S2396
Physics - Quality assurance and auditing
ESTRO 2026
and specificity of our PSQA workflow using the PTW Octavius 4D / VeriSoft system on a Varian TrueBeam 6 MV FF for the detection of clinically relevant dosimetric deviations in SAbR plans. Material/Methods: The SEAFARER team developed a SAbR case (20 Gy in one fraction) adapted to our linac : a Varian TrueBeam 6 MV WFF equipped with a Millennium 120 MLC. Eleven modified plans were generated by introducing intentional modifications (MLC, MU, rotation error, dose and geometric shifts) to simulate delivery uncertainties near tolerance limits.PSQA of the reference plan was prepared on the PTW Octavius phantom with the SRS+ insert and the 1600 SRS detector array, employing a 1,25 mm calculation grid. All twelve plans were delivered and measured on the linac and compared to the reference plan calculations using a global gamma analysis with 2% and 2mm criteria and a 10% dose threshold.Dosimetric deviations were quantified as the percentage reduction in target coverage (PTV D90) and the percentage increase in the near-maximum spinal cord (SC) (D0.03cc) for each of the eleven modified plans. Results: The analysis included the 11 modified plans and the original reference plan. Seven of the modified plans were classified as “fail,” while four were validated as “pass.” Among the four passing plans, the DVH parameters (PTV D90% and spinal cord D0.03cc) showed only minor variations compared with the reference plan. However, one plan exhibiting a 7.2% deviation in the spinal cord D0.03cc was not detected by the PSQA. The analysis included the 11 modified plans and the original reference plan. Deviations ranged from − 8.8% to +9.2% for PTV D90%, and from − 14.2% to +37% for spinal cord D0.03cc. Conclusion: The SEAFARER 2025 results demonstrate that Octavius 4D PSQA with a 2 % / 2 mm gamma index provides reliability for spinal SAbR, with excellent specificity and strong sensitivity for detecting clinically significant dosimetric errors.However, the presence of a false negative highlights that the gamma index alone is insufficient for clinical safety. Incorporating DVH-based analysis—especially for the spinal PRV—is essential.This study contributes to international efforts to standardize and quantify PSQA performance in stereotactic radiotherapy. References: [1] SEAFARER – Un nouveau concept pour la validation de l’assurance qualité spécifique au patient en radiothérapie, pour les essais cliniques et la pratique clinique.[2] Lehmann, J., Hussein, M., Barry, MA, Siva, S., Moore, A., Chu, M., ... & Clark, CH (2022). Radiothérapie et oncologie, 171, 121-128. Keywords: SEAFARER, Octavius 4D, Stereotactic radiotherapy
into the existing clinical workflow to continuously validate trajectory log calibration (Figure 2), and the ability to verify treatment fidelity against critical Dose- Volume Histogram (DVH) limits. This advanced integration positions MobiusFX as a central tool in the evolution towards fully adaptive and personalized radiotherapy.
Figure 2: EPID image of MLC QC image (left) and log- file fluence map (right) as a means of validating machine log calibration References: 1.Simon Stevens et al., “IPEM Topical Report: Guidance for the Clinical Implementation of Online Treatment Monitoring Solutions for IMRT/VMAT,” Physics in Medicine and Biology 68, no. 18 (September 21, 2023). 2.Christina E. Agnew, Denise M. Irvine, and Conor K. McGarry. Correlation of phantom-based and log file patient-specific QA with complexity scores for VMAT. 15(6):204–216. Publisher: Journal of Applied Clinical Medical Physics. Keywords: On-treatment monitoring, MobiusFX, Trajectory-log Experience feedback on the assessment of PSQA Sensitivity and Specificity for Spinal SAbR in the SEAFARER International Program Hadjar Rached 1 , Fatiha Benlakhdar 1 , Sihem Aizeti 1 , Mohamed Amine Benseddik 1 , Laure Vieillevigne 2,3 , Sybelle Deloule 4 , Merieme Cheriguene 1,5 , Abdelbaki Boukerche 1,5 1 Radiation therapy, EHSO Amir Abdelkader CAC d’Oran, Oran, Algeria. 2 Radiation therapy, Centre de Recherches en Cancérologie de Toulouse, Toulouse, France. 3 Medical Physics, Univ Toulouse, oncopole Claudius Regaud IUCT-Oncopole, Toulouse, France. 4 PTW-France, PTW-France, France, France. 5 Medicine, University of Oran 1, Oran, Algeria Digital Poster 4367 Purpose/Objective: The SEAFARER international program [1,2] offers a unique opportunity to benchmark and evaluate the performance of PSQA systems in SAbR. Our objective was to share our experience and assess the sensitivity
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