An EGSnrc investigation of the correction factor for ion chambers in kilovoltage x rays

Abstract

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.

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.…”

Dependence with air density of the response of the PTW SourceCheck ionization chamber for low energy brachytherapy sources

“…On the other hand, the construction materials may also play a significant role in the chamber response. Depending on the design and materials of the chamber, and on the photon energy, the result may be either an overcorrection or an undercorrection, as shown by La Russa et al 14,15 And so, while the HDR1000 Plus exhibits an overresponse to the low energy photon source irradiation, the SourceCheck shows an underresponse. Following radiography of the SourceCheck chamber, it appears to be no metallic elements present except for an aluminum frame surrounding the active volume.…”

Accuracy of Spencer‐Attix cavity theory and calculations of fluence correction factors for the air kerma formalism

“…The K fl correction replaces the K SA correction 5,6 discussed above because we are only dealing with 60 Co beams and not low-energy beams where other aspects of the theory break down. In particular, we assume that the fraction of response due to photon interactions in the cavity is negligible, which cannot be assumed for low-energy photons, 5,22,23 and that the departure from SA conditions is primarily due to the perturbation of the charged particle fluence by the presence of the cavity. Thus, Eq.…”