S1699
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
and reducing handling time without compromising dosimetric accuracy.Monte Carlo validation supports the clinical feasibility of this substitution, representing a practical optimization for monthly imaging QA workflows. Keywords: Monte-Carlo Simulation, kV-Imaging, Equivalence Digital Poster 4938 Scintillation-based real-time dosimetry system for FLASH radiotherapy applications Brett Velasquez 1 , Shannon Holmes 2 , Emil Schueler 1 , Sam Beddar 1 1 Radiation Physics, MD Anderson Cancer Center, Houston, Tx, USA. 2 Standard Imaging, Standard Imaging, Middleton, Wi, USA Purpose/Objective: Ultra-high dose rate (FLASH) radiotherapy requires dosimetry systems capable of accurate and real-time measurements under extreme beam conditions. Conventional detectors fail at these dose rates due to saturation and slow response. This work is aimed to develop and characterize a scintillation- based dosimetry system using BCF12 scintillators and silica optical fibers with improved electronic interface for precise dose measurement in FLASH beams. Material/Methods: Prototype detectors were constructed using 1.5 × 1 mm BCF12 scintillators. 1 mm diameter silica optical fibers were used for improved radiation hardness compared with PMMA. Signal acquisition employed fast photodiodes and high-speed electronics. Characterization was performed using both 7 MeV and 16 MeV electron FLASH beams. Key parameters evaluated included dose per pulse (0.2–48 Gy/pulse), pulse repetition frequency (10–180 Hz), pulse width (0.5–4 μs), and accumulated dose up to 28.5 kGy. Linearity, temporal resolution, and radiation damage recovery were assessed. Results: The scintillator dosimetry system demonstrated linear response to dose per pulse up to 48 Gy/pulse (R² > 0.99) and stable performance across PRF values from 10 to 180 Hz. No dependence on pulse width was found, with consistent behavior across the tested range. Integrated dose response remained linear for both low and high dose-per-pulse settings. Initial signal degradation was observed at −1.86%/kGy for the blue channel, with partial recovery (17%) after three days. Radiation damage to silica fibers was significantly lower than PMMA fibers, supporting their use for long-term FLASH applications. The electronics resolved individual pulses with sub-microsecond
temporal resolution, enabling real-time monitoring. Conclusion: BCF12 scintillators coupled with silica optical fibers provides a robust, linear, and high-speed dosimetry solution for FLASH radiotherapy. Their performance across a wide range of beam parameters and their radiation hardness positions this system as a promising candidate for both machine QA and future in vivo dose verification in clinical FLASH treatments. Keywords: FLASH RT, scintillator, real-time dosimetry Mini-Oral 4962 Fetal dose estimation in breast photon radiotherapy during pregnancy: first experimental results from the European SONORA project Federica Guida 1 , Mercedes Horvat 2 , Hrvoje Brkić 3,4 , Francesca De Monte 1 , Marijke De Saint Hubert 5 , Vladimir Dufek 6 , Mladen Kasabašić 3,7 , Željka Knežević 2 , Marija Majer 2 1 Medical Physics Department, Veneto Institute of Oncology IOV – IRCCS, Padua, Italy. 2 Radiation Chemistry and Dosimetry Laboratory, Ruđer Bošković Institute (RBI), Zagreb, Croatia. 3 Faculty of Medicine (MEFOS), Department of Biophysics and Medical Physics, J. J. Strossmayer University Osijek, Osijek, Croatia. 4 Faculty of Dental Medicine and Health, Department of Biophysics, Biology and Chemistry, J. J. Strossmayer University Osijek, Osijek, Croatia. 5 Belgian Nuclear research centre, SCK CEN, Mol, Belgium. 6 Medical exposure section, National Radiation Protection Institute- SURO, Prague, Czech Republic. 7 Department of Medical Physics, University Hospital Center Osijek, Osijek, Croatia Purpose/Objective: Accurate fetal dose assessment and optimization are essential in radiotherapy (RT) treatments for pregnant patients to balance maternal benefit against potential fetal risk. However, lack of harmonization of clinical practice results in large differences in this estimation. Within the European SONORA project (Towards safe, optimized and personalized radiology and radiotherapy procedures for pregnant patients), Working Package 4.1 focuses on estimating fetal dose during breast RT for photon and proton therapy techniques in all stages of pregnancy. This study reports the first experimental results for photon RT obtained using a realistic second-trimester phantom. Material/Methods: An anthropomorphic phantom representing a pregnant woman at 18th week of gestation (TENA-II) was developed from CT and MRI scans of a real patient [1]. Following clinical guidelines, organs at risk and target volumes (whole left breast (WB), breast boost (BB), and breast + supraclavicular lymph
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