An MRI shows a 2.2 cm acoustic neuroma (vestibular schwannoma) centered at the internal auditory canal with extension into the cerebellopontine angle cistern. The Koos grading for this tumor would be:
- A Koos Grade I
- B Koos Grade II
- C Koos Grade III ✓
- D Koos Grade IV
Correct answer: C. Koos Grade III
Explanation
The Koos grading system for vestibular schwannomas: Grade I — intracanalicular (within IAC); Grade II — small tumor with CPA extension ≤1 cm not reaching the brainstem; Grade III — medium tumor with CPA extension 1–2.5 cm, not touching the brainstem; Grade IV — large tumor >2.5 cm in CPA touching/compressing the brainstem. A 2.2 cm tumor in the CPA that has not yet compressed the brainstem fits Koos Grade III. This grading guides treatment choice (observation/stereotactic radiosurgery vs. microsurgery).
Reference: Dhingra Diseases of Ear, Nose and Throat, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.