High-Dose Stereotactic Re-Irradiation of Recurrent High-Grade Gliomas: Clinical Outcome and Experience with AI-Based Target Volume Simulation

Background/Objectives: Despite multimodal therapeutic concepts, treatment of recurrent malignant gliomas remains challenging. Stereotactic radiosurgery (SRS) may be a possible safe and effective non-invasive salvage treatment. In this study, we aim to investigate the SRS treatment outcomes using partly 18F-Fluorethylthyrosine (FET)-PET-imaging sequences for SRS treatment planning focusing on overall survival, event-free survival, and the incidence and factors influencing radiation necrosis (RN) occurrence. Additionally, we evaluated the potential application of AI-based tumor segmentation. Methods: We conducted a retrospective analysis of patients with recurrent malignant glioma treated with single-fraction or hypofractionated SRS at our institution. The outcomes assessed included local control, overall survival (OS), and local event-free survival (LEFS, defined as the interval until tumor recurrence or the onset of RN). We also performed a simulation analysis to assess the potential of AI-based tumor segmentation. Results: The study included 27 patients with a median age of 57 years and 41 lesions. The median OS post-SRS was 9.6 months and an LEFS of 5.2 months. Factors positively influencing OS and LEFS included the gross tumor volume (GTV) of the lesions before SRS therapy, presence of an IDH mutation, and lomustine treatment post-SRS. The incidence of RN post-SRS was 31.7%. RN was confirmed histopathologically in 15.4%, based on MRI in 46.2% and by FET-PET in 38.5% of lesions. In a simulation analysis, AI-based tumor segmentation reliably delineated all lesions, requiring only minimal manual adjustments to define target volumes. Conclusions: High-dose SRS is a feasible salvage treatment for small-volume recurrent high-grade gliomas, achieving local control and survival outcomes comparable to other re-irradiation strategies. IDH mutation, smaller tumor volume, and lomustine therapy were associated with improved survival. RN occurred frequently, particularly in periventricular lesions. AI-based tumor segmentation showed promise in well-defined satellite recurrences, but remains limited in cavity-adjacent lesions, underlining the need for expert review and ¹⁸FET-PET imaging.