RBIO-04. OUTCOMES OF RADIOSURGERY FOR WHO GRADE 2 MENINGIOMAS: THE ROLE OF KI-67 INDEX IN GUIDING THE TUMOR MARGIN DOSE

Abstract The management of WHO grade 2 meningiomas is complicated by their diverse clinical behaviors. Stereotactic radiosurgery (SRS) can be an effective management option. Literature on SRS dose selection is limited, but suggests a higher dose is better for tumor control. We aimed to characterize the predictors of post-SRS outcomes that can help guide planning and management. We reviewed a cohort of consecutive patients with pathologically-proven WHO grade 2 meningiomas who underwent SRS at a single institution between 2011 and 2023. Ninety-nine patients (median age 62 years) underwent SRS, eleven of whom received hypofractionated SRS in 5 fractions. Twenty-two patients had received prior irradiation. The median follow-up was 49 months. The median overall survival (OS) was 119 months (95%CI 92–NA) with estimated 5- and 10-year survival of 83% and 27% respectively. The median progression-free survival (PFS) was 40 months (95%CI 32–62) with 3- and 5-year rates at 54% and 35% respectively. The median locomarginal PFS was 63 months (95%CI 51.8-NA) with 3- and 5-year rates at 65% and 52%. Nine (9%) patients experienced adverse events, 2 CTCAE grade 3 and 7 grade 2, consisting of worsening neurologic deficit from edema. In the single-session SRS cohort, Ki-67 was a significant predictor of both OS and intracranial PFS. Tumors with Ki-67 >10% had ~10 times risk of locomarginal progression compared to Ki-67 ≤10% (p<0.001). In tumors with Ki-67 >10%, margin dose ≥14 Gy was associated with significantly better tumor control, but not for tumors with Ki-67 ≤ 10%. Other significant predictors of tumor control were prescription dose, tumor volume, and location. The management of WHO grade 2 meningiomas requires a multi-modality approach. This study demonstrated the value of a targeted SRS approach in patients with limited disease, and further establish predictive biomarkers that can guide planning through a personalized approach.