Prevalence and Clinicoradiopathological Characterization of H3 K27-Altered Diffuse Midline Gliomas in Adults—A Retrospective Observational Study

<b>Background/Objectives</b>: Diffuse midline glioma (DMG), H3 K27M-altered, represents a rare group of gliomas arising in midline structures of the central nervous system. Historically regarded as a pediatric entity, it is now increasingly recognized in adults. Although its relative prevalence among all midline diffuse gliomas and its clinical-radiological characteristics are well defined in children, these tumors remain less characterized in adults, and comparative evaluations with H3 K27 wildtype midline diffuse gliomas are limited. <b>Methods</b>: Consecutive adult patients with histopathologically confirmed diffuse glioma (WHO grade ≥ 2) diagnosed between 2016 and 2025 were retrospectively screened for midline tumor location, with systematic revision of imaging and pathology. For identified midline diffuse gliomas, comprehensive clinical, imaging, and immunohistochemical data were collected, and a detailed morphometric analysis was performed. H3 K27 alteration status was established immunohistochemically, with supplementary immunostaining when necessary. Descriptive and comparative analyses were conducted. <b>Results</b>: A total of 5% of the 541 adult diffuse gliomas were midline, and 23% of IDH wildtype midline gliomas were consistent with DMG, H3 K27-altered (all H3 K27M-mutant). The affected patients were significantly younger, and these tumors predominantly involved the thalamus and mesencephalon. Morphometric analyses revealed trends toward fewer high-grade features in H3 K27-altered tumors, with composite scores demonstrating significant discriminatory ability. The overall survival was not significantly different between groups but showed associations with ring-like enhancement as well as adjuvant and salvage therapies in the overall midline cohort. <b>Conclusions</b>: This study provides population-based prevalence estimates for DMG, H3 K27M-altered, and complements the limited literature with comparative clinical-radiological and morphometric data of potential prognostic relevance.