P13.19.B PUSHING THE LIMITS: CARBON ION RE-IRRADIATION FOR RECURRENT MENINGIOMAS FOLLOWING PREVIOUS HIGH-DOSE PHOTON RADIOTHERAPY

Abstract BACKGROUND The management of recurrent meningiomas following prior high-dose photon radiotherapy remains a substantial clinical challenge due to limited treatment options and concerns regarding cumulative toxicity. Carbon ion radiotherapy, characterized by its steep dose gradient and superior relative biological effectiveness (RBE), offers a promising salvage modality in this context. METHODS We retrospectively analyzed 64 patients treated with carbon ion re-irradiation for recurrent intracranial meningiomas between 2011 and 2024. Treatment was administered in 13-17 fractions, with a per-fraction dose of 3 Gy (RBE), corresponding to an EQD2 range of 48.8-63.8 Gy (RBE). Dose prescription was individualized based on the time interval since prior radiotherapy and cumulative dose within the target volume. Tumor progression was assessed according to the Response Assessment in Neuro-Oncology (RANO) criteria. RESULTS The median follow-up was 18 months (range: 3-112 months). All patients had previously received high-dose photon radiotherapy, with a median dose of 54 Gy. The median interval between the initial and re-irradiation treatments was 68.5 months (range: 12-295 months). The majority of patients had WHO grade 2 meningiomas (n=41), followed by grade 1 (n=16) and grade 3 (n=7). The 2-year local control rate was 57.7%, and the 2-year overall survival rate was 93.6%. Radiation necrosis occurred in 17 patients (26.6%), all initially managed with corticosteroids; three required escalation to bevacizumab. CONCLUSION Carbon ion re-irradiation appears to be a feasible and effective salvage treatment for recurrent meningiomas following high-dose photon radiotherapy. Early results demonstrate promising local control with an acceptable toxicity profile. Ongoing analyses, including volumetric assessment and molecular profiling, aim to refine risk stratification and optimize patient selection.