ESTRO 2025 - Abstract Book

S3675

Physics - Quality assurance and auditing

ESTRO 2025

Germany. 7 Department of Radiation Oncology, University Medical Center, Freiburg, Germany. 8 Saphir Radiosurgery, Saphir, Kiel, Germany. 9 Clinic and Polyclinic for Radiotherapy and Radiation Oncology, LMU, Munich, Germany. 10 'Department of Biomedical Sciences, Radiology & Radiotherapy Sector, University of West Attica, Athens, Greece Purpose/Objective: We evaluated the infrastructure and protocols for single-isocenter multi-target (SIMT) stereotactic radiosurgery (SRS) across 23 radiotherapy centers in Germany, Austria, and Switzerland. Our analysis focused on the accuracy of dose delivery and the clinical quality of treatment plans. The aim is to provide a comprehensive overview of current practices and establish a foundation for future recommendations and guidelines to ensure the safe and effective implementation of SIMT-SRS. Material/Methods: At each center, we conducted an end-to-end test using a 3D-printed, anthropomorphic head phantom equipable with various detector inserts: radiochromic films, TLDs and polymer gel (MRI-evaluated). Based on a reference CT, five target structures representing brain metastases were delineated. All 23 centers conducted CT imaging, co registration with the reference CT, treatment planning, and treatment delivery, following their in-house SIMT-SRS protocols. To assess the accuracy of dose delivery, we segmented and compared the prescription isodose levels between the measured and planned doses and evaluated centroid distances, Hausdorff distances, and Dice coefficients. Additionally, conformity and gradient indices were calculated for each target volume. The standardized reference case allowed for a direct comparison of dose profiles across centers, enabling them to evaluate the clinical plan quality and performance within the study group. We gathered the parameters describing the infrastructure and SIMT-SRS protocol at each center and performed statistical analyses to identify whether specific parameters or techniques significantly influenced the achieved results.

Results:

In the pattern-of-care analysis, we found consensus (agreed by at least 80% of centers) for three parameters: non coplanar treatment, 6 degree-of-freedom setup correction capability and a CT slice thickness of 1mm or lower. The other parameters, including intrafractional imaging use, showed higher variability. Comparing measured to planned dose across all centers, 88% of targets were irradiated within our test uncertainty plus individual PTV margins. The