A Rare Case of CIC-DUX4 Sarcoma in a Mediastinal Lymph Node Diagnosed by Fine Needle Aspiration Cytology.

CIC-rearranged sarcomas (CDS) are rare, aggressive undifferentiated small round cell tumors recently classified as a distinct entity by the World Health Organization. These tumors are defined by CIC gene fusions, most commonly CIC::DUX4, and are associated with poor prognoses and diagnostic challenges, particularly in limited cytological samples. We report the case of a 37-year-old male who presented with respiratory symptoms and was found to have mediastinal lymphadenopathy. Fine-needle aspiration (FNA) of a right paratracheal lymph node revealed hypercellular smears of dyscohesive tumor cells with high nuclear-to-cytoplasmic ratios, prominent nucleoli, and cytoplasmic vacuoles. Immunohistochemistry showed positivity for WT1, CD99, and vimentin, while RNA sequencing confirmed a CIC::DUX4 fusion, establishing the diagnosis of CDS. This case highlights the critical role of integrating cytopathology, immunohistochemistry, and molecular diagnostics in identifying CDS, particularly in atypical anatomical sites. Early and accurate diagnosis is essential for guiding clinical management in this rare and aggressive sarcoma subtype.