S2379
Physics - Quality assurance and auditing
ESTRO 2026
large treatment volume, leading to field overlap regions that must be precisely verified to ensure dose uniformity and patient safety. This work describes a standardized and comprehensive quality control (QC) protocol developed for the evaluation and verification of the dose distribution in the overlap regions of TBI- VMAT treatments. Material/Methods: The global treatment plan was divided into contiguous isocenter pairs, each defining a specific overlap region (positions A, B, etc.). Verification plans were created within the Eclipse treatment planning system using the Octavius II phantom. Each treatment field was calculated independently, and the corresponding dose distributions were exported for comparison. Gafchromic films were positioned between solid water slabs inserted in the Octavius II phantom with the homogeneous TC phantom base, according to pre- defined overlap distances.After irradiation on the linear accelerator, the films were digitized using the SilverFast scanning software under controlled conditions to ensure reproducibility. The DoseLab software was used to convert the mean pixel value (MPV) to optical density (OD), and subsequently to absorbed dose (cGy) through calibration curves priorly. Measured and calculated dose maps were aligned and compared using gamma analysis criteria using DoseLab. Results: The implemented QC procedure allows for precise evaluation of dose uniformity in field overlap regions. Preliminary results showed a high agreement between measured and calculated dose distributions using gamma analysis (3%, 3mm), confirming the accuracy of field matching and the reliability of the VMAT delivery for TBI. Conclusion: This protocol provides a robust and reproducible framework for quality assurance in field overlaps in VMAT-based TBI treatments. It enhances treatment accuracy, safety, and reproducibility, supporting clinical implementation in complex multi-isocenter radiotherapy treatment plans. The methodology is also adaptable to other extended-field techniques requiring junction or overlap verification. References: DoseLab Pro, User manual v6.3, MOBIUS Medical Systems.Mancosu et al., In-vivo dosimetry with Gafchromic films for multi-isocentric VMAT irradiation of total marrow lymph-nodes: a feasibility study, Radiation Oncology (2015) 10:86.Pierce et al., Extended SSD VMAT treatment for total body irradiation, J Appl Clin Med Phys 2019; 20:1:200– 211.Cananoglu et al., Planning and dosimetric evaluation of three total body irradiation techniques: Standard SSD VMAT, Extended SSD VMAT and Extended SSD Field-in-Field, Radiat Environ Biophys 62,
Conclusion: This QA framework successfully validated the clinical sCT, demonstrating accurate dose reproduction and full compliance with gamma and dose guidance criteria. These results support the safe integration of the clinical sCT into daily MRI-only oAMRgRT workflows.The research leading to these results has received funding from AIRC under MFAG 2024 - ID. 31024 project. Keywords: synthetic CT, online QA, oAMRgRT Total Body Irradiation with VMAT: A Quality Control Protocol for Field Overlap verification A. G. Dias 1,2 , S. Gonçalves 1,2 , L. T. Cunha 1,2 , D. Pinto 1,2 , J. Lencart 1,2 1 Medical Physics Department, Portuguese Oncology Institute of Porto (IPO-Porto), Porto, Portugal. 2 Medical Physics, Radiobiology and Radiation Protection Group, IPO Porto Research Centre (CI-IPO), Portuguese Oncology Institute of Porto (IPO-Porto), Porto Comprehensive Cancer Center (Porto.CCC) & Rise@CI- IPOP (Health Research Network), Porto, Portugal Digital Poster 3363 Purpose/Objective: Total Body Irradiation (TBI) is a key component in conditioning regimen prior to hematopoietic stem cell transplantation. The adoption of Volumetric Modulated Arc Therapy (VMAT) for TBI enables improved dose conformity and enhances patient comfort but requires multiple isocenters due to the
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