Comparison of air‐kerma strength determinations for HDR ¹⁹²Ir sources

Abstract

Purpose: To perform a comparison of the interim air-kerma strength standard for high dose rate (HDR) 192 Ir brachytherapy sources maintained by the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) with measurements of the various source models using modified techniques from the literature. The current interim standard was established by Goetsch et al. in 1991 and has remained unchanged to date. Methods: The improved, laser-aligned seven-distance apparatus of the University of Wisconsin Medical Radiation Research Center (UWMRRC) was used to perform air-kerma strength measurements of five different HDR192 Ir source models. The results of these measurements were compared with those from well chambers traceable to the original standard. Alternative methodologies for interpolating the 192 Ir air-kerma calibration coefficient from the NIST air-kerma standards at 137 Cs and 250 kVp x rays (M250) were investigated and intercompared. As part of the interpolation method comparison, the Monte Carlo code EGSnrc was used to calculate updated values of A wall for the Exradin A3 chamber used for air-kerma strength measurements. The effects of air attenuation and scatter, room scatter, as well as the solution method were investigated in detail. Results: The average measurements when using the inverse N K interpolation method for the Classic Nucletron, Nucletron microSelectron, VariSource VS2000, GammaMed Plus, and Flexisource were found to be 0.47%, À0.10%, À1.13%, À0.20%, and 0.89% different than the existing standard, respectively. A further investigation of the differences observed between the sources was performed using MCNP5 Monte Carlo simulations of each source model inside a full model of an HDR 1000 Plus well chamber. Conclusions: Although the differences between the source models were found to be statistically significant, the equally weighted average difference between the seven-distance measurements and the well chambers was 0.01%, confirming that it is not necessary to update the current standard maintained at the UWADCL.

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

“…The AKS measured with this technique was used to establish the calibration factor for reference well chambers at UWADCL. [910] The well-type ionization chambers are used in general to determine the AKS in the clinical environment to achieve greater precision due to reproducible geometry. As the detection volume of the well-type ionization chamber is large, it produces high ionization current that can be measured with high precision.…”

Importance of HDR source calibration and measurement

“…[2] In addition, the primary values of air kerma for the UW250-M beam and the 137 Cs beam were changed by NIST (and other primary laboratories). Rasmussen determined the calibration coefficient for a flat energy response chamber using…”

“…This source has been in use for over 25 years and was first calibrated at the University of Wisconsin -Madison. [4] The latest summary of the calibration of this source is given by Rasmussen, et al [2] The weighted average energy of this source is 397 keV. [4] The output quantity for this source is air kerma strength (AKS) or reference air kerma rate (RAKR); both quantities are the same at 1 m. The air kerma strength is expressed in vacuum at a distance, generally 1 cm.…”

Determination of contributions of scatter and distance error to the source strength of 192Ir HDR brachytherapy source

“…The overall uncertainty of the measurement was determined by performing an uncertainty budget. Rasmussen et al [6] compared the interim air-kerma strength standard established by Goetsch et al [4] for HDR 192 Ir brachytherapy sources with measurements of the various source models using modified techniques from the literature. They found the differences between the source models to be statistically significant but confirmed that it is not necessary to update the current interim standard of seven distance method.…”

“…The in-air measurement of source strength using cylindrical ionization chamber requires the determination of scatter radiation from walls and floor of the room and also the offset in positioning of the chamber. Few studies [4][5][6][7][8][9][10] have been reported for determining the room scatter contribution from an 192 Ir HDR brachytherapy source. Goestch et al [4] described a method for calibrating 192 Ir HDR brachytherapy afterloading systems.…”