Histologically indolent but widely disseminated diffuse midline glioma H3K27-altered revealed by autopsy.

Diffuse midline glioma, H3K27-altered is a highly aggressive central nervous system tumor classified as grade 4 in the 2021 World Health Organization Classification of Tumors of the Central Nervous System. This malignancy typically arises in midline structures such as the thalamus, brainstem, and spinal cord, and is characterized by its infiltrative capacity. Despite the use of magnetic resonance imaging to evaluate tumor extension, its microscopic dissemination can extend far beyond radiological detection. We report the case of a 46-year-old woman with a thalamic diffuse midline glioma harboring an H3K27M mutation. She underwent endoscopic resection followed by standard radiotherapy and chemotherapy with temozolomide. Despite salvage treatments, she died 33 months after initial therapy. Autopsy revealed widespread microscopic infiltration involving the brainstem, cerebellum, and spinal meninges, extending from the cervical to lumbar regions. Notably, these lesions were not detected by MRI or routine histopathology. This case highlights the extensive infiltrative capacity of diffuse midline glioma with H3K27M mutation, demonstrating its ability to permeate central nervous system structures while preserving tissue architecture.