• Yi-Hua Wang, Jian Gu, Juan-Han Yu, Lin Fu, Qing-Chang Li, Xue-Shan Qiu, and En-Hua Wang.
• Department of Pathology, The First Affiliated Hospital of China Medical University, Shenyang, P.R. China.
• Medicine (Baltimore). 2022 Jun 17; 101 (24): e29448.

IntroductionDiffuse midline glioma with H3-K27M mutation is an infiltrative high-grade glioma, with predominantly astrocytic differentiation.Patient ConcernsA 54-year-old Chinese woman presented with memory loss for a month and walking instability for 15 days.DiagnosisMagnetic resonance imaging showed a mass shadow of isometric T1 and slightly longer T2 with mild mixed signals in the third ventricle of the suprasellar region. Histologically, the tumor was primarily sheet-like, with many "anucleate areas" composed of long and thin fibrillary processes of the bipolar cells, which formed "whorls." The neoplastic nuclei were ovoid and moderate in size. The tumor showed brisk mitotic activity and vascular proliferation, with no necrosis. In addition to histone H3K27M mutation, immunohistochemical staining showed that the tumor cells were positive for glial fibrillary acidic protein, oligodendrocyte transcription factor 2, alpha-thalassemia/mental retardation syndrome X, S-100 and Vimentin. The "anucleate areas" were positive for glial fibrillary acidic protein and negative for synaptophysin. The Ki-67 proliferation index was about 10%. Molecular genetic analyses detected H3F3A K27M mutation, but no mutations in IDH1 or IDH2, TERT promoter mutations, MGMT promoter methylation, KIAA1549-BRAF fusion or deletion of 1p/19q were found. Based on these findings, the patient was diagnosed as diffuse midline glioma with H3-K27M mutation in the third ventricle, corresponding to WHO grade 4.InterventionsA craniotomy with total excision of the tumor was performed.OutcomesAfter surgery, she was routinely treated with temozolomide for chemotherapy and synchronous radiotherapy. It has been 11 months now, and the patient is living well.ConclusionThis case report provides information on the microscopic morphological features of diffuse midline glioma with H3K27M mutation, which can help pathologists to make a definitive diagnosis of this tumor.Copyright © 2022 the Author(s). Published by Wolters Kluwer Health, Inc.

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