BACKGROUND: To evaluate the efficacy of radiotherapy (RT) for World Health Organization (WHO) grade 1 and 2 intracranial meningiomas, focusing on the impact of postsurgical tumor volume on treatment outcomes. METHODS: Adult patients (aged ≥18 years) with WHO grade 1 or 2 intracranial meningiomas who received RT between January 1, 2019 and April 1, 2022 were identified. Exclusion criteria encompassed known extracranial tumors, preoperative RT or chemotherapy, and insufficient RT modality data. Patients were treated according to the international guidelines. Tumor recurrence was identified on magnetic resonance imaging with a follow-up period until April 2024. Kaplan-Meier estimates for progression-free survival (PFS) calculations and Cox proportional hazard models were performed to evaluate the impact of tumor volume and other covariates on PFS. RESULTS: Among 113 patients with intracranial meningiomas who received RT, 103 met the inclusion criteria. Of these, 84.5% received photon-based and 15.5% proton-based treatment. The cohort was predominantly female (72.8%) with a mean age of 59 years. The 2-year and 5-year PFS rates were 95.6% and 90% for grade 1 tumors, respectively, and 83.3% for grade 2 tumors. Tumors >
21 cm CONCLUSIONS: Tumor volume was identified as a key prognostic factor for PFS in WHO grade 1 and 2 intracranial meningiomas treated with RT. A critical postsurgical volume threshold of 21 cm