An experimental and computational investigation of the standard temperature‐pressure correction factor for ion chambers in kilovoltage x rays

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 theP T Pcorrection 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 ofP T P, including the effect of impurities and the sensitivity to chamber dimensions. For an unfiltered 60 kV beam, theP 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 ofN Knormalized 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.