S1659
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
accurate in vivo dosimetry remains challenging due to detector saturation and recombination. Alanine dosimeters and Gafchromic EBT-XD films have each proven suitable under FLASH conditions, but their combined in vivo use has not been validated. This study evaluates the accuracy and potential dose perturbation of hybrid alanine–EBT-XD assemblies for preclinical FLASH experiments in mini pigs. Material/Methods: Custom in vivo dosimeter stacks were used, combining alanine pellets with EBT-XD films to mimic realistic in vivo skin placement conditions. Dosimeters were calibrated under conventional and FLASH electron beams, traceable to the absorbed dose to water. Irradiations were performed using a 9MeV electron beam from the FLASHKNife (Theryq, France) in conventional and FLASH modes at 10Gy and 20Gy using 2cm and 5cm applicators. A buildup layer of 16mm water equivalent thickness was placed on top of setup to ensure that measurements were taken at the depth of maximum dose determined from the percentage depth dose curve.Doses recorded by EBT- XD and/or alanine were compared with simultaneous reference measurements obtained from a FLASH diamond detector (PTW, Freiburg, Germany). Phantom measurements assessed potential skin-interface dose perturbations from the combined detectors. Results: All detectors showed a near-linear dose response in Conventional (monitor units) and FLASH (pulse number and width) modes. As the study focuses on inter-detector agreement, these results are omitted. Across all investigated conditions (10–20Gy; 2cm and 5cm applicators), hybrid alanine–EBT-XD measurements demonstrated a mean relative difference of 1.6% ±1.7% compared with the diamond reference. The largest deviation (−3.8%) occurred for the 2cm, 10Gy FLASH configuration, likely due to steeper surface dose gradients and increased lateral electron scatter within the smaller field, which amplify positioning sensitivity of the detectors. All other measurements remained within ±2% of the diamond- only baseline. The diamond detector exhibited stable readings under all configurations. Figure 1 shows the results for the different conditions. Our results confirmed that the presence of the alanine and film layers did not measurably perturb the local dose distribution.
Figure A: Planning CT scan with water inserts in place (no detectors present)Figure B: MR image acquired during treatment set-up (central farmer chamber insert not in place) Conclusion: The water-filled phantom addresses key limitations of MRI-Linac audits by providing MR visibility, water- equivalent conditions, and compatibility with existing dosimetry tools. It enables robust evaluation of adaptive workflows and supports consistent intercomparison across centres, contributing to the safe and effective implementation of MRI-guided
radiotherapy. References:
Jacco de Pooter et al. (2021). Reference dosimetry in MRI-linacs: evaluation of available protocols and data to establish a Code of Practice. Phys. Med. Biol. 66 05TR02. Keywords: MRI-Linac, dosimetry audit, adaptive radiotherapy
Digital Poster 2915
Evaluation of hybrid alanine-EBT-XD film in vivo dosimeters for preclinical FLASH radiotherapy Dorien De Ridder 1,2 , Iris Apale 1,3 , Milan Compernolle 4 , Brigitte Reniers 4 , Racell Nabha 1,3 , Marlies Boussaer 1,3 , Sven Van Laere 1,3 , Mark De Ridder 1,3 , Jan Gutermuth 2 , Thierry Gevaert 1,3 1 Department of Radiotherapy, UZ Brussel, Brussels, Belgium. 2 Department of Dermatology, UZ Brussel, Brussels, Belgium. 3 Research Centre for Digital Medicine, Vrije Universiteit Brussel, Brussels, Belgium. 4 NuTeC, Uhasselt, Hasselt, Belgium Purpose/Objective: FLASH radiotherapy offers strong potential for sparing normal tissue while maintaining tumor control. Yet,
Made with FlippingBook - Share PDF online