S390
Clinical - CNS
ESTRO 2026
contrast enhancement occurred in 20 of 79 patients (25.3%) and were initially managed with corticosteroids in all cases; 4 patients with radiation necroses required additional treatment with bevacizumab. Molecular characterization via DNA methylation profiling was performed in 18/79 patients, with a higher-risk profile in 78% of all tested patients. Conclusion: Carbon ion re-irradiation is a feasible salvage option for patients with recurrent meningiomas after previous high-dose photon radiotherapy. Early outcomes demonstrate encouraging disease control with manageable toxicity. Ongoing analyses of the cumulative spatial dose distribution and volumetric parameters, alongside sequencing-based molecular predictors, are underway to further refine risk stratification and outcome prediction. Keywords: Re-radiotherapy, carbon-ion therapy, meningioma Biomaterial changes after re-irradiation of recurrent glioblastoma – early results of the prospective “ALA-RDT in GBM” phase I/II trial (NCT05590689) Niklas B Pepper 1 , Michael Müther 2 , Michael Oertel 1 , Christian Thomas 3 , Enrico Küllenberg 3 , Stefan Gravemeyer 1 , Fabian M Troschel 1 , Dorothee C Spille 2 , Hans T Eich 1 , Walter Stummer 2 1 Dapartment of radiation oncology, University hospital of Münster, Münster, Germany. 2 Department of neurosurgery, University hospital of Münster, Münster, Germany. 3 Dapartment of neuropathology, University hospital of Münster, Münster, Germany Purpose/Objective: Re-irradiation (re-RT) for recurrent glioblastoma (rGBM) becomes increasingly established as part of multimodal treatment, although RT-dose is limited in most cases due to prior treatment. Radiosensitizers have been explored to increase the effectiveness of re- RT but have yet to demonstrate efficacy in the clinical setting despite promising pre-clinical data1. While 5- ALA is well established for fluorescence-guided resection of glioma2, novel data also reveal the radiosensitizing capabilities of its metabolite protoporphyrin IX (PPIX)3. Material/Methods: Digital Poster Highlight 4437 The phase I/II prospective “ALA-RDT in GBM”-trial4 investigates 5-ALA, an endogenous amino acid that is metabolized to PPIX as a radiosensitizer in the setting of re-RT for recurrent GBM. Patients receive an increasing number of RT-fractions (up to 8 fractions) as radiodynamic therapy (RDT) after prior oral application of 5-ALA. In two study cohorts, patients
generally well tolerated without severe adverse events. Conclusion: Conclusions: Oncological outcome was not compromised by reduced margins. These findings support margin de-escalation in WHO grade III gliomas. Keywords: WHO III glioma, margin reduction, recurrence Pushing therapeutic boundaries: carbon ion re- irradiation for recurrent meningiomas after previous high-dose photon radiotherapy. Maximilian Y. Deng 1 , Lisa Marie Seidel 1 , Fabian Allmendinger 1 , Thomas Held 1 , Sebastian Regnery 1 , Lukas Bauer 1 , Line Hoeltgen 1 , Felix Sahm 2 , Semi Harrabi 1 , Klaus Herfarth 1 , Jürgen Debus 1 , Laila König 1 1 Department of Radiation Oncology, Heidelberg University Hospital, Heidelberg, Germany. 2 Department of Neuropathology, Heidelberg University Hospital, Heidelberg, Germany Digital Poster Highlight 4409 Purpose/Objective: Management of recurrent meningiomas following high-dose photon radiotherapy remains a clinical challenge due to limited therapeutic options and cumulative toxicity concerns. Carbon ion radiotherapy displays a steep dose gradient with an enhanced relative biological effectiveness (RBE), representing a promising modality in the salvage setting. Material/Methods: This study presents a retrospective analysis of 79 patients treated from 2011 to 2025 with carbon ion re- irradiation for recurrent intracranial meningiomas. Carbon ion re-irradiation was delivered in 13–17 fractions with a single dose of 3 Gy (RBE) (EQD2: 48.8 - 63.8 Gy (RBE)). Dose prescription during re-irradiation was typically based on the latency period since prior photon irradiation and the prior dose exposure within the target volume. Progression was determined according to the guidelines by the Response Assessment in Neuro-Oncology Working Group
(RANO). Results:
Median follow-up was 19 months. All patients had previously received high-dose photon radiotherapy, with a median dose of 54 Gy (range: 50.4-68 Gy). Median interval between initial radiotherapy and carbon ion treatment was 68 months (range: 10–295 months). Most patients were initially categorized as CNS WHO grade 2 (n=53), followed by WHO grade 1 (n=18) and WHO grade 3 (n=8). The estimated 2-year local tumor control rate was 61.3%, and the 2-year overall survival (OS) rate was 93.6%. Radiation-induced
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