The effect of ambient pressure on well chamber response: Experimental results with empirical correction factors

Abstract

For some air-communicating well-type chambers used for low-energy brachytherapy source assay, deviations from expected values of measured air kerma strength were observed at low pressures associated with high altitudes. This effect is consistent with an overcompensation by the air density correction to standard atmospheric temperature and pressure (P(TP)). This work demonstrates that the P(TP) correction does not fully compensate for the high altitude pressure effects that are seen with air-communicating chambers at low photon energies in the range of 20-100 keV. Deviations of up to 18% at a pressure corresponding to an approximate elevation of 8500 ft for photon energies of 20 keV are possible. For high-energy photons and for high-energy beta emitters in air-communicating chambers the P(TP) factor is applicable. As expected, the ambient pressure does not significantly affect the response of pressurized well chambers (within 1%) to either low- or high-energy photons. However, when used with beta emitters, pressurized chambers appear to exhibit a slight dependence on the ambient pressure. Using measured data, the response and correction factors were determined for three models of air-communicating well chambers for low-energy photon sources at various pressures corresponding to elevations above sea level. Monte Carlo calculations were also performed which were correlated with the experimental findings. A more complete study of the Monte Carlo calculations is presented in the accompanying paper, "The effect of ambient pressure on well chamber response: Monte Carlo calculated results for the HDR1000 Plus."

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.…”

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

“…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.…”

“…Griffin et al. documented correction factors for several seeds and several chambers, reporting coefficients for an altitude correction factor for three low‐energy seed models combined with three different well‐type ionization chambers . Tornero‐López et al.…”

Dosimetric evaluation of newly developed well-type ionization chamber for use in the calibration of brachytherapy sources

“…With high-energy photons and beta emitters, the k TP factor is applicable in air-communicating well-type ionization chambers. [14] The well-type ionization chambers are provided with a calibration factor for a particular type of source. The calibration factor is dependent on the source design compared to in-air calibration with Farmer-type ionization chamber.…”