Rare osseous mastoid invasion of secondary gliosarcoma: illustrative case.

backgroundGliosarcomas are rare isocitratedehydrogenase-wildtype glioblastomas characterized by their glial and sacromatous components. A gliosarcoma is primary if the tumor arose de novo and secondary if the tumor arose from a previous glioblastoma. Here, the authors describe a unique case of a secondary gliosarcoma invading the mastoid and affecting the horizontal segment of the facial nerve. OBSERVATIONS: A 69-year-old female presented to the emergency department with 2 weeks of right-sided facial weakness. Six years earlier, she had undergone a gross-total resection for a right temporal glioma and was treated with chemoradiotherapy postoperatively. Pathology confirmed isocitrate dehydrogenase-negative, O6-methylguanine-DNAmethyltransferase-methylated glioblastoma. At current presentation, MRI demonstrated a right temporal tumor that invaded the mastoid. After surgical debulking, the tumor was confirmed to be a secondary gliosarcoma. CT of the temporal bone showed significant tegmen destruction and direct invasion of the horizontal segment of the facial nerve. LESSONS: Facial weakness was a symptom of the secondary glioscarcoma tumor invading the mastoid and affecting the horizontal segment of the facial nerve. To the authors' knowledge, this symptom has not been previously reported and thus adds to the limited knowledge of secondary gliosarcoma presentations. https://thejns.org/doi/10.3171/CASE25179.