S1831
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Digital Poster 1387 Dosimetric evaluation of the impact of a frameless immobilization system in brain Linac-based stereotactic radiotherapy Denis Panizza 1,2 , Valeria Faccenda 1,2 , Stefano Arcangeli 2,3 , Elena De Ponti 1,2 1 Medical Physics, Fondazione IRCCS San Gerardo dei Tintori, Monza, Italy. 2 School of Medicine and Surgery, University of Milan Bicocca, Milan, Italy. 3 Radiation Oncology, Fondazione IRCCS San Gerardo dei Tintori, Monza, Italy Purpose/Objective: Frameless immobilisation systems are increasingly adopted in cranial stereotactic radiotherapy to enhance patient comfort and streamline clinical workflow. However, their structural components, particularly midline mask attachments, may introduce beam attenuation and affect dose accuracy at the isocentre. This study aimed to quantify the dosimetric impact of a frameless immobilisation system with midline thermoplastic mask fixation and to determine whether specific beam entrance angles should be avoided during treatment planning. Material/Methods: The Solstat (A) and Solstice (B) systems (CQ medical) with the Lucy phantom (Standard Imaging) were used to simulate a patient’s head setup. The systems comprised a rigid Kydex-T (A) and carbon fibre (B) head support together with a polycarbonate frame securing the reinforced thermoplastic mask along the midline. Five coplanar SRS plans were recalculated in Monaco using the Monte Carlo algorithm (Elekta) at the centre of a cylindrical volume (0.015 cc) positioned at the phantom isocentre. By comparing the doses under various planning conditions, such as including or excluding the supports in the external contour, assigning the system electron density (ED) directly from the CT-to-ED curve in the TPS or using a formula to correct for non-human materials, and introducing rotational (1°, 2°, and 3°) or translational (1–3 mm) offsets of the immobilisation supports relative to the phantom, the dosimetric impact of the supports and patient positioning errors within the masks was assessed. Results: The dose to the isocenter varied significantly by up to 7% when the supports were removed from the external contour. Dose discrepancies at the isocentre between the two methods used for ED calculation were less than 1%. The dosimetric impact of rotational misalignments was minimal for both devices and showed no consistent trend in any direction or with increasing rotation. Dosimetric discrepancies for translational misalignments ranged from 1.2% for 1mm to 1.5% for 3mm, with the effect being more
89.7% (anorectum). Reducing the benchmark set to five plans did not significantly affect DGC-based evaluations, unlike DVH-based assessments.
Figure: For bladder and anorectum their reference situations BS20 for both aDVH and aDGC (green) compared against BS5(small) (red) and BS5(large) (blue) when respectively focusing on the five smallest or largest ROI volumes only. Conclusion: DGC-based plan evaluation offers a reliable, volume- independent method for assessing organ sparing in prostate SBRT. It enables automated detection of suboptimal plans and remains robust even with reduced benchmark sizes. Importantly, this approach supports integration into automated radiotherapy workflows, providing a scalable and interpretable tool for quality assurance of treatment plans generated by AI-driven or template-based planning systems. References: Gregoire V, MacKie TR. Dose prescription, reporting and recording in intensity-modulated radiation therapy: A digest of the ICRU report 83. Imaging Med 2011;3:367–73. https://doi.org/10.2217/iim.11.22.[2] Nitsche M, Brannath W, Brückner M, Wagner D, Kaltenborn A, Temme N, et al. Comparison of different contouring definitions of the rectum as organ at risk (OAR) and dose-volume parameters predicting rectal inflammation in radiotherapy of prostate cancer: Which definition to use? Br J Radiol 2017;90:1–12. https://doi.org/10.1259/bjr.20160370.[3] Sung KH, Choi YE. Dose gradient curve: A new tool for evaluating dose gradient. PLoS One 2018;13:1–16. https://doi.org/10.1371/journal.pone.0196664. Keywords: plan quality, automated workflows
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