Early Volumetric Response After Stereotactic Radiosurgery in Treatment-Naïve Koos Grade III-IV Cystic Vestibular Schwannomas.

BACKGROUND: The role of stereotactic radiosurgery (SRS) for large vestibular schwannomas (VS) remains controversial, particularly for patients with cystic VS, which can grow rapidly and are often managed by microsurgical resection. OBJECTIVE: This study aimed to evaluate the volumetric response of cystic VS compared with solid VS following SRS in patients with treatment-naïve Koos grade III-IV tumors. METHODS: This retrospective study included patients with newly diagnosed Koos grade III-IV VS treated with SRS, with > 6 months of radiological follow-up. Tumors were classified as cystic or solid based on pretreatment MRI. Inverse probability of treatment weighting (IPTW) was applied to adjust for baseline differences, and multivariable weighted linear regression identified predictors of volumetric response at the last follow-up MRI. Tumor volumetric changes were also assessed at early (3-6 months), intermediate (12-18 months), and late (>24 months) follow-up. Local control (LC) was defined as no need for additional treatment (surgical resection or repeat SRS). RESULTS: Among 261 SRS-treated VS patients, 76 (15 cystic, 61 solid) met inclusion criteria. The mean radiological follow-up was 34.0 months. After IPTW adjustment, baseline tumor volume was a significant predictor of volume reduction (p <0.01). Cystic VS demonstrated significantly greater tumor volume reduction during early and intermediate follow-up than solid VS (Mann-Whitney U: p = 0.029 and p = 0.031; IPTW-adjusted: p < 0.01 and p = 0.024, respectively). This difference was not observed at late follow-up (Mann-Whitney U: p = 0.064; IPTW-adjusted: p = 0.780). The LC rate was 100% in cystic and 98.4% in solid VS. CONCLUSION: Large cystic VS exhibit significantly greater early tumor shrinkage following SRS, with excellent local control comparable to solid VS. These findings suggest that SRS is a viable, less invasive treatment option for selected patients with large cystic VS.