Malignant Peripheral Nerve Sheath Tumor With Primitive Neuroepithelial Differentiation in an Adult: A Case Report

Primitive neuroepithelial differentiation in malignant peripheral nerve sheath tumors (MPNSTs) has been reported in children but is extremely rare in adults. The authors report the case of a 70-year-old woman who presented with swelling of the right leg of 1-month duration. Fine-needle aspiration cytology was suggestive of a benign peripheral nerve sheath tumor. Histopathological examination of the excised mass revealed a MPNST with spindle-cell areas and a few round-cell areas with rosettes. The spindle cells showed positive immunoreactivity for S-100 protein and vimentin and negativity for desmin, confirming their nerve sheath origin. The round cells were immunoreactive for synaptophysin and chromogranin, indicating primitive neuroepithelial differentiation. These cells did not stain for CD99, which is consistently expressed by the cells of primitive neuroectodermal tumors (PNETs) of bone and soft tissue but not by central nervous system (CNS) PNETs or medulloblastomas. In this case, the PNET-like focus resembled a CNS-PNET.