S2025
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Material/Methods: Ten prostate SABR plans were created using the Eclipse treatment planning system. Conventional VMAT plans were generated with two full arc fields: one rotating clockwise and the other counterclockwise, both using a single fixed collimator angle for each direction. The same cases were replanned using the VC_VMAT technique, where the full arc was divided into maximum nine partial arcs (40° per sector). For each partial arc, the collimator angle was adjusted based on the beam's eye view (BEV) of the PTV and adjacent organs at risk (OARs), resulting in variable collimator angles across the 360° rotation.Dosimetric evaluation of both techniques included target coverage (V100%, D1cc), conformity index (CI), gradient index (GI), and OAR sparing. Plan efficiency metrics such as monitor units (MU), control point complexity, treatment delivery time, and fluence verification via gamma index were also analyzed. Results: Both techniques demonstrated comparable results in terms of PTV coverage, CI, GI, and D1cc values. However, VC_VMAT plans required 24% (±4%) fewer monitor units compared to conventional VMAT plans, likely due to optimized collimator angles that reduced MLC complexity during arc optimization. A reduction in the number of small control points was also observed in VC_VMAT plans.Fluence verification showed high gamma pass rates of 99% (±1%) for both techniques, confirming plan deliverability. Treatment delivery times, measured using the automatic delivery option on the Varian Edge linear accelerator, revealed that VC_VMAT plans required an additional 10 seconds on average compared to conventional VMAT plans. Conclusion: The VC_VMAT technique demonstrated dosimetric equivalence to conventional VMAT while offering significant reductions in monitor units and improved MLC optimization, potentially enhancing the efficiency of linac performance. However, the additional time required to define optimal collimator angles for each partial arc remains a limitation. Automating this process through scripting could facilitate the clinical adoption of VC_VMAT, particularly after further data collection and refinement of workflow processes. References: Fan, J., Wang, L., Jiang, Z., & Zhang, H. (2018). Dynamic collimator angle optimization in VMAT for improved plan quality and delivery efficiency.Medical Physics, 45(6), 2563-2572.Gao, H., Li, X., Chen, L., & Wang, J. (2020). Variable collimator VMAT for head and neck radiotherapy: A dosimetric and delivery efficiency study.Radiotherapy and Oncology, 147, 1-7.Zhang, P., Happersett, L., Yang, Y., Yamada, Y., Mageras, G., & Hunt, M. (2015). Optimization of collimator trajectory in volumetric modulated arc therapy: Development and evaluation for paraspinal SBRT.International
Conclusion: Multi-TPS dose summation of Brachy-like SBRT and VMAT 40Gy/20F achieves favorable HR-CTV coverage while adhere to international OOI criteria. Keywords: Cervical cancer, MRgRT, brachytherapy-like SBRT
Digital Poster 5068
Dosimetric and Delivery Efficiency Comparison of Variable Collimator VMAT versus Conventional VMAT in Prostate SABR Mahmoud Younes, Gvantsa Tonia GenesisCare UK, Cromwell centre, London, United Kingdom Purpose/Objective: This study aims to evaluate the dosimetric impact and deliverability of the Variable Collimator Volumetric Modulated Arc Therapy (VC_VMAT) technique in prostate stereotactic ablative radiotherapy (SABR) plans. Specifically, the study aimed to compare VC_VMAT with conventional VMAT by assessing target coverage, organ-at-risk (OAR) sparing, plan efficiency metrics (such as monitor units and control point complexity), and treatment delivery feasibility on a linear accelerator. The goal was to determine whether VC_VMAT could achieve comparable dosimetric outcomes while improving delivery efficiency through optimized collimator angles.
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