Determination of the Fricke G value for HDR 192Ir sources using ionometric measurements

High Dose Rate (HDR) brachytherapy using 192Ir is widely accepted as an important treatment option, and it thus requires an accurate dosimetry standard. However, a dosimetry standard for the direct measurement of absolute dose to water is currently not available. The dose to water conversion is calculated via the dose rate constant Λ and several correction factors accounting for the scatter, attenuation, and anisotropy of the dose distribution, among other effects. Two potentially useful procedures have been reported, including one by Sarfehnia et al. [3,4], which used a water-based calorimeter with an uncertainty of 1.9% for k=1, and a second by Austerlitz et al. and de Almeida et al., which used Fricke dosimetry with estimated uncertainties of 3.9% for k=1 and 1.4% for k=1, respectively. Chemical dosimetry using a standard FeSO4 solution has shown potential to be a reliable standard of absorbed dose for the HDR 192Ir source. A major uncertainty is associated with the G values reported by Fregene, which had a numerical value of 1.1 %. However, that reference provided very little detail of the experimental procedures for the 192Ir source. The G value may be obtained by using a calorimeter or ionometric measurements. In the absence of calorimetric data, this paper makes an attempt to measure the G value for the HDR 192Ir sources using ionometric measurements and recommendations from dosimetry protocols. (author)