S2232
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
(right/triangles). The DVH shows near-complete curve overlap, confirming the dosimetric accuracy of the sCT based plan for PTVs (merged yellow, individual blue), GTVs (red) and other structures. Conclusion: The same-day MRI-only simulation workflow for SRS is clinically feasible and dosimetrically robust. It enhances targeting precision by utilizing up-to-date anatomy, which may enable the safe adoption of a 0 mm PTV margin. This would result in a 38 ± 8% treatment volume reduction compared to a 1 mm margin, thus sparing healthy brain tissue. A clinical trial of adaptively delivered Linac-based SRS for volatile brain metastases to assess this will commence shortly. References: 1.Grishchuk D et al. ISRS Technical Guidelines for Stereotactic Radiosurgery: Treatment of Small Brain Metastases ( ≤ 1 cm in Diameter). Pract Radiat Oncol. 2023 May 1;13(3):183–194.2.Shaw E, Scott C, Souhami L, Dinapoli R, Kline R, Loeffler J, Farnan N. Single dose radiosurgical treatment of recurrent previously irradiated primary brain tumors and brain metastases: final report of RTOG protocol 90-05. Int J Radiat Oncol. 2000 May 1;47(2):291–298.3.Masitho S, Putz F, Mengling V, Reißig L, Voigt R, Bäuerle T, Janka R, Fietkau R, Bert C. Accuracy of MRI-CT registration in brain stereotactic radiotherapy: Impact of MRI acquisition setup and registration method. Z Med Phys. 2022 Nov;32(4):477-487. Keywords: SRS, Brain Metastases, Synthetic CT Digital Poster 2522 From motion to dose: Accumulation of AI-predicted dose evaluates margins designed for MR-Linac adaptive workflows Nina Tilly 1,2 , Samuel Fransson 3 , David Tilly 1,2 1 Medical Radiation Physics, Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden. 2 Department of Medical Physics, Uppsala University Hospital, Uppsala, Sweden. 3 Department of Surgical Sciences, Uppsala University, Uppsala, Sweden Purpose/Objective: We constructed reduced motion-based PTV margins for MR-Linac prostate radiotherapy to be clinically introduced.The aim of this study was to dosimetrically evaluate the adequacy of these reduced motion-based margins using accumulation of AI-predicted dose. Material/Methods: Reduced PTV margins were derived from displacements recorded from plan to verification MR images and analyses of intra-fraction motion traces for patients previously treated with conventional margins.
treatment position on the day of treatment. An FDA- approved deep-learning-based synthetic CT (sCT), combined with an in-house digital model of the MRI- invisible fixation device, replaces the dedicated CT simulation for dose calculation. A preliminary treatment plan, prepared using prior diagnostic MRI, is adapted to match treatment-day anatomy by re- segmenting targets and organs-at-risk for a final optimization. The HyperArc VMAT plan is re-optimized in Eclipse using the sCT and delivered on a Varian TrueBeam Linac the same day. Commissioning included retrospective dosimetric comparisons of sCT and CT plans and tests verifying positioning accuracy by registering CBCT images to the sCT.
Figure 1:Schematic of the workflow. Results:
Dosimetric analysis demonstrated a D95% dose difference of -0.041 ± 0.718% between sCT and CT plans. Computed γ -indices were 99.59 ± 0.36% (1% dose deviation (DD), 1 mm dose-to-agreement (DTA), 10% threshold, local dose normalization) and 100% (3% DD, 2 mm DTA, 10% threshold, global normalization). CBCT-to-sCT alignment for patient setup achieved sub-millimeter accuracy, comparable to the CT-based workflow.
Figure 2:Dosimetric validation for a representative clinical treatment plan. Side-by-side plan comparison on CT (left/squares) and sCT with fixation model
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