Superficial malignant peripheral nerve sheath tumor: a rare and challenging diagnosis.

We reviewed the clinicopathologic features of 5 cases of malignant peripheral nerve sheath tumor (MPNST) manifesting in superficial locations associated with cutaneous neurofibromas (4 cases) or superficial peripheral nerve (1 case). Four cases had spindle cell morphologic features and were at least focally positive for S-100 protein, whereas the associated benign neural elements had more extensive S-100 immunoreactivity. The single epithelioid case was diffusely and strongly positive for S-100 protein. Melanoma markers, epithelial membrane antigen, glial fibrillary acidic protein, neurofilament, pancytokeratin (AE1/AE3), CD34, smooth muscle actin, and desmin were negative in all cases. There were no local recurrences, but 3 patients died of metastatic disease within 2 to 30 months (median, 21 months). MPNSTs can occur in a superficial location and may have an aggressive clinical course. Immunohistochemical markers are helpful in excluding other lesions in the differential diagnosis. However, identification of a benign precursor or origin from a nerve may be the most definitive way to properly classify these rare lesions.