S380
Clinical - CNS
ESTRO 2026
replanning. At radiologic recurrence (RANO HGG 2.0), the recurrent GTV (rGTV) was delineated on co- registered MRIs to quantify spatial overlap with the CTV and summed RT dose. rGTV coverage and dose (mean, D90, D98, D100) were compared among contouring groups using Kruskal–Wallis test, and progression-free (PFS) and overall survival (OS) were analyzed by Kaplan–Meier with log-rank comparison. Results: Among the 98 treated patients, we found NF in 24, FNC in 27, and FC in 47. Baseline performance status, MGMT methylation, RT prescription and completeness of resection were comparable among groups.Recurrences (n=69) were predominantly central (65–72%, Table 1) with no significant difference between the groups (p=0.717). The median local (central or marginal) rGTV coverage within the CTV was 97.9% (IQR 92.2-100) and did not vary significantly across delineation strategies (p=0.105, Figure 1A). Estimated rGTV dose metrics were also similar (Table1, Figure 1B).Median PFS was 10.5, 8.0, and 8.6 months for NF, FNC, and FC, respectively (p=0.279). Median OS was 16.9, 15.8, and 13.4 months, respectively (p=0.389).
progressed; three underwent surgery with histologically confirmed necrosis. One patient was re- irradiated and achieved partial response. No correlation between PTV margin and tumor control was observed. Conclusion: Fractionated, image-guided SRT with reduced or negative GTV–PTV margins respecting the pseudocapsule boundary is feasible and effective for acoustic neuroma. The inherent peripheral dose gradient provides clinically relevant coverage of the pericapsular region, likely contributing to maintained tumor control while minimizing brainstem toxicity. This strategy supports individualized, anatomy-based target definition in modern SRT planning. Keywords: Acoustic neuroma, Pseudocapsule Proffered Paper 3420 Influence of FLAIR Inclusion on Recurrence Patterns and Outcomes in Glioblastoma: Results from a Prospective Adaptive Radiotherapy Trial (UNITED) Suman Ghosh 1 , Hany Soliman 1 , Chia-Lin (Eric) Tseng 1 , Hanbo Chen 1 , Deepak Dinakaran 1 , Kang Liang Zeng 1 , James Stewart 2 , Mark Ruschin 2 , Arjun Sahgal 1 , Jay Detsky 1 1 Radiation Oncology, Odette Cancer Centre, Sunnybrook Health Sciences Centre, Toronto, Canada. 2 Medical Physics, Odette Cancer Centre, Sunnybrook Health Sciences Centre, Toronto, Canada Purpose/Objective: The inclusion of the peritumoral FLAIR hyperintense region (FHR) within the clinical target volume (CTV) for glioblastoma (GBM) varies between guidelines and amongst radiation oncologists. Using data from a prospective Phase 2 trial employing a small margin, MR-guided adaptive radiotherapy (RT) approach, we evaluated the influence of treated versus untreated FHR on recurrence patterns and outcomes. Material/Methods: Patients with newly diagnosed GBM enrolled in this single-arm prospective trial1 (NCT04726397) between April 2021 and May 2023 underwent chemoradiation with weekly online adaptation on an MR-LINAC platform. Gross tumor volume (GTV) included only the resection cavity and residual tumor. The CTV consisted of a 5-mm isotropic expansion around the GTV, with optional inclusion of FHR beyond 5mm at the clinician’s discretion. Subsequently, three categories of CTV were classified: no FHR beyond 5mm outside GTV (NF), FHR beyond 5mm but not included in CTV (“FHR- not-chased” or FNC), and FHR seen and included in CTV (“FHR-chased” or FC). Contrast-enhanced MRI at fraction 1 and every fifth fraction guided adaptive
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