ESTRO 2025 - Abstract Book

S2590

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

References: Wright et al.“Non-Conventional Ultra-High Dose Rate (FLASH) Microbeam Radiotherapy Provides Superior Normal Tissue Sparing in Rat Lung Compared to Non-Conventional Ultra-High Dose Rate (FLASH) Radiotherapy.”Cureus,2021 https://doi.org/10.7759/cureus.19317 Potez et al.“Effects of Synchrotron X-Ray Micro-Beam Irradiation on Normal Mouse Ear Pinnae.”Int.J.Radiat.Oncol.Biol.Phys.,2018 https://doi.org/10.1016/j.ijrobp.2018.02.007 Trappetti et al.“Towards Melanoma in Situ Vaccination with Multiple Ultra-Narrow X-Ray Beams”Cancer Lett.,2024 https://doi.org/10.1016/j.canlet.2024.217326 Laissue et al.“Tolerance of Normal Rabbit Facial Bones and Teeth to Synchrotron X-Ray Microbeam Irradiation.”Radiat.Res.,2021 https://doi.org/10.1667/RADE-21-00032.1

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Poster Discussion Online 2D dose verification required in UHDR-FLASH preclinical research: challenges and possibilities Verdi Vanreusel 1,2 , Stephen Brown 3,4,5 , shujat Ali 5 , Dirk Verellen 2,6 , Luana de Freitas Nascimento 1 1 RDA, SCK CEN, Mol, Belgium. 2 AReRO, University of Antwerp, Antwerp, Belgium. 3 Radiation Oncology, Henry Ford Health, Detroit, USA. 4 Radiation Oncology, Michigan State University, Detroit, USA. 5 Medical Physics, Wayne State University, Detroit, USA. 6 Medical Physics, Iridium Netwerk, Wilrijk, Belgium Purpose/Objective: The increasing interest in FLASH-RT has lead to both the development of new, and the conversion of existing linear accelerators to enable ultra-high dose rate (UHDR) irradiations for preclinical research. Dosimetry in this setting remains challenging with several crucial aspects missing. This work shows the challenges and need for real-time 2D UHDR dosimetry, and presents a solution in the context of preclinical irradiations in a non-homogeneous electron beam. Material/Methods: An experimental camera-scintillation combination, and radiochromic film, were used to investigate the spatial dose distribution of two UHDR electron LINACS: a converted Varian Trilogy™ (16 MeV; 25x25 cm²) and a Sordina IORT Technologies ElectronFlash (9 MeV; Ø=12 cm). The challenges of real-time 2D dosimetry with scintillating coatings were assessed by irradiation of 1) an ex-vivo rat brain with the Varian Trilogy™, with the skull at an SSD of 80 cm, using a field size of 2x2 cm² and delivering 30 pulses of 0.39 Gy at an average dose rate (DR) of 44.9 Gy/s; and 2) a 3D printed mouse phantom with the ElectronFlash, using an SSD of 138.7 cm, a circular field with a diameter of 120 mm and delivering 10 pulses of 1.98 Gy at a DR of 198 Gy/s. Results: The field homogeneity, assessed using film is shown in Figure 1 for the gaussian shaped Varian Trilogy™ field and the flattened ElectronFlash. The coating showed a linear dose response in the flattened field (R²>0.999) and linear response with pulses in the gaussian field (R²=0.998). Calibration in the non-homogeneous field (Trilogy™) is challenging and requires a 2D reference dosimeter and perfect alignment. As an example, the intensity difference between 2 regions of interest, separated by 3 cm, is 18.31% in the gaussian field (Trilogy™) and 0.65% in the flattened field (ElectronFlash). The ex-vivo use of the flexible coatings is shown in Figure 2. The left panel shows the rat brain with coating (top), overlayed with the signal (middle) and with overlying film (bottom). The right panel shows the 3D printed mouse phantom (top), with the flexible coating (middle), overlayed with the signal (bottom). The flexible coating was capable of assessing the signal per pulse in each point. Rotation of the rat resulted in a maximal response difference of 3.2%.