ESTRO 2025 - Abstract Book

S3609

Physics - Quality assurance and auditing

ESTRO 2025

Figure 1 illustrates the Gamma evaluation for Halcyon (o) and Ethos (x) as a function of cumulative irradiated MU, with each data point representing 50 MUs. Figure 2 shows the median field center signal difference. Three notable characteristics emerged: firstly, a systematic shift to a lower measured signal level with increasing MUs, producing a characteristic ‘hockey stick’ pattern, which secondly partly resets even after brief irradiation breaks. Thirdly superimposed oscillation is visible (with an approximate frequency of 1 min -1 ). Conclusion: This study identifies a strong dependency of the dose response signal on exposure time for PD QA for Varian Halcyon and Ethos LINACs. Other groups observed and investigated similar effects (McDermott2004 (1), McDermott2006 (2)).This behavior directly affects patient specific QA procedures. Additional investigations, including further measurements and insights from Varian regarding EPID calibration and correction of ghosting effects, are necessary to understand and address the underlying causes of these observations fully.

Keywords: PD QA, Gamma evaluation, EPID stability

References: (1) McDermott LN, Louwe RJ, Sonke JJ, van Herk MB, Mijnheer BJ. Dose-response and ghosting effects of an amorphous silicon electronic portal imaging device. Med Phys. 2004 Feb;31(2):285-95. doi: 10.1118/1.1637969. PMID: 15000614. (2) McDermott LN, Nijsten SM, Sonke JJ, Partridge M, van Herk M, Mijnheer BJ. Comparison of ghosting effects for three commercial a-Si EPIDs. Med Phys. 2006 Jul;33(7):2448-51. doi: 10.1118/1.2207318. PMID: 16898447.

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Digital Poster Dosimetry audit in PROTECT – a European randomized phase III-trial comparing proton and photon therapy in esophageal cancer patients Mai L Ehmsen 1 , Camilla S Byskov 2 , Hanna R Mortensen 1,3 , Rebecca Bütof 4 , Richard Canters 5 , Gilles Defraene 6 , Karin Haustermans 6,7 , Maria F Jensen 1 , Arturs Meijers 8 , Christina T Muijs 9 , Ditte S Møller 2 , Marianne Nordsmark 2 , Pieter Populaire 6,7 , Gloria Vilches-Freixas 5 , Lars Nyvang 2 , Liliana Stolarczyk 1 , Sebastian Makocki 4 , Sara Broggi 10 , Francesco Fracchiolla 11 , Andrea Martignano 11 , Kenneth Poels 6 , Séverine Rossomme 12 , Stefan Vasiliniuc 13 , Lone Hoffmann 2 1 Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark. 2 Department of Oncology, Aarhus University Hospital, Aarhus, Denmark. 3 Department of Clinical medicine, Aarhus University, Aarhus,