Automating Position and Timing Quality Assurance for High Dose Rate Brachytherapy Using Radioluminescent Phosphors and Optical Imaging

Abstract

procedures for both systems are entirely different. The Axxent system uses a well-type chamber calibrated in terms of Air Kerma Strength (AKS). The calibration system for the Intrabeam device is performed in terms of absorbed dose to water utilizing a soft X-ray ionization chamber in a water. The calibration factor of the chamber is derived from Air Kerma calibrations and a conversion into Dw according to ICRU 17. For the development of the primary standard device and investigations of the calibration chain characterizations of both sources were required: X-ray emission spectra were measured at various azimuthal and radial angles, relative 3Ddose distributions in distances below 3 cm were determined with radiochromic gels. Scintillator and X-ray storage film measurements were used for complement measurements. Monte-Carlo-simulations were performed to mimic the characteristics of the sources, for the characterization of the utilized measuring devices, and to calculate conversionand correction factors for the primary standard. The German (PTB) and Czech (CMI) national collaborated in this project, founded in the framework of the European Metrology Research Programme (EMRP) (project: MetrExtRT, http://radiotherapy-emrp.eu/). The prototype of the primary standard is based on the ‘‘Grovex II’’, an extrapolation chamber in a phantom of water-equivalent material, which is already utilized at PTB as a primary standard device for I-125 seeds. However, the phantom material, RW1, is less suitable for the radiation fields of EBXs as due to the higher amount of low-energy photons much more disturbing lowenergy fluorescence-photons of the ingredients of RW1, Mg and Ca, are induced. Therefore, the phantom material needed to be substituted and after rebuilding, the chamber needed to be recommissioned. Results: The results of the measurements for the characterization of the sources and the complementary Monte Carlo simulations will be presented with a special emphasis on the information required for the realization and dissemination of the desired quantity absorbed dose to water. The procedure of the realization will be exemplified and compared to that of the realization of the absorbed dose to water for I-125 seeds including a discussion of the uncertainties. Conclusions: The realization of the absorbed dose to water for EBXs is ongoing. In a further step consistent procedures or protocols for the dissemination of the quantity needs to be established. Thereby, fundamental issues are the establishment of criteria to be met by devices defines as EBX and a consistent protocol for all EBX devices is desirable or practicable.