H3K27M-mutant spinal cord and hemispheric tumor with prolonged survival: illustrative case.

backgroundH3K27M-altered diffuse midline glioma (DMG) is a recently classified tumor in the 2021 WHO classification of central nervous system tumors. Pediatric patients with H3K27M-altered DMG have a universally poor prognosis, with a median survival of 9-15 months. The prognosis of adult patients with the same tumor type is more variable, with rare cases of prolonged survival. OBSERVATIONS: The authors report the case of a 32-year-old male who presented with Brown-Séquard syndrome secondary to a heterogeneously enhancing intramedullary mass extending from T7 to T10. The lesion was resected, and the patient was diagnosed with an H3K27M-altered DMG. The patient presented again 29 months later with a nonenhancing right-sided temporal mass. This was again determined to be an H3K27M-mutated glioma with a molecular signature similar to the intramedullary mass. He received postoperative radiation therapy after each surgery and continued to survive well at the 42-month follow-up. LESSONS: The authors report the rare case of an adult-onset H3K27M midline glioma in the spinal cord and subsequent dissemination to the right temporal lobe with prolonged survival. This suggests that H3K27M-altered DMGs are heterogeneous entities with variable prognoses based on location and age. https://thejns.org/doi/10.3171/CASE24668.