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

Abstract

Variations of the altitude and changes in the weather conditions may produce significant density corrections, and that effect should be taken into account. This effect is chamber-dependent, indicating that a specific calibration is necessary for each particular chamber. To our knowledge, this correction has not been considered so far for SourceCheck ionization chambers, but its magnitude cannot be neglected in clinical practice. The atmospheric pressure and temperature at which the chamber was calibrated need to be taken into account, and they should be reported in the calibration certificate. In addition, each institution should analyze the particular response of its SourceCheck ionization chamber and compute the adequate correction factors. In the absence of a suitable pressure chamber, a possibility for this assessment is to take measurements at different altitudes, spanning a wide enough air density range.

Technical Note: Empirical altitude correction factors for well chamber measurements of permanent prostate and breast seed implant sources

“…These results are contrary to those observed by Tornero‐López et al. for the plane‐parallel well chamber, where a chamber‐specific under‐response was reported . These disagreements suggest that the geometry of the chamber is an important factor in the characterization of this response.…”

“…The final seed measured was the Nucletron selectSeed (I‐125; Elekta AB, Stockholm, Sweden). This seed has a documented altitude correction factor with the PTW 34051 SourceCheck chamber (PTW, Freiburg, Germany); this chamber, however, is a plane‐parallel well chamber, and therefore a different chamber‐specific correction factor would be expected in this study due to the use of a chamber with cylindrical geometry.…”

“…This is done using the P TP correction factor:where standard temperature and pressure in North America are traditionally taken as T o = 22.0 °C and P o = 760 torr, respectively. This factor, however, relies on the assumptions of Bragg–Gray cavity theory, which have been shown to break down for low‐energy brachytherapy seeds . In particular, it is assumed that all particles cross the volume of air within the chamber.…”

“…This factor, however, relies on the assumptions of Bragg-Gray cavity theory, which have been shown to break down for low-energy brachytherapy seeds. [3][4][5] In particular, it is assumed that all particles cross the volume of air within the chamber. At very low energies, however, this is an invalid assumption.…”

Supplement 2 for the 2004 update of the AAPM Task Group No. 43 Report: Joint recommendations by the AAPM and GEC‐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.…”