S2347
Physics - Quality assurance and auditing
ESTRO 2026
the “EPID vs. EPID” tool in Epiqa was used to evaluate the gamma agreement (with 3%/2mm criteria, 10% threshold cutoff) comparing each arc of all sessions from the second onwards to those from the first session. Additionally, the dose was recalculated on the HyperSight CBCT image of each treatment day. Results: Gamma agreement was assessed for each patient and each session, with values exceeding 95% for 94% of the analysed sessions. In the plot below, gamma agreement is shown for one selected patient, for each arc, alongside dose recalculation results.
Conclusion: This study proposed two effective and robust MLC QA integration methods that can reduce partial radiation scattering. When the strip spacing exceeds a certain threshold, the algorithm's positional accuracy is almost unaffected by the strip position or spacing. Keywords: MLC QA, EPID, Picket fence, FWHM
Digital Poster 1508 EPID dosimetry for in-vivo constancy QA
Marco Fusella 1 , Francesca Dusi 1 , Antonella Fogliata 2 1 Radiation Oncology, Abano Terme Hospital, Abano Terme, Italy. 2 Radiotherapy and Radiosurgery, Humanitas Research Hospital IRCCS, Milan-Rozzano, Italy Purpose/Objective: In-vivo quality assurance based on EPID images relies on two transit dosimetry methods, the forward- and back-projection approaches. The forward-projection method compares the in-vivo acquisition with a predicted image obtained from a specific algorithm, while the back-projection method recomputes the dose distribution from the EPID acquisition back into the patient’s CT. Both approaches involve several approximations and uncertainties that can obscure actual variations in the delivered dose. A simpler method to monitor treatment quality in vivo is to compare the in-vivo EPID acquisition of each treatment session (from the second session onward) directly to the first session’s acquisition, rather than to a predicted image. This work aims to analyse this in- vivo constancy approach using the “EPID vs EPID” tool from the Epiqa software for EPID-based PSQA, applied to 10 patients treated on a Halcyon linac for prostate cancer. Material/Methods: Ten patients were selected from the institutional database: all were treated on a Varian Halcyon linac, where EPID images are automatically acquired during the treatment. The patients received adjuvant radiotherapy for prostate cancer, delivering 50 Gy in 25 fractions over the whole pelvis. The treatment involved 3-arc VMAT, preceded by kV-CBCT with HyperSight for accurate positioning. For each patient,
In this case, two sessions exhibited lower agreement. The discrepancy was attributed to an anatomical change (an air pocket in the bowels) visible in the Figure below comparing the planning CT (synthetic CT) and the fraction 6 CBCT (HyperSight). The dose recalculation confirmed reduced accuracy in those sessions, with corresponding gamma evaluations.
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