Gamma Knife radiosurgery delivered to internal auditory canal glioblastoma under a presumed diagnosis of vestibular schwannoma: illustrative case.

BACKGROUND: Primary glioblastoma (GBM) involving the internal auditory canal (IAC) is exceedingly rare. Only 6 prior cases have been reported, initially misdiagnosed as benign or nerve sheath pathology and treated surgically before receiving radiation therapy. The diagnostic utility of [¹⁸F]fluorodeoxyglucose positron emission tomography (FDG-PET) in these tumors is uncharacterized. OBSERVATIONS: A 39-year-old man presented with rapidly progressive right hearing loss, facial palsy (House-Brackmann grades III-V within 1 month), and gait disturbances. MRI showed a right cerebellopontine angle (CPA) mass extending into the IAC, a small contralateral intracanalicular lesion, and a nonenhancing frontal lesion. FDG uptake was confined to the CPA lesion. Under a working diagnosis of vestibular schwannoma with atypical features, possibly Wishart-type neurofibromatosis type 2, Gamma Knife radiosurgery (marginal dose 12 Gy) was delivered. Eight days later, deterioration prompted biopsy with difficult hemostasis. Histopathological analysis revealed IDH-wildtype GBM with a gliosarcomatous component. Adjuvant therapy was declined. The patient died 2.7 months postoperatively with leptomeningeal dissemination. LESSONS: This is the seventh reported high-grade glioma involving the IAC and the first in which radiosurgery preceded histopathological diagnosis. Strong FDG uptake in CPA lesions with rapidly progressive clinical features should prompt consideration of malignancy and tissue diagnosis before radiation-based treatment. https://thejns.org/doi/10.3171/CASE26668.