Case report of a rare TCF3::BEND2-fused primary intracranial neuroepithelial neoplasm in a female child.

Astroblastoma, MN1-altered, is a rare circumscribed glial neoplasm defined by alterations in the MN1 gene and often the MN1::BEND2 fusion. However, it is noted that a subset of these tumors lacks the classic astroblastic perivascular pseudorosettes, which can complicate diagnosis. We report a diagnostically challenging case in a 6-year-old female presenting with gait disturbance and limb weakness. MRI identified a well-defined fourth ventricular roof mass causing obstructive hydrocephalus. Histology post-resection revealed a solid tumor composed of epithelioid cells arranged in sheets and nests, exhibiting abundant cytoplasm, nuclear atypia, and prominent grooves, but no astroblastic pseudorosettes or significant mitosis. Immunohistochemically, tumor cells were negative for GFAP and Olig2, showed only focal EMA expression, but exhibited diffuse PR positivity. Methylation profiling did not match any established WHO CNS5 subtype, while targeted NGS identified a defining TCF3::BEND2 in-frame fusion. This case supports the existence of a phenotypically distinct variant within the spectrum of BEND2-rearranged neuroepithelial tumors, characterized by the TCF3::BEND2 fusion. Although molecularly related to MN1-altered astroblastoma, it displays distinct histological and immunophenotypic features. These findings further support the role of BEND2 as a potential oncogenic driver and underscore the importance of recognizing this fusion-defined tumor as a distinct variant within the astroblastoma-like spectrum.