ESTRO 2023 - Abstract Book

S228

Saturday 13 May

ESTRO 2023

Purpose or Objective Brachytherapy (BT) treatment planning systems (TPS) are indispensable tools for computing the dose distribution and treatment time of brachytherapy patient plans. For a safe treatment it is essential that source data, calculation parameters, and planning functionalities affecting the TPS output are modelled and/or validated properly. Currently there are only a few dedicated societal reports available to guide users through the commissioning process of a BT TPS. The GEC- ESTRO BRAPHYQS working group in cooperation with the AAPM is working on a report with practical recommendations for TPS commissioning for HDR/PDR BT as well as LDR seed implant BT TPS. Materials and Methods The BRAPHYQS working group was extended with further experts in the field for this project. After extensive literature research and internal discussions, an outline and sections were defined. Due to the high availability and use worldwide, this report focuses on TG-43-based algorithms only; for advanced model-based dose calculation algorithms, the users are referred to existing reports such as TG-186. Five main subtopics have been defined to guide the commissioning process: (a) geometry and imaging, (b) source specification, (c) dose calculation accuracy and representation, (d) (library) applicator specification, and (e) output/data transfer and reporting. Results A set of commissioning tests was designed and summarized in tables per topic, including the frequency, tolerance, and possible tools and resources to be used. A CT-based DICOM set of a phantom with well-defined structures was prepared, allowing users to test image import, display and manipulation, as well as geometric properties and functionalities of the TPS. This DICOM set will be made available through the GEC-ESTRO/BRAPHYQS website. A range of imaging modalities used in modern BT is covered such as CT, MRI, and US. Furthermore, a selection of extensive tests is described in greater detail within the Appendices. The recommended tests in this report can be used at the time of TPS commissioning as well as after software upgrades or for annual constancy checks. Conclusion BRAPHYQS is finalizing the recommendations to validate the calculational accuracy and performance of HDR/PDR and LDR BT TPSs. This report will provide a comprehensive set of practical tools (tables/checklists, phantom data, detailed tests) to ensure high quality BT dosimetry calculations. It is aimed to finalize and publish the report in 2023. OC-0293 Intrinsic energy dependent response of inorganic scintillation detectors for in vivo dosimetry P. Georgi 1 , Å. Carlsson Tedgren 2 , L. Persson 3 , J. Graversen Johansen 1,4 1 Aarhus University, Department of Clinical Medicine, Aarhus, Denmark; 2 Karolinska Hospital, Department of Medical Physics and Nuclear Medicine, Stockholm, Sweden; 3 Swedish Radiation Safety Authority, National Regulation Department, Stockholm, Sweden; 4 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective Inorganic scintillation detectors are emerging as a tool for in vivo dosimetry in brachytherapy (BT). The response of any detector depends on the energy spectrum of the incident radiation. While absorbed dose energy dependence, mainly caused by water inequivalence, can be obtained from Monte Carlo (MC) simulation the dose-to-signal efficiency or intrinsic energy dependence cannot. This study aims to characterize the intrinsic energy dependence of fiber-coupled inorganic scintillation detectors in X-ray beam qualities relevant for BT (< 1 MeV). Materials and Methods 3 point-like detectors made from fiber-coupled cuboid ZnSe:O-based scintillators (1 0.5x0.5x2 mm3 and 2 0.5x0.5x1 mm3) were calibrated at a National Metrology Laboratory for ionizing radiation. The calibration was done in terms of air kerma free in air, K _air, in 13 X-ray beam qualities, Q , from 25 to 300 kVp (BIPM/CCRI 25-250 kV and ISO 4037 N-series), and in terms of dose-to-water, D _w, in a Co-60 beam, Q _0. For each quality, the relative intrinsic energy dependence, R , was determined relative to Q _0 as where M was the detector read-out. The mean dose to the detector, D _det, relative to K _air and D _w, were obtained with the MC code Topas (Geant4) using X-ray spectra obtained with the SpekPy software and laboratory filtration data. The angular dependence of the detector response was measured for all detectors in a 25 kVp (0.20 mm Al HVL) beam, and for a single detector in a 135 kVp (0.35 mm Cu HVL) beam, by rotating the detectors along the fiber's longitudinal axis, which was perpendicular to the beam direction. Results R was constant within 5% for all detectors above 30 keV, indicating negligible variation in typical BT beam qualities e.g., the Ir-192 spectrum, fig. 1. Below 30 keV the detectors showed noticeable variation, likely due to differences in detector material and geometry not captured by the simulations using nominal dimensions. All detectors showed angular dependence in the 25 kVp quality varying with 20-40% over the measured angles, fig. 2. The dependence was asymmetric, assumed to result from non-uniform radiation damage and asymmetric detector geometry. In 135 kVp the signal variation dropped to below 3%.