A pediatric low-grade sinonasal mesenchymal tumor harboring a novel CHD9::BEND2 fusion.

BEND2-rearranged neoplasms represent a newly emerging group of tumors with variable morphology, involving multiple tissue types and anatomical sites such as the central nervous system, bone, soft tissue, and salivary glands. Here, we describe a novel CHD9::BEND2 fusion in a low-grade spindle cell mesenchymal tumor arising in the sinonasal tract of a 5-year-old girl. Histologically, the tumor exhibited ovoid to spindle-shaped cells with moderate atypia, numerous thin-walled vessels, and focal necrosis. Immunohistochemistry did not indicate a specific line of differentiation, showing focal SATB2 positivity and a low proliferative index. The CHD9::BEND2 fusion was identified by whole transcriptome sequencing and confirmed by RT-PCR, Sanger sequencing, and break-apart FISH. This case broadens the clinicopathological and molecular spectrum of BEND2-associated tumors, suggesting a potential oncogenic role of CHD9::BEND2 in pediatric mesenchymal neoplasia.