S1638
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Switzerland. 2 PTW-Freiburg, PTW-Freiburg, Freiburg, Germany. 3 Medscint Inc, Medscint Inc, Quebec, Canada Purpose/Objective: Ultra-high-dose-rate (UHDR) radiotherapy (RT), referred to as Flash-RT, represents a novel treatment modality with the potential to widen the therapeutic window by enhancing normal tissue sparing while maintaining tumor control. Translating this technique into clinical practice poses challenges in monitoring and recording the delivered dose. This work presents the development and characterization of an in-vivo dosimetry methodology that combines passive and active detectors, which was investigated and then applied for dose recording and reporting for the prospective Skin-Flash I clinical trial (NCT06549439). Material/Methods: In the Skin-Flash I, seven patients were treated using a modified C-arm linear accelerator (VarianTrueBeam with FLEX extension) capable of delivering UHDR electron beams (9 MeV, 1.08 Gy per pulse, mean dose rate of 216 Gy/s). Four complementary detector systems were assessed to ensure accurate dose verification during treatment. The two passive devices were optically stimulated luminescence detectors (OSLDs) and radiochromic films. The active devices were a plastic scintillator and a diamond detector. As shown in Figure 1, the detectors were either located directly on the patient’s skin (film) or positioned on a 3D-printed mount to record integrated (OSLDs) and time-resolved dose (scintillator, diamond). The OSLDs were calibrated against a reference ionization chamber and tested under different irradiation settings, varying gantry angle, dose-per-pulse and number of pulses. The active detectors assessed real- time beam stability: the scintillator monitored pulse- to-pulse variations, while the diamond was also able to capture intra-pulse fluctuations.
useful for identifying potential beam irregularities in case of suboptimal beam tuning. Finally, film dosimetry was confirmed to be a valuable and well established method to measure skin dose and field size, which passed >95% the 5%/3mm gamma analysis comparison with the planned dose for all delivered fractions.
Figure 2. Pulse-by-pulse dose accumulation. Conclusion:
This study reports a reliable method to integrate passive and active dosimetry systems to monitor UHDR dose delivery in-vivo. Although this multi- detector setup requires relevant resources and coordination, it provides redundancy, temporal resolution, and robustness, representing a practical pathway toward early clinical implementation of UHDR radiotherapy. Keywords: FLASH, Melanoma, UHDR Digital Poster 1493 Jaw-collimated rectangular small field output factors Antonella Fogliata 1 , Luca Pellegri 2,3 , Antonella Stravato 4,5 , Luca Cozzi 6 1 Radiotherapy and Radiosurgery, Humanitas Research Hospital IRCCS, Milan-Rozzano, Italy. 2 Radiotherapy, CHU Tivoli, La Louvière, Belgium. 3 Physics, University of Liège, Liège, Belgium. 4 Medical Physics, A.O. San Giovanni-Addolorata, Rome, Italy. 5 Medical University, UniCamillus University, Rome, Italy. 6 Ra, Humanitas Research Hospital IRCCS, Milan-Rozzano, Italy Purpose/Objective: The accuracy of the small field output factor (FOF) has gained increasing interest. According to the IAEA Code of Practice TRS-483, precise estimation of the equivalent square field size (ESF) is crucial for FOF measurement in small fields. Recently, a formula for ESF applicable to elongated fields was published [1], based on MLC-shaped fields, addressing the limitations of TRS-483 [2], which pertains mainly to nearly square fields. This work aims to evaluate the ESF formula’s applicability to rectangular small fields and very elongated jaw-collimated fields, where collimator backscatter significantly impacts results (e.g. 1x40 cm2 field).
Figure 1. Schematic view of the setup. Results: Using twelve chips OSLD consistently yielded an agreement with the ion chamber with an uncertainty of ±2.9% (k=1).Therefore, OSLDs were the primary detector for dose reporting. The scintillator and diamond detector readings for pulse-to-pulse stability agreed within ±1%, enabling reconstruction of intra- fraction dose accumulation (Figure 2). The diamond detector enabled assessing the intra-pulse stability,
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