ESTRO 2022 - Abstract Book

S244

Abstract book

ESTRO 2022

fDiamond is a diamond detector based on the microDiamond (mD, PTW, Germany) and successfully re-designed for UHDR by changing both the size of the active volume and the boron concentration in the diamond doped layer. For measurements in UHDR a PTW Tandem electrometer and an external box were provided by PTW, to avoid detector instantaneous currents exceeding the specifications of the electrometer. Measurements were performed at 7 and 9 MeV with different applicators ranging from 10 to 120 mm 2 . The instantaneous dose rate (DR) and the dose per pulse (DPP) were up to 3 MGy/s and 13 Gy/pulse, respectively. All measurements were performed in a modified MP3-XS water tank using the MEPHYSTO mc 2 software (PTW, Germany). Absolute dosimetry is performed in reference conditions, using the 100 mm 2 applicator, placing the water tank in contact with the applicator at 106 cm from the linac exit window and with the detector at d max . For relative dosimetry an out-of- field PTW Advanced Markus electron chamber was used as reference chamber. Absolute and relative dosimetry were performed by installing the fDiamond in the water tank using the same holder as the mD. The 3D movement of the water tank is controlled from the console room. Results Our results confirm that fDiamond can accurately measure dose up to at least 13 Gy/pulse. Response is linear with DPP within 5%. PDD were in less than one minute and compare well with the films (within 1 mm for R50 and 10 % overal, where the film uncertainty plays the largest role). Accurate positioning of the detector in the central field axis is performed via the software, more easily than with passive dosimeters. Conclusion fDiamond is a potential game changer for commissioning of UHDR system for Flash RT, as it allows dose measurements up to at least 13 Gy/pulse. It is able to measure real-time PDD and profiles in a water tank with accurate positioning using the PTW clinical software and potentially up to DPP of 26 Gy. This work is part of the 18HLT04 UHDpulse project which received funding from the EMPIR programme. G. Verona Rinati 1 , G. Felici 2 , F. Galante 2 , A. Gasparini 3 , L. Giuliano 4 , S. Heinrich 4 , M. Pacitti 2 , G. Prestopino 1 , V. Vanreusel 3,5 , D. Verellen 3 , C. Verona 1 , M. Marinelli 1 1 University of Rome "Tor Vergata", Dpt. Industrial Engineering, Rome, Italy; 2 Sordina IORT Technologies S.p.A., S.I.T., Aprilia, Italy; 3 Iridium Netwerk, Antwerp University, Faculty of Medicine and Health Sciences, Antwerp, Belgium; 4 Institut Curie, University Paris-Saclay, PSL Research University, Orsay, France; 5 SCK CEN, Research in dosimetric applications, Mol, Belgium Purpose or Objective FLASH radiation therapy (RT) is a promising technique for cancer treatment making use of radiation beams delivered at high and ultra-high dose rate (> 40 Gy s -1 ). However, in these irradiation conditions, some limitations hinder the translation of FLASH-RT into clinical practice, including technical issues related to the dosimetric characterization of FLASH beams. In particular, all commercially available active real time dosimeters (i.e., ionization chambers, semiconductor detectors, and scintillators) have shown to be unsuitable in ultra-high dose per pulse (UH-DPP) conditions, primarily due to saturation problems and nonlinearity of their response. The purpose of this study is to perform a systematic experimental verification to address the major limitations in the microDiamond (mD) device design affecting its response linearity in UH-DPP conditions, and to develop a novel diamond detector specifically designed and optimized for FLASH-RT applications. Materials and Methods A commercial mD detector and several diamond Schottky diode detector prototypes with different layouts were produced at Rome Tor Vergata University in cooperation with PTW-Freiburg. The rationale behind the design of the fabricated prototypes was to explore two key parameters affecting device linearity under UHDR beams, namely the device sensitivity and the series resistance of the Schottky diode. The produced diamond prototypes were characterized at two Electron UHDR beam facilities equipped with ElectronFLASH Linac (SIT S.p.A., Italy). The typical experimental setup is shown in Figure 1. The linearity of the detector response was tested by varying the DPP up to a maximum value of about 26 Gy/pulse. This was performed by changing the pulse duration, the applicator size and the SSD. Gafchromic films were used as reference dosimeters. Results The response of the commercial mD was found to completely saturate in UH-DPP regimes. However, by properly changing the fabrication parameters of the diamond Schottky junction it was possible to extend the linearity range of its response. In particular, a reduction of the series resistance of the detector and/or a reduction of its sensitivity allowed to operate at much higher DPPs. This was achieved by increasing the doping level of the p-type layer of the device, leading to a lower series resistance and by reducing the sensitive volume of the detector. Finally, two optimized prototypes were produced, showing a linear response from conventional dose-rate up to 26 Gy/pulse within a 5% uncertainty (Figure 2). Conclusion The obtained experimental results demonstrated that properly designed diamond detectors can be suitable for UH-DPP FLASH radiation therapy dosimetry. The present work is part of the 18HLT04 UHDpulse project (http://uhdpulse-empir.eu/) which has received funding from the European Metrology Programme for Innovation and Research (EMPIR) programme, co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme. OC-0284 Development of a novel diamond based Schottky diode detector for FLASH radiotherapy dosimetry