ESTRO 2023 - Abstract Book

S778

Monday 15 May 2023

ESTRO 2023

saturation of the electrometer due to the extremely high peak current (up to 100’s of mA). Doses per pulse (DPP) ranging from few cGy up to 5 Gy were delivered on a pulse duration ranging from 1 us up to 4 us (up to 1.2 MGy/s instantaneous dose rate). Results Figure 1 (left) shows an optimal linearity (R2=0.999) of the SiC detector (5x5 mm2 and 10x10 mm2, 10 um thick) response as a function of the DPP. Alanine detectors were employed as a dose rate independent reference for the measurement of the delivered dose. Pulse shape time measurements at high time resolution (ns) were also performed (Figure 1, right), by connecting the detectors to a fast oscilloscope showing the high sensitivity of such detectors to the eventual intra-pulse small variations of the instantaneous dose rate which is one of the crucial parameters for the preclinical investigations of the FLASH effect.

Conclusion The results clearly demonstrated the dose-rate independence of newly developed SiC detectors at FLASH regime and their capability of providing the intra-pulse real-time monitoring of the instantaneous dose rate at high time resolution. Moreover, both large area SiC sensors and 2D configurations with high spatial resolution can be easily realized representing a unique a promising perspective for the QA and dosimetry measurements of UHDR beams. OC-0931 Monte Carlo modelling of a Small-body Portable Graphite Calorimeter for ultra-high dose rate beams J. Cotterill 1 , R. Thomas 1,2 , A. Subiel 1 , N. Lee 1 , D. Shipley 1 , H. Palmans 1,3 , A. Lourenco 1,4 1 National Physical Laboratory, Medical Radiation Science, London, United Kingdom; 2 University of Surrey, Engineering and Physical Science, Guildford, United Kingdom; 3 MedAustron Ion Therapy Center, Medical Physics, Wiener Neustadt, Austria; 4 University College London, Medical Physics and Biomedical Engineering, London, United Kingdom Purpose or Objective Accurate dosimetry in ultra-high dose rate proton beams is complicated as reference dosimeters such as ionisation chambers require large ion recombination corrections. The National Physical Laboratory (UK) has built a Small-body Portable Graphite Calorimeter (SPGC) which may offer an improvement in accuracy on current secondary standard dose measuring devices because its response is dose-rate independent. Additionally, it is more practical to operate than a primary standard. A detailed model of the SPGC has been built in TOPAS (v3.6.1), shown in Figure 1, to derive the k_imp and k_gap correction factors used to convert the dose to its graphite core to a dose to homogeneous graphite. k_imp corrects for the non-graphite constituents of the calorimeter, such as core thermistors and surrounding Styrofoam. The k_gap correction factor accounts for the presence of an air gap between the core and its surrounding jacket.

Materials and Methods The SPGC comprises a graphite core with built-in thermistors to measure the temperature rise for a given delivered dose. The core is surrounded by a graphite jacket, with an air gap to minimise the heat transfer between the components. The jacket is embedded in Styrofoam to better thermally isolate the body from the environment. Three modelled geometries of the calorimeter are required to determine the correction factors: a full geometry including non-graphite components, a pure-graphite geometry where all non-graphite components are modelled as graphite, and a compensated geometry where the air-gaps are filled with graphite, but the build-up depth to the core is kept constant. The ratio between the dose measured in the core in these geometries gives the correction factors. The geometries were simulated using a 5cm x 6cm field of 249.7MeV protons with a Gaussian energy spread (standard