Background: Patients with vestibular schwannomas (VS) are either assigned to watchful waiting, microsurgical resection, or radiosurgery. Decision making on how to proceed is based on parameters such as age, tumor growth, loss of hearing, and the tumor's Koos grading.
Methods: In order to correlate Koos grading with tumor volume, patient records of 235 patients with VS who underwent Gamma Knife radiosurgery (GKRS) were retrospectively reviewed.
Results: From 1994 to 2009, 235 consecutive patients underwent GKRS for sporadic VS at the Zurich Gamma Knife Center. Median follow up was 62.8 ± 33.0 months. Of the 235 tumors, 32 (13.6 %) were graded Koos I with a volume of 0.25 ± 0.3 cc; 71 (30.2 %) were graded Koos II with a volume of 0.57 ± 0.54 cc; 70 (29.8 %) were graded Koos III with a volume of 1.82 ± 1.88 cc; and 62 (26.4 %) were graded Koos IV with a volume of 4.17 ± 2.75 cc. Tumor progression was defined as a volume increase > 20 % at 2 years or later following GKRS. Overall tumor progression occurred in 21/235 (8.9 %) patients at 3.4 ± 0.9 years. Tumor progression did not differ statistically significantly in the various Koos grades: 1/32 (3.1 %) patients with VS Koos Grade I, 7/71 (9.8 %) patients with VS Koos Grade II, 6/70 (8.6 %) patients with VS Koos Grade III, and 7/62 (11.3 %) patients with VS Koos Grade IV.
Conclusion: To our knowledge, this is the first work correlating the various Koos grades of VS to their respective tumor volumes. In our patients, tumor volumes of VS Koos Grade IV were limited because all of our patients were eligible for radiosurgery. In our series, the outcome following GKRS for patients with VS Koos Grade IV tumors did not differ from patients with VS Koos Grades I-III. We therefore suggest to limit Koos Grade IV VS to tumor volumes < 6 cc that may be eligible for radiosurgery, and introduce an additional VS Grade V for large VS with tumor volumes of > 6 cc that may not be eligible for radiosurgery.