S2277
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
validated for visible lesion cases (Fig1.b). Large artifacts on a kidney patient treated on a Varian Linac, (Fig1.c) motivated a transfer to the Unity (Fig1.d). Overall, optimized margins permit a reduction of 37.5±9.4% of the PTV volume. Dose escalation was possible for 13 patients (79 fractions); on average, the IGTV D90% was increased by 7.1±8.3%. The average duty cycle was 70±14%. CMM was able to track all surrogates successfully. The average overall treatment time was 77±28 minutes with 0-3 baseline-shift plans. Median follow-up was 2.6 months (min0–max6.2). No patient experienced an acute adverse event of grade II or higher. No local control failure was noted.
Poster Discussion 4932
A prospective abdominal SBRT treatment study using exhale gating combined with optimized margin and Comprehensive Motion Management Eric Poulin 1,2 , Thomas Reynaud 3 , Luc Gingras 1,2 , François Fabi 4 , Bernard Lachance 1 , Frédéric Lacroix 1 , Janelle Morrier 1 , Raphael Mathieu 4 , Olivier Fillion 1 , Jason St-Hilaire 1 , Isabelle Thibault 4,3 , Laurence Bernier 4,3 , Mathieu Leclerc 4,3 , William Foster 4,3 , Elizabeth Guimond 4,3 1 Service de physique et de radioprotection, CHU de Québec – Université Laval, Québec, Canada. 2 Département de physique, génie physique et d’optique, Université, Laval, Canada. 3 Centre de Recherche, CHU de Québec – Université Laval, Axe Oncologie, Québec, Canada. 4 Service de radio- oncologie, Département de médecine spécialisée, Faculté de Médecine, CHU de Québec – Université Laval, Québec, Canada Purpose/Objective: MR-guided adaptive radiotherapy can benefit patients through daily MR imaging and treatment adaptation. The goal of this study is to present our initial results treating abdominal lesions with comprehensive motion management (CMM), optimized patient- specific margins and exhale gating. Material/Methods: Nineteen patients (94 fractions) were recruited in this prospective study (April to October 2025) using CMM with Unity. Pancreas, liver, adrenal gland, kidney and lymph nodes were treated. Reference plans were created on a 1.5T Sola 3D T2 space triggered scan, where 3D T1 post gadolinium, including exhale and inhale breath-hold scans, were registered. 4D CT and cine MRI were used to evaluate tumor motion and gating feasibility. GTV was delineated on the T2, the CT 50% phase and the 3D T1 exhale post-gadolinium. IGTV margin is a 3-mm GTV isotropic expansion when movement <1cm, 5mm inferiorly for movement between 1-2cm or 7mm >2cm. ICTV is the gating envelope and equals the IGTV. PTV of 3mm is used with an Adapt-To-Shape workflow. Daily T2 images are registered rigidly to the T2 reference scan, rotations are then applied during contouring. Volume reduction was evaluated by applying tumor motion (0-90%) to generate an ICTV with a 5mm margin for PTV, simulating treatment on a conventional Linac. The mean dose prescription was 40Gy (range 30-54Gy, 3-5 fractions) and dose escalation to IGTV was performed at each adapted fraction when possible. Toxicities were reported using CTCAEv6. Results: The proposed workflow allows liver treatments where the tumor is not visible on T2 navigated scan, by registering the liver and GTV (Fig1.a); this was
Conclusion: CMM with optimized margins allows treating
abdominal sites with a reduced treatment volume in a clinically adequate time with no grade II or higher toxicities. IGTV dose boosting up to 7% on average was possible. Keywords: MRI-Linac, MRgRT, Adaptive Radiotherapy
Digital Poster 4984
From motion to margin: Incorporating the acceptance frequency of a secondary position adaptation in MR-Linac adaptive workflows David Tilly 1,2 , Samuel Fransson 3 , Martin Lundmark 2 , Adam Johansson 1,2 , Katja Klaman 2 , Ulf Isacsson 1,2 , Calin Radu 1,4 , Anna AK Flejmer 1,4 , Ariadni Kyriakogiannaki 1,4 , Nina Tilly 1,2 1 Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden. 2 Medical Physics, Uppsala University Hospital, Uppsala, Sweden. 3 Surgical Sciences, Uppsala University, Uppsala, Sweden. 4 Oncology, Uppsala University Hospital, Uppsala, Sweden Purpose/Objective: The aim of this study was to define reduced PTV margins and implement an online decision-support strategy for ultra-hypo-fractionated prostate MR-Linac treatments. The decision-support system was designed to balance PTV margin size with the frequency of additional positional adaptations (ATP) following the initial adaptation to shape.
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