Extrarenal malignant rhabdoid tumor infiltrating the brachial plexus and spinal canal.

Extrarenal malignant rhabdoid tumors are rare, aggressive lesions that primarily affect infants and children with characteristic SMARCB1/INI1 mutations. While rhabdoid tumors are most commonly found in the kidneys and central nervous system, they have been reported in virtually every soft tissue in the body. A 20-year-old previously healthy male presented with a 4-month history of left upper extremity weakness and pain and a 1-week history of lower extremity weakness. MRI showed a combined intradural and extradural mass arising from the C8 root and involving the lower trunk of the brachial plexus. CT guided biopsy followed with onsite cytologic analysis initially concerning for plasma cell etiology. Full body PET/CT showed no evidence of metastases. The tumor was surgically resected and debulked. Extensive immunohistochemical testing on the resected specimen revealed a diagnosis of extrarenal malignant rhabdoid tumor. Recurrence invading paraspinous muscles, left lateral paravertebral space, left apical lung, and brachial plexus was noted within 2 months. Radiation and chemotherapy were initiated. Here we present imaging findings, image-guided biopsy, surgical management, and pathologic diagnosis of a rare case of extrarenal, extracranial malignant rhabdoid tumor of the brachial plexus and surrounding paraspinous muscles in a young adult male.