S2388
Physics - Quality assurance and auditing
ESTRO 2026
limit. A comparable shift in the results was observed for the median dose measured with the Delta4+, the average GPR was 94.3 ± 6,4 %. The largest differences between TPS-calculated and IC-measured doses were found for prostate plans. For larger PTV volumes, the differences between the TPS and IC were smaller. However, the opposite trend was observed for the Delta4+ gamma analysis, with the lowest passing rates obtained for the largest volumes. Despite the variation in treatment techniques employed, analogous observations were recorded.Table 1 Summary of QA verification results.
locations. The ratio of the planning target area to the film area was not constant at all film locations therefore, investigation into the effect of film area on gamma pass rate is required to harmonize the gamma analysis between film positions. The results indicated that the multiple met treatment plans returned lower passing rates. Independently quantifying dose offsets and spatial offsets was necessary as an alternative to gamma analysis. Keywords: Stereotactic Radiosurgery, Dosimetry Digital Poster 3860 Systematic dose shift detected in QA results of Ethos verification plans Krzysztof Doma ń ski, Paulina Weso ł owska, Bart ł omiej Sadowski, Anna Zawadzka, Marta Fillmann Medical Physics Department, Maria Sk ł odowska-Curie National Research Institute of Oncology, Warsaw, Poland Purpose/Objective: The Ethos radiotherapy system was introduced for patient treatment at our hospital in 2024. The machine and the treatment planning system (TPS) were commissioned in accordance with manufacturer and international guidelines. The commissioning confirmed good TPS–accelerator agreement and readiness for clinical use. This study aims to present the differences between TPS-calculated and measured doses for clinical IMRT and VMAT Ethos plans. Material/Methods: A 65 treatment plans (53 IMRT and 12 VMAT) were generated in the Ethos TPS using 6MV FFF photon beams (28 prostate, 18 lung, 11 gynecology, 5 bladder, 3 other) in accordance with predefined clinical protocols. All plans were verified using ionization chamber (IC) measurements in Solid Water HE (Sun Nuclear) and a Delta4+ (ScandiDos) phantom. For IMRT, point dose measurements were performed at a 0° gantry angle in a Solid Water HE (Sun Nuclear) with a Semiflex 3D IC (PTW) at 10 cm depth; VMAT plans were measured in a Mobius Verification Phantom (MVP) using same IC preserving plan geometry. Both phantoms had assigned in TPS material – water. All plans also underwent Delta4+ measurements with original geometry. The mass density of the Delta4+ was assigned as 1.1745 g/cm3 and the matrix was calibrated against measurements using Semiflex 3D IC [1]. Local gamma passing rate (GPR) was determined using criteria of 3%/3mm, 20% threshold and 3% median dose difference. Results: The results are presented in Table 1, the mean difference between TPS and IC measurements was - 1.6 ± 0.8 % (max-2.9%). All deviations were within 3%
Conclusion: The findings demonstrated a systematic
overestimation of the TPS calculated dose, which varies depending on the treatment site/PTV volume and was highest for smaller targets. The overestimation was within the 3% acceptance criterion for all evaluated plans, however inconsistencies were detected between measurements with the Delta4+ phantom and IC results. These findings highlight the importance of multi-detector verification to ensure accurate patient-specific QA. References: [1] Density scaling of phantom materials for a 3D dose verification system, Radiation Oncology Physics, 5 April 2018, DOI: 10.1002/acm2.12357 Keywords: Ethos, multi-detector QA Quality assurance of the prospective randomized phase II trial STEREOPAC for borderline resectable pancreatic cancer: results of the Belgian Dummy Run Christelle Bouchart 1,2 , Sara Q Poeta 3 , Ines Joye 4,5 , Geneviève Van Ooteghem 6,7 , François Lallemand 8 , Karin Stellamans 9 , Nicolas Christian 10 , Maxime Deslé 11 , Fréderic Duprez 12,13 , Jens Van Loon 14 , Mark De Ridder 14 , Lorraine Donnay 15 , Arnaud Dhenin 15 , Thomas Guiot 16 , Ana Veron Sanchez 17 , Jean-Luc Van Laethem 18 1 Radiation Oncology, HUB Institut Jules Bordet, Brussels, Belgium. 2 Laboratory of Experimental Gastroenterology, Université Libre de Bruxelles, Brussels, Belgium. 3 Physic, HUB Institut Jules Bordet, Brussels, Belgium. 4 Radiation Oncology, Iridium Netwerk, Antwerp, Belgium. 5 MIPRO, University of Antwerp, Antwerp, Belgium. 6 Radiation Oncology, Cliniques universitaires Saint-Luc, Brussels, Belgium. Digital Poster 3979
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