Surgical Outcomes in Large Vestibular Schwannomas: Proposed Koos Grade V and Radiological Risk Features

In the multimodality era, surgery is largely reserved for large vestibular schwannomas (VSs), with high expectations for safety and durable tumor control. The Koos grading system is simple and widely used, but conventional Koos grade IV encompasses heterogeneous risk profiles. We evaluated whether a simple subgrade-proposed Koos grade V, defined by clear midline reach/crossing-improves risk stratification for perioperative risk, facial nerve outcomes, and long-term need for additional treatment.

We retrospectively analyzed 435 consecutive patients undergoing primary microsurgical resection of sporadic Koos grade III to V VSs (2016-2020). Preoperative tumor size and radiological risk features (peritumoral edema, ventricular enlargement, and hypervascularity) were assessed. Outcomes included extent of resection, major perioperative complications, facial nerve function, and additional treatment (stereotactic radiosurgery and/or reoperation). Time-to-event and parsimonious multivariable analyses were performed among Koos grade IV to V tumors.

Radiological risk features clustered in higher grades and were most frequent in grade V. Major perioperative complications occurred in 7.1% overall and were more frequent in grade V (16.9%) despite high mean resection rates (95.8% overall; 95.0% in grade V). Long-term favorable facial nerve function was maintained in approximately 95% across grades. During a median 81-month follow-up, additional treatment (mainly stereotactic radiosurgery) was required in 12.9% overall and increased stepwise from grades III to V (6.1%, 10.1%, and 28.1%); reoperation occurred in 2.1% overall and predominantly in grade V (7.9%). Among grades IV-V, proposed grade V was associated with additional treatment, whereas hypervascularity showed the strongest association with major perioperative complications.

Proposed Koos grade V, defined by midline extension, is simple, likely reproducible, and identifies a high-risk subset of large VSs with clustered adverse imaging features, higher complication rates, and greater long-term need for additional treatment, while preserving high resection rates and favorable facial nerve outcomes.