Malignant peripheral nerve sheath tumor: the utility of fascicular biopsy and teased fiber studies.

We present the case of a 67-year-old man with a malignant peripheral nerve sheath tumor (MPNST) affecting the brachial plexus. He presented with progressive right upper extremity paresthesias, numbness, weakness, and severe pain. Nerve conduction studies/electromyography demonstrated a right lower and middle trunk predominant brachial plexopathy. Three-tesla magnetic resonance imaging of the brachial plexus showed a soft tissue mass with central necrosis and cystic changes and irregular contrast enhancement. Positron emission tomography showed increased fluorodeoxyglucose uptake within the mass. Targeted fascicular nerve biopsy revealed hypercellular tumor, featuring atypical cells with mitotic figures and limited immunoreactivity for S-100 protein. The findings were those of an MPNST. The effects on the variably involved fascicles were also seen in teased fiber preparations, paraffin sections, and through immunohistochemistry. This case illustrates the presentation of this rare type of tumor, and characteristic neuroimaging and pathologic features of MPNSTs.