ESTRO 2025 - Abstract Book

S2613

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

2722

Digital Poster MR-guided proton therapy: Suitability of semiconductor detectors for reference dosimetry Hermann Fuchs 1 , Eleonora Paolocci 1,2 , Hugo Palmans 3,4 , Dietmar Georg 1 1 Department of Radiation Oncology, Medical University of Vienna, Vienna, Austria. 2 Department of Physics, Sapienza Università di Roma, Rome, Italy. 3 Medical Physics, MedAustron Iontherapy Center, Wr. Neustadt, Austria. 4 Medical Physics, National Physical Laboratory, Teddington, United Kingdom Purpose/Objective: In photon beam therapy, hybrid MR-linear accelerator systems have been integrated into clinical practice over the past several years. Proton beam therapy (PT) allows a higher degree of dose conformality compared to photon beam therapy; consequently, it potentially benefits even more from the incorporation of MR guidance. From a medical physics perspective, establishing reliable dosimetry methods within this complex environment is a prerequisite for advancing pre-clinical and clinical studies. Ion chambers have already been shown to be affected by the magnetic field, requiring additional correction factors. In this work, we focused on the suitability of using solid state detectors for reference dosimetry in magnetic fields. Initial measurements [1] showed a change of response for a semiconductor detector, but the detector was oriented orthogonal to the incident proton beam, which does not reflect the clinical protocols. Material/Methods: Experiments were performed in a synchrotron-based facility. 7 magnetic field strengths (-1, -0.5, -0.25, 0, 0.25, 0.5, and 1 T) were employed using a resistive dipole electromagnet, positioned such that the isocenter of the ion therapy beam line and the magnet iso center coincided. Homogeneous, mono-energetic energy 10x10 cm² fields were irradiated using 97.4, 152, and 252.7 MeV scanned proton beams, respectively. A microDiamond (TM60019) and a microSilicon (TM60023) detector (both PTW, Freiburg, Germany) were positioned in a water phantom at a water equivalent depth of 2 cm oriented in the beam direction. 20 repetitions per energy and field strength were performed. A drift correction was applied for the microSilicon detector. Results: Differences in chamber response due to the presence of a magnetic field were found to be up to 0.3% and 0.9% for the microDiamond and microSilicon detector, respectively (see Figure 1). Both detectors’ change in response with energy was comparable for all situations and found to be very small. The microSilicon detector exhibited a considerable drift during the measurement campaign.