ESTRO 2020 Abstract Book

S15 ESTRO 2020

Electronic Architectures Laboratory LCAE, Saclay, France ; 6 Centre national de la recherche scientifique CNRS, Centre Etudes Nucléaires de Bordeaux Gradignan CENBG, Gradignan, France ; 7 Australian Nuclear Science and Technology Organisation, Centre for Accelerator Science, Lucas Heights, Australia ; 8 Laboratoire de physique corpusculaire de Caen LCP, Centre national de la recherche scientifique CNRS, Caen, France ; 9 University Paris Saclay- CNRS, Imagerie et Modélisation en Neurobiologie et Cancérologie IMNC, Orsay, France ; 10 CEA Laboratoire TIRO & Université de Nice-Sophia Antipolis, University Côte d’Azur, Nice, France ; 11 Institut Méditerranéen de Protonthérapie, Centre Antoine-LACASSAGNE, Nice, France Purpose or Objective In microdosimetry (microdosimetric kinetic model) precise measurements of the lineal energy of particles are used to model the relative biological effectiveness (RBE) of clinical beams. The aim of this work was to measure the microdosimetric distributions of different ion beams, namely proton, carbon and silicon, using a novel diamond- based microdosimeter with well-defined micro-sensitive volumes (µSV). Material and Methods DIAµDOS Guard-Ring microdosimeter is an scCVD (single crystal chemical vapor deposition) diamond-membrane- based microdosimeter with cylindrical-shaped µSVs of 60 µm diameter and 12 µm thickness. During the irradiation with particle beams, the diamond microdosimeters were connected to low noise electronics allowing the detection of lineal energies of less than 10 keV/µm. The diamond microdosimeters provide very high spatial resolution and are more tissue-equivalent when compared with other existing alternative technologies. The response of DIAµDOS Guard-Ring microdosimeters was studied in 290 MeV/u carbon and 230 MeV/u silicon passive ion beams, and 200 MeV proton and 290 MeV/u carbon scanning therapy beams at heavy-ion medical accelerator in Chiba (HIMAC) in Japan, Institute Curie Proton Therapy Center in Orsay (CPO) in France and Gunma University Heavy Ion Medical Center (GHMC) in Japan, respectively. The diamond microdosimeters were positioned at various depths in a water phantom or behind a solid-water wall along the central axis of the ion beam. The microdosimetric lineal energy spectra were measured for pristine Bragg peak (BP). Geant4 simulations were performed in order to compare the simulated spectra to the experimental Figure 1 (A) shows the measured energy spectra of a passive 290 MeV/u carbon ion beam at HIMAC in Japan. The insert in the right corner represents the measured absorbed dose for this ion beam. Characteristic spectra for the corresponding positions in the Bragg peak can be observed. In Figure 1 (B) and (C) the microdosimetric spectra of the carbon beam are shown. A general trend can be observed in which there is an increase in lineal energy at Bragg peak and its distal part. These characteristics have been observed for all measured ion types and energies. results. Results

Conclusion The diamond DIAµDOS Guard-Ring microdosimeter with its well-defined micro-sensitive-volumes has shown its ability in characterization of particle radiation fields and can measure relevant physical parameters for the later determination of the RBE. Good agreement was observed between the experimental and simulation results, confirming the potential application of this microdosimeter in particle therapy. PH-0046 Characterization of ionization chambers in magnetic fields for MR guided proton beam therapy H. Fuchs 1 , F. Padilla Cabal 1 , L. Fetty 1 , D. Georg 1 , H. Palmans 2 1 Medical University of Vienna, Department of Radiation Oncology & Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria ; 2 MedAustron Iontherapy center & National Physical Laboratory, Medical Physics, Wr. Neustadt & London, Austria Purpose or Objective MR-linac systems have become commercially available, are currently clinically explored and provide promising possibilities for adaptive radiation therapy. In proton beam therapy, research towards the integration of an MR and a beam line has already started. In clinical practice, air- filled ionization chambers (IC) are the most important dosimetry systems. The necessity of magnetic-field related correction factors for ICs has been reported for MR-linacs. For proton beam therapy, the performance of ICs in magnetic fields has not yet been studied. This work characterizes the performance of ICs in proton beam therapy in the presence of external magnetic fields. Material and Methods A resistive dipole magnet with a pole gap of 12.5cm provided a magnetic field oriented orthogonal to the beam path. Magnetic field strengths were verified to be within 4 mT of the target value using a portable hall probe. The magnet was placed at the isocenter of a horizontal research beam line. A thimble-type Farmer chamber (TM300013, 0.6mm³, PTW, Freiburg, Germany) and a plane-parallel Roos chamber (TM34001, 0.35cm³, PTW, Freiburg, Germany) were investigated. Read out was performed using a UNIDOS webline (T10021, PTW, Freiburg, Germany). The