An Unusual Case of H3K27-Altered Diffuse Midline Glioma Presenting as a Third Ventricular Mass

Diffuse midline gliomas (DMGs) are rare central nervous system tumors, typically arising from astrocytes. The H3K27-altered DMG subtype is defined by a specific histone protein mutation that disrupts normal cellular regulation and promotes unchecked proliferation. Due to their aggressive nature, the World Health Organization (WHO) universally classifies these as grade IV, regardless of histological appearance. While DMGs are most commonly described in pediatric populations and overwhelmingly originate in the pons, their occurrence and clinical progression in adults remain underexplored. This report highlights a rare presentation of an H3K27-altered DMG, presenting as a third ventricular mass in a 30-year-old man, emphasizing its atypical location and the challenges it presents for diagnosis and management.