Diffuse midline glioma, H3-K27M mutant: Awareness leads to identification

Midline astrocytic neoplasms have distinct molecular characteristics, quite different from astrocytic neoplasms with similar morphology but not located in the midline. It is imperative that neuropathologists should be aware of the existence of these tumors, so they can be correctly diagnosed. Here, we discuss the case of a 14-year-old boy who presented with acute onset of vomiting followed by loss of consciousness. Subsequent magnetic resonance imaging revealed an ill-defined exophytic lesion arising from the brainstem and extending into the left cerebellopontine angle, with areas of hemorrhage and patchy restricted diffusion. The tumor was resected. Microscopy revealed medium-sized tumor cells in diffuse sheets, having round nuclei, granular chromatin, and scant cytoplasm. Microvascular and endothelial cell proliferation in small necrotic areas were seen. Mitosis was 0–1 per high-power field. By routine histopathological analysis, all features were consistent with the diagnosis of glioblastoma. Tumor cells were immunopositive for glial fibrillary acidic protein and isocitrate dehydrogenase-1 mutation (R132H), immunonegative for p53, and retained alpha thalassemia/mental retardation syndrome X-linked. It also showed a strong immunopositivity for H3-K27M mutation. A diagnosis of a diffuse midline glioma with H3-K27M mutation corresponding to the World Health Organization Grade IV was made. This case highlights the importance of exploring signature mutations in well-defined tumor categories such as H3-K27M-mutant diffuse midline glioma.