On MRI, a vestibular schwannoma fills the cerebellopontine angle cistern and contacts the brainstem but does not indent or displace it, and the fourth ventricle remains midline. According to the Koos classification, this tumour is:

• A Koos Grade 2
• B Koos Grade 4
• C Koos Grade 3 ✓
• D Koos Grade 1

Explanation

The Koos classification is based on extension into the cerebellopontine angle: Grade 1 is purely intracanalicular, Grade 2 extends into the CPA cistern without brainstem contact, Grade 3 fills the CPA cistern and touches the brainstem without deforming it, and Grade 4 compresses the brainstem and displaces the fourth ventricle. A tumour touching but not displacing the brainstem with a normal fourth ventricle fits Grade 3 exactly; Grade 4 requires actual compression.

Reference: Cummings Otolaryngology: Head and Neck Surgery, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.