Exploring Tumor Size as a Predictor of Treatment Success/Failure Following Stereotactic Radiosurgery in Vestibular Schwannoma

Objective: Stereotactic radiosurgery (SRS) achieves high tumour control in vestibular schwannoma (VS), yet treatment failure occurs in a subset of patients. Tumour size is a proposed predictor, but prior studies show conflicting results and are often limited by small cohorts or short follow-up. We evaluated its prognostic value in a large, single-centre cohort with consistent treatment and long-term follow-up. Methods: This is a retrospective cohort study. The patient collective was identified by a prospectively kept registry. Only solitary VSs were included, VSs associated with schwannomatosis were systematically excluded. Patients with radiographic follow-up of less than two years were systematically excluded to rule out the known phenomenon of pseudoprogression. Volumetric measures were carried out in gadolinium enhanced magnetic resonance imaging. KOOS Classification was used additionally to classify tumour size. Clinical data were collected retrospectively. Results: The study cohort included N =928 VS patients treated with SRS between 1998 and 2019, who met the above-mentioned inclusion criteria. Mean follow-up time was 6.37 (±3.96) years. The rate of treatment failure was different in the different KOOS-subgroups with the lowest rate in KOOS I at 4%, 10% in KOOS II, the highest in KOOS III with 13% and 10% in KOOS IV. Mean time toprogression was 4.49 (±2.64) years overall, with the longest mean time to progression in KOOS I at 5.46 (±4.14) years, 4.97 (±2.85) years in KOOS II, 4.52 (±2.70) years in KOOS III, and 4.75 (±4.50) years in KOOS IV. Paddick Conformity Index (PCI) increased with increasing tumour size, but showed a worse predictive performance compared to smaller tumours. Conclusions: There is a correlation of treatment success/failure to tumour volume: The rate of treatment success decreases with increasing tumour size after SRS in VS. Paddick Conformity Index’s performance to predict treatment failure decreased in increasing tumour size. SRS treatment strategy for large VS for should be adapted in order to improve treatment response.