TU-C-108-10: Development of An Absorbed Dose to Water Primary Standard for HDR Ir-192 Brachytherapy Based On the Fricke Dosimetry System

Purpose: To develop a Fricke based absorbed dose standard for HDR Ir-192 brachytherapy that will give a significantly lower uncertainty on the dose to water at 1 cm compared to methods based on air-kerma standards. Methods: A ring shaped PMMA Fricke holder was developed to allow measurements to be conducted in a water phantom with the Fricke volume centred at 1 cm from the source. The seed was positioned at the ring centre to allow for a uniform irradiation geometry. The radiation chemical yield of Ferric ions, or G-value, was determined at 250KVp X-ray and at Co-60 and a linear interpolation of effective photon energies allowed the determination of the G-value at Ir-192. Using this interpolated G-value the radiation induced change in the Fricke solution's optical density could then be related to the absorbed dose to water at 1 cm. Results: The G-value experiments yielded values for 250 KVp X-ray and Co-60 of 1.56 umol/J and 1.61 umol/J respectively. The uncertainty in the G-value for Ir-192 photon energies is estimated to be around ±1%. The absorbed dose to water using the Fricke system was calculated and compared with the value obtained via an air-kerma measurement and TG-43 agreement was obtained within the combined uncertainties. An uncertainty analysis of the Fricke system indicates an overall uncertainty of 1.2% to 1.5% (for dose at 1 cm) is achievable. Conclusion: This work demonstrates the ability to reduce the standard uncertainty for Ir-192 HDR brachytherapy sources by using a Fricke chemical dosimeter that measures the absorbed dose to water directly and allows measurements to be conducted in the quantity of interest. The overall uncertainty is on par, or better than current water calorimetry based methods.