An experimental and computational investigation of the standard temperature-pressure correction factor for ion chambersin kilovoltage x rays

• Published in: Medical physics (Lancaster) 2007-11, Vol.34 (12), p.4690-4699
• Main Authors: La Russa, Daniel J., McEwen, Malcolm, Rogers, D. W. O.
• Format: Article
• Subjects: air pressure; correction factors; EGSnrc; ion chamber dosimetry; Monte Carlo; temperature-pressure correction; x ray
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.2799580

For ion chambers with cavities open to the surrounding atmosphere, the response measured at a given temperature and pressure must be corrected using the standard temperature-pressure correction factor ( P T P ) . A previous paper based solely on Monte Carlo simulations [ D. J. La Russa and D. W. O. Rogers , Med. Phys. 33 , 4590-4599 ( 2006 ) ] pointed out the shortcomings of the P T P correction factor when used to correct the response of non-air-equivalent chambers for low-energy x-ray beams. This work presents the results of several experiments that corroborate these calculations for a number of ion chambers. Monte Carlo simulations of the experimental setup revealed additional insight into the various factors affecting the extent of the breakdown of P T P , including the effect of impurities and the sensitivity to chamber dimensions. For an unfiltered 60 kV beam, the P T P -corrected response of an NE 2571 ion chamber measured at 0.7 atm was 2.5 % below the response measured at reference conditions. In general, Monte Carlo simulations of the experimental setup using EGSnrc were within 0.5 % of measured values. EGSnrc-calculated values of air kerma calibration coefficients ( N K ) at low x-ray energies are also provided as a means of estimating the level of impurities in the chambers investigated. Calculated values of N K normalized to the value measured for a 250 kV beam were obtained for three chambers and were within 1 % of experiment with one exception, the Exradin A12 in a 50 kV beam.