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Table 1 Koos classification system

From: Early magnetic resonance image volumetric changes of vestibular schwannoma after Gamma Knife radiosurgery: a prospective study of 18 cases

Grade Definition
1 Intracanalicular
2 Tumor in the cerebellopontine angle but not reaching the pons
3 Tumor reaching the pons, perhaps deforming it but not shifting it
4 Tumor deforming the pons and shifting the fourth ventricle