Diffuse pediatric-type high-grade glioma, H3-wildtype and IDH-wildtype, of the adult cerebellum: a report of 2 cases.

The 2021 World Health Organization (WHO) Classification of Tumors of the Central Nervous System, 5th Edition, defines diffuse pediatric-type high-grade glioma, H3-wildtype and IDH-wildtype (dpHGG, H3-/IDH-WT) as a molecularly distinct entity, primarily diagnosed by DNA methylation profiling. Although this tumor predominantly occurs in children, its clinicopathological characteristics in adults are poorly understood. Here, we report two adult patients with cerebellar tumors that were histologically consistent with glioblastoma, but were reclassified as dpHGG, H3-/IDH-WT, RTK1 subtype based on genome-wide DNA methylation analysis. Both patients presented with large infiltrative cerebellar masses and experienced an aggressive clinical course with early postoperative progression. Histologically, the tumors exhibited high cellularity, marked cytological atypia, brisk mitotic activity, and microvascular proliferation without necrosis. Molecular analyses revealed PDGFRA amplification and homozygous deletion of CDKN2A in both cases, while neither showed EGFR amplification. These findings suggest that a subset of morphologically defined cerebellar glioblastomas in adults represents the dpHGG, H3-/IDH-WT, RTK1 subtype. Recognition of this underappreciated manifestation is important for accurate tumor classification, and further accumulation of well-characterized cases is required to clarify its clinical significance.