Absorbed dose to water standard for 192Ir HDR sources using Fricke Dosimetry

Abstract

The Fricke solution is a chemical dosimeter that is based on the oxidation of ferrous ions to ferric ions in the solution after interaction with ionizing radiation. It is composed of 96% water (by weight), and its density is thus remarkably similar to that of water. In addition, studies show that the Fricke dosimeter can be used as a primary dosimeter in the determination of the absorbed dose to water for high dose rate (HDR) 192Ir brachytherapy. The Radiological Sciences Laboratory of the University of Rio de Janeiro State (LCR/UERJ) has been investigating the use of the Fricke dosimeter in various applications for more than ten years, particularly in the area of radiotherapy. This review paper presents important improvements in recent years by the LCR/UERJ in the determination of the absorbed dose to water for 192Ir sources. This includes a newly designed irradiation vessel, a new reading device, a description of the need for careful temperature control during irradiation and reading, a more accurate calculation of the correction factors and the results of an intercomparison with the National Calibration Laboratory of Canada. Careful preparation of the Fricke solution is one of the most critical steps in the process. Over the years, the LCR/UERJ has tested different methods of preparing the solution and the final procedure is presented. Regarding the irradiation vessel, a molded double-walled, spherical flask for the Fricke solution was first constructed and used to measure the absorbed dose to water. However, as it was difficult to manipulate the spherical flask, a second design also made with PMMA was molded as a cylinder, with a central tube where the source was centrally positioned. Different methodologies have been reported in the determination of the G-value, a key parameter in Fricke dosimetry, and herein, two different methodologies used by the LCR are reviewed. For the absorbed-dose-to-water determination for 192Ir sources, the overall combined uncertainty associated with the measurements is estimated to be less than 1% for k = 1. Thus, the obtained uncertainties for the determination of the absorbed dose to water using Fricke dosimetry are lower than those obtained using the standard protocols. With respect to clinical practice, this could improve the accuracy in the calculation of the dose delivered to the patients. Overall, the results show that Fricke dosimetry is a reliable system to measure absorbed dose to water as a standard for HDR 192Ir.