CIC-rearranged sarcoma of the arm presenting as a soft tissue mass with regional lymphadenopathy and tumor thrombus.

Capicua transcriptional repressor (CIC)-rearranged sarcoma is an aggressive and rare small round cell tumor predominantly affecting the soft tissues of extremities, often with early metastasis and poor prognosis. We report a case of a 46-year-old female presenting with a CIC-rearranged sarcoma of the right arm, notably complicated by extensive tumor thrombus involving the brachial, axillary, and subclavian veins, extending into the internal jugular vein, and regional lymphadenopathy. While tumor thrombus in CIC-rearranged sarcoma has been described in several abdominal cases, this is the first case described in the extremity, to the best of our knowledge. This case highlights the challenges in diagnosing this rare entity, the critical role of molecular confirmation, and the unusual presentation of widespread venous involvement, impacting staging and treatment considerations.