Nf1-Associated Midline High-Grade Astrocytoma With Piloid Features Arising in the Dorsal Thalamus: A Diagnostic Consideration

High-grade astrocytoma with piloid features (HGAP) is a recently defined brain tumor entity primarily diagnosed via DNA methylation profiling due to its histological overlap with glioblastoma and pilocytic astrocytoma. We report a 26-year-old man with neurofibromatosis type 1 (NF1) presenting with a high-grade midline lesion involving the thalamus and mesencephalon. Histopathological examination demonstrated a high-grade glial neoplasm with loss of ATRX expression by immunohistochemistry. While HGAP typically occurs in the posterior fossa, NF1-associated cases show a marked predilection for midline locations, complicating the differential diagnosis with other high-grade gliomas. In this case, DNA methylation profiling confirmed the diagnosis with high confidence. Prognostically, HGAP demonstrates an intermediate survival profile, with a 5-year overall survival rate of approximately 50%, which is superior to IDH-wild-type glioblastoma but inferior to conventional pilocytic astrocytoma. Although standardized treatment guidelines are lacking, identifying targetable molecular alterations remains essential for management.