An MRI of a 55-year-old with progressive unilateral sensorineural hearing loss shows a vestibular schwannoma that fills the cerebellopontine angle cistern and abuts the pons without displacing it. According to the Koos grading system, this tumour is classified as:

• A Koos grade II
• B Koos grade III ✓
• C Koos grade IV
• D Koos grade I

Explanation

In the Koos classification, grade I tumours are purely intracanalicular, grade II extend into the cerebellopontine angle without contacting the brainstem, grade III fill the cistern and reach the brainstem without displacement, and grade IV displace the brainstem and cranial nerves. Contact without displacement defines grade III exactly. Grade IV requires visible compression or displacement of the pons, which this scan does not show.

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

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Written and medically reviewed by the StethoPrep medical team.