ESTRO 2025 - Abstract Book

S3619

Physics - Quality assurance and auditing

ESTRO 2025

particularly small considering the actual ±1 mm tolerance for weekly MLC QC tests. Based on this analysis, it would be possible to establish a lower tolerance of 0.5 mm, as currently recommended by guidelines [3].

Keywords: MLC, Statistics, Behavior charts

References: [1] MEYERS, SM, Balderson MJ, Létourneau D. Evaluation of Elekta Agility multi-leaf collimator performance using statistical process control tools. Med Phys., 2019, 20:100-108. [2] PAWLICKI, T., Whitaker, M. and Boyer, A.L. Statistical process control for radiotherapy quality assurance. Med. Phys., 2005, 32: 2777-2786. [3] SEFM. Guide for quality control and safety in clinical use linear accelerators. Rev Fis Med,. 2024; 25:77-122.

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Mini-Oral EPID-based in-vivo dosimetry for online-adaptive radiotherapy of the rectum: feasibility and QA criteria Gary Razinskas, Robert Schindhelm, Johannes Kraft, Marcus Zimmermann, Jörg Tamihardja, Andrea Wittig Sauerwein, Sonja Wegener Department of Radiation Oncology, University Hospital Würzburg, Würzburg, Germany Purpose/Objective: Cone-beam CT (CBCT)-based online-adaptive radiotherapy (oART) enables personalized treatment plans tailored to daily anatomical variations, improving dose delivery precision. Current quality assurance (QA) methods, however, rely heavily on vendor-provided secondary dose calculations, which cannot independently verify beam delivery accuracy, detect synthetic CT errors, or identify intra-fractional anatomical changes. In-vivo dosimetry (IVD) using electronic portal imaging devices (EPID) offers a measurement-based QA approach for online-adaptive workflows. This study evaluates the feasibility and performance of EPID-based IVD in CBCT-based online-adaptive radiotherapy for rectal cancer and its effectiveness in monitoring beam delivery and patient-specific parameters. Material/Methods: Transmission EPID images were analyzed from 10 patients (50 fractions) with locally advanced rectal cancer undergoing neoadjuvant CBCT-based, AI-assisted oART on an Ethos linear accelerator (Varian Medical Systems, Palo Alto, CA, USA). Each patient received hypofractionated short-course radiotherapy delivered as multifield intensity modulated radiotherapy (IMRT) with a prescribed fraction dose of 5 Gy. EPID exit beam images were evaluated per treatment field using SunCHECK (Sun Nuclear Corporation, Melbourne, FL, USA), comparing the measured fluence to a predicted fluence map generated through the forward propagation method. Gamma pass rates (GPR) were calculated under varying criteria (3%/3mm to 2%/2mm) and low-dose thresholds (10%-20%). Angular dependencies were assessed through in-air and in-phantom test plans. Anatomical changes, such as intra-fractional gas accumulation, were analyzed for their impact on dose distribution and gamma results. Results: IVD for oART treatments achieved consistently higher GPR than reported for standard image-guided radiotherapy [1]. Using 3%/3mm and a 20% threshold, the average GPR across patients was 95.7% ± 1.8%. Inter-patient variability ranged from 91.4% to 98.0%. Session-specific variability correlated with anatomical changes like gas accumulation, which reduced GPR but had minimal impact on dose metrics (e.g., deviations in target volume D 98% <2%). GPR showed no temporal trends but varied across fields, with reduced rates for lateral and anterior-posterior beam angles (Figure 1). Angular dependencies, confirmed through phantom and in-air measurements, highlighted geometric and beam-modeling contributions to the observed variations.