ESTRO 2025 - Abstract Book

S2622

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

Conclusion: EPIbeam demonstrated good performance and sensitivity. It is an effective tool for routine pretreatment verification in radiotherapy.

Keywords: EPID pre-treatment verifications

References: Dogan N, Mijnheer BJ, Padgett K, et al. AAPM Task Group Report 307: use of EPIDs for patient-specific IMRT and VMAT QA. Med Phys. 2023; 50(8): e865-e903

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Digital Poster Characterization of a commercial EPID-based in vivo dosimetry system Marta Casati 1 , Cinzia Talamonti 2 , Livia Marrazzo 2 , Silvia Calusi 1 , Margherita Zani 1 , Chiara Arilli 1 , Antonella Compagnucci 1 , Stefania Pallotta 2 1 Medical Physics Unit, Careggi University Hospital, Florence, Italy. 2 Department of Experimental and Clinical Biomedical Sciences, University of Florence, Florence, Italy Purpose/Objective: This study evaluates the commercial EPID-based dosimetry system EPIgray for 6MV beams, assessing its performance in in-vivo dosimetry under various conditions, including small field sizes and inhomogeneities. The investigation focuses on the system’s repeatability and response in challenging conditions using phantoms. Ten VMAT plans were delivered to a cylindrical phantom to test system performances with clinical plans. Material/Methods: VersaHD LINACs are equipped with iviewGT EPIDs and Dosisoft EPIgray version 2.0.10.20 software for in-vivo dosimetry. Monaco TPS was used to calculate dose-to-medium distributions (predicted dose). EPIgray reconstructs dose at the prescription point and in 50 points randomly spaced in an auto-generated volume (Autovol). The deviation between reconstructed and predicted dose is calculated in the prescription point (DRPp) and averaged over the 50 points in the Autovol (DRP50). Gamma passing rate (3% 3mm global) is computed for the 50 points. The following tests were conducted: • Repeatability with a Quasar Penta-Guide phantom irradiated with a static beam 10cm x10cm and a VMAT plan, delivered to the phantom: 5 times sequentially (short-term) and once a day over 1 month (long-term, 2static and 16VMAT acquisitions). • EPIgray dosimetric accuracy on plastic phantoms (fig.1), with small fields and inhomogeneities: o Solid water (SW) slabs (10cm, 20cm, 30cm thickness) were irradiated with square (2, 3, 4, 5, 10, 15, 20cm field size (FS)). o SW slabs with 2cm lung-equivalent insert were irradiated with 2, 5, 10, 20cm FS; o PMMA cylindrical phantom was irradiated with: ▪ 4 static beams 10cm FS, gantry 270°, 0°, 90° and 180°; ▪ 10VMAT plans.