S2392
Physics - Quality assurance and auditing
ESTRO 2026
The corresponding 10° sub-arc TPS dose planes of the patient plan on the same phantom are used to determine ratio matrices between measured and planned dose planes. Each ratio matrix is back- projected through the patient CT along the beam direction of its sub-arc to reconstruct an estimated delivered 3D dose distribution in the patient anatomy [2]. For validation, this method was applied to a published methodology for patient-specific QA benchmarking using a SABR spine plan with a series of ten plans containing artificially introduced linac delivery errors [3]. Each measured plan was reconstructed using the error-free TPS dose as reference. Changes in the DVH metrics PTV D90% and spine PRV D0.03cc were quantified relative to the error-free plan and compared to the expected changes between the TPS dose distributions for the error plans relative to the error-free plan. Results: DVH-based analysis of the EPID-reconstructed dose distributions demonstrated strong correlation with TPS-calculated changes in PTV D90% and spine PRV D0.03cc across all error plans (Figure 1). The measured changes observed in spine PRV D0.03cc deviated slightly from the unity line, possibly reflecting the high sensitivity of near-maximum-dose parameters to local dosimetric variations. Using a tolerance of ±5% change for both DVH metrics to classify plans as pass or fail yielded sensitivity and specificity of 1.0.
replaced filters showing condensation, validating the finding. Conclusion: This study achieved calibration of DICOM external contours and VisionRT-generated surfaces. The proposed QA metrics revealed variations in camera performance, suggesting the need for standardization to improve accuracy. Moreover, roughness analysis identified a faulty camera. These metrics can enhance surface reconstruction accuracy, support standardized QA procedures, and ultimately improve treatment quality and patient safety. References: [1] Al - Hallaq, H. A., Cerviño, L., Gutierrez, A. N., Havnen - Smith, A., Higgins, S. A., Kügele, M., ... & Tomé, W. A. (2022). AAPM task group report 302: surface - guided radiotherapy. Medical physics, 49(4), e82-e112.[2] Freislederer, P., Batista, V., Öllers, M., Buschmann, M., Steiner, E., Kügele, M., ... & Lehmann, J. (2022). ESTRO-ACROP guideline on surface guided radiation therapy. Radiotherapy and Oncology, 173, 188-196.[3] Daneshmand, M., Helmi, A., Avots, E., Noroozi, F., Alisinanoglu, F., Arslan, H. S., ... & Anbarjafari, G. (2018). 3d scanning: A comprehensive survey. arXiv preprint arXiv:1801.08863. Keywords: Surface, noise, similarity Digital Poster 4108 Bridging EPID dosimetry and patient dose: clinically meaningful VMAT delivery verification with DVH analysis Therese S Standen 1 , Natalie Kong 1 , Raj David 2,3 , Jose A Baeza-Ortega 2 , Lauren May 2 , Joerg Lehmann 1,2 , Peter B Greer 1,2 1 Department of Radiation Oncology, Calvary Mater Newcastle Hospital, Newcastle, Australia. 2 School of Information and Physical Sciences, University of Newcastle, Newcastle, Australia. 3 Department of Radiation Oncology, Central Coast Cancer Centre, Gosford, Australia Purpose/Objective: To develop and validate a method for verification of VMAT plans delivered to the EPID and reconstructed in the patient CT dataset, enabling DVH-based analysis of patient plan quality assurance. This will result in clinically meaningful verification of VMAT plan delivery. Material/Methods: The VMAT delivery verification method utilises single- frame DICOM images acquired in-air of the patient plan with the EPID. The single-frame images are summed to create sub-arc integrated images subtending 10°, and converted to dose in a virtual flat water phantom for each sub-arc image using a previously validated EPID fluence-to-dose model [1].
Figure 1 Observed DVH changes in error plans relative to error-free plan for measured versus TPS dose
distributions. Conclusion:
Current patient-specific VMAT verification techniques have been shown to lack sensitivity and specificity to significant dosimetric errors [3]. The proposed DVH- based EPID verification provides a clinically meaningful, efficient, and automatable approach for comparing measured and planned dose distributions, enhancing verification of clinical plans as well as auditing and benchmarking of centres and techniques. References: 1. King BW, Morf D and Greer P B 2012. Development and testing of an improved dosimetry system using a backscatter shielded electronic portal imaging device
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