; D017785:Photons; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D013696:Temperature; D014965:X-Rays
D003037:Cobalt Radioisotopes; D007477:Ions; D006108:Graphite; D000535:Aluminum