Malignant primitive neuroectodermal tumor presenting as a scalp nodule.

We report a case of a 20-year-old woman who presented with a 3-year history of a stable cystic nodule on the scalp. Light microscopy of the excised nodule demonstrated a malignant small round cell undifferentiated neoplasm. Immunohistochemical studies suggested a neural crest origin, while ultrastructural examination revealed characteristics of schwannian differentiation. Both of these special techniques were essential in establishing the diagnosis of a malignant epithelioid schwannoma of superficial tissue (neurotropic melanoma, desmoplastic melanoma). We discuss the differential diagnoses and describe the immunohistochemical and ultrastructural characteristics of this tumor. This case illustrates the difficulty of diagnosing this rare malignant tumor, which masqueraded as a benign-appearing scalp lesion.