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

Abstract

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(TP)). 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(TP) 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(TP), including the effect of impurities and the sensitivity to chamber dimensions. For an unfiltered 60 kV beam, the P(TP)-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.

Supplement 2 for the 2004 update of theAAPMTask Group No. 43 Report: Joint recommendations by theAAPMandGEC‐ESTRO

“…Several papers have examined this issue for both lowenergy photon-emitting brachytherapy sources and for kilovoltage x rays. 31,32,33,34,35 For water, the recommended composition of H 2 O and mass density of 0.998 g/cm 3 are unchanged from the TG-43U1 report. The values of brachytherapy dosimetry parameters are generally provided in terms of dose to water in water -the reference medium for brachytherapy dose calculations.…”

The Bland–Altman analysis: Does it have a role in assessing radiation dosimeter performance relative to an established standard?

“…The uncertainties for the depth dose measurements were estimated using the International Organization for Standardization (ISO) methodology for determining uncertainties in measurements (ISO, 1995;La Russa et al, 2007). The uncertainties that were considered in these measurements include variations in the dose output from the x-ray unit, geometric uncertainty in the positioning of the ionisation chamber and treatment applicator, the resolution of the motors on the water tank and the variation in the mass energy absorption coefficient of water-to-air as a function of depth in water.…”

Monte Carlo calculations of , the beam quality conversion factor

“…The geometry is based on the original chamber design by Aird and Farmer. 15 The NE2571 model is based on the model used by La Russa et al 16 The PTW 31010 chamber is based on the model given by Wulff et al 4 The ionization chambers that are used are given in Table I, along with the wall and electrode materials and chamber dimensions. The Exradin blueprint dimensions are proprietary so the specifications from the user's manual are given, although the simulations of Exradin ion chambers use blueprint models.…”