ESTRO 2025 - Abstract Book

S3608

Physics - Quality assurance and auditing

ESTRO 2025

1241

Digital Poster Dose response, ghosting and QA Performance in Portal Dosimetry for Varian Halcyon and Ethos LINACs Stefan Menger, Gary Razinskas, Anne Richter, Robert Schindhelm Department of Radiation Oncology, University Hospital Würzburg, Würzburg, Germany Purpose/Objective: Portal dosimetry (PD) on the Halcyon and Ethos (Varian) LINACs is a valuable QA solution for IMRT and VMAT utilizing the onboard electronic portal imaging device (EPID). Integrated within the Aria system, PD allows for comprehensive dose-image comparison and gamma evaluation, making it the standard QA solution in our clinic for the Halcyon system. However, we observed that PD outcomes might depend on saturation and ghosting effects of the EPID panel during clinical workflows. This study systematically investigates the performance of two distinct EPIDs through controlled flood field irradiations. Material/Methods: We generated a Halcyon 6X FFF treatment plan in Eclipse (V18.0), consisting of 50 identical treatment fields (25 × 25 cm²) with 50 monitor units (MU) per field. This plan was then converted into a verification plan to calculate the predicted dose per field. The treatment plan was irradiated three consecutive times on both Halcyon and Ethos LINACs in machine QA mode, one hour after the last treatment of the day. Calculation and measurements were evaluated in ARIA’s Portal Dosimetry module using a global gamma criterion of 1%/1mm and an 80% threshold to exclude penumbra effects. Additionally, signal difference between calculated and measured field center signals were extracted from the DICOM data.

Results: