S878
Clinical - Mixed sites & palliation
ESTRO 2026
Medicine, Alexandria University, Alexandria, Egypt. 3 Radiotherapy Physics Department, Guy’s and St Thomas’ NHS Foundation Trust, London, United Kingdom. 4 Radiology Department, Guy’s and St Thomas’ NHS Foundation Trust, London, United Kingdom Purpose/Objective: Brainstem metastases (BSM) are rare but present significant clinical challenges. Concerns over treatment-related toxicity have led many UK linac- based stereotactic radiosurgery (SRS) centres to favour hypofractionated SRS (hSRS) over single-fraction SRS for BSM. We report on consecutive patients treated with single-fraction, linac-based, SRS at a UK tertiary referral centre between January 2018 and June 2025. Material/Methods: The GTV encompassed the contrast-enhancing lesion, with a uniform 1mm isotropic expansion to create PTV. SRS was delivered using single-isocentre, non- coplanar VMAT, prescribed to deliver 100% of the prescription dose to 98–100% of the PTV.Local control (LC), neurological toxicity, and overall survival (OS) were assessed. The primary endpoint was lesion-level LC. Safety endpoints were symptomatic radionecrosis (RN) and CTCAEv5.0 grade ≥ 3 neurological toxicity. Prognostic factors (age, sex, KPS, prior WBRT, SACT, primary tumour type) were analysed, and dose- volume correlates of RN evaluated. Results: Twenty-five lesions in 20 patients received SRS: 24 at 16Gy and one at 14 Gy (Figure 1). Median age was 60 years (IQR57–65). Primary tumours were breast (n = 11, 44%), lung (n = 6, 24%), renal cell carcinoma (n = 5, 20%), and melanoma (n = 3, 12%). Ten lesions (40%) followed prior WBRT, with a median interval of 23 months (IQR17.5–40.2). Median maximal-GTV- diameter was 6 mm (IQR 3.9–11.2) and median PTV volume 0.30cc (IQR 0.10–1.40).Median follow-up was 13.7 months. Local (in-field) recurrence occurred in 1/25 (4%), yielding 1-year LC of 92.9%. At 12 months, OS was 74.5% (Figure 2); median OS (95%CI) 27.3 months (14.7–49.3). On univariate analysis, larger GTV, PTV, and GTV diameter correlated with poorer OS; clinicodemographic factors showed no association.Symptomatic RN occurred in 2/25 (8%) lesions; grade ≥ 3 neurotoxicity (vomiting) in 1/25 (4%); no treatment-related mortality. Exploratory trends showed higher brainstem D0.035cc and larger GTV- diameter among RN cases, though limited events precluded statistical validation.During the same period, four BSM patients received hSRS (21Gy/3fractions), with median maximal-GTV-diameter 24mm (IQR 23.4–28.4) and PTV volume 6.0cc (IQR 4.6– 10.1). One in-field recurrence occurred; median OS 7.1
Conclusion: SFRT demonstrates excellent feasibility, safety, and promising efficacy for bulky tumors, particularly for rapid palliation and local control. However, the lack of standardized dosimetric definitions—especially regarding VPDR—limits cross-study comparability. Future investigations should prioritize harmonized dose prescription protocols, robust prospective trials, and the exploration of synergistic combinations with systemic therapies such as immunotherapy. References: 1. Koka K, Verma A, Dwarakanath BS, et al. (2022) Technological Advancements in External Beam Radiation Therapy (EBRT): An Indispensable Tool for Cancer Treatment. CMAR Volume 14: 1421–1429.2. Galunic Bilic L, Santek F, Grah JJ, et al. (2023) Efficacy and toxicity of infradiaphragmal radiotherapy fields in lymphoma patients: a single-centre experience. Radiol med 128: 492–500.3. Alterio D, Zaffaroni M, Bossi P, et al. (2023) Reirradiation of head and neck squamous cell carcinomas: a pragmatic approach— part I: prognostic factors and indications to treatment. Radiol med 129: 160–173.4. Hardcastle N, Tomé WA, Foo K, et al. (2011) Comparison of Prostate IMRT and VMAT Biologically Optimised Treatment Plans. Medical Dosimetry 36: 292–298. Keywords: SFRT, Bulky Tumors, GRID, Lattice Digital Poster 1939 efficacy and safety of single-fraction linac-based stereotactic radiosurgery for brainstem metastases: A UK institutional experience Mohammad Emarah 1,2 , Omar Al-Salihi 1 , Vishal Manik 1 , Lucy Brazil 1 , Angela Swampillai 1 , Sheila Hassan 1 , Fahmida Khan 1 , Kirsty Blythe 3 , Justin Lam 3 , Asif Mazumder 4 , Mark Macdonald 4 , Kazumi Chia 1 1 Guy's Cancer Centre, Guy’s and St Thomas’ NHS Foundation Trust, London, United Kingdom. 2 Faculty of
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