Cytologic features of malignant peripheral nerve sheath tumor

OBJECTIVE: To study the cytomorphologic features of malignant peripheral ne rve sheath tumor (MPNST), including the epithelioid cell variant, and to es tablish differential diagnostic features with benign neurogenic tumors and other sarcomas. STUDY DESIGN: Cytologic smears from primary, recurrent and metastatic tumor s in 10 patients with MPNST were reviewed. Three patients had neurofibromat osis 1 (NF1), and in two others the tumor arose from a preexisting neurofib roma. Immunocytochemical evaluation of S-100 protein was performed in four cases. A complete pathologic study was available in all cases. To assess th e validity of morphologic recognition, a blinded study, including eight cas es of spindle MPNST among smears from histologically proven schwannomas, sy novial sarcomas, leiomyosarcomas, malignant fibrous histiocytomas and lipos arcomas, was performed. RESULTS: Neurogenic differentiation was recognizable in four cases (differe ntiated), while the other four (anaplastic) were indistinguishable from oth er pleomorphic sarcomas. The presence of elongated, slender, often wavy nuc lei and less commonly a delicate, fibrillary metachromatic stroma were feat ures suggestive of nerve sheath differentiation. Other cytologic, as well a s clinical, features permitted their identification as malignant. Two cases of epithelioid MPNST disclosed large, polygonal to plasmocytoid tumor cell s without specific cytologic features. S-100 immunoexpression was positive in two of the four cytologic samples tested. CONCLUSION: Although no morphologic findings are specific to MPNST, the abo ve-mentioned cytologic features may suggest, in differentiated cases, its n eurogenic differentiation. On the basis of morphologic features alone, the diagnosis of anaplastic and epithelioid MPNST is not possible, and immunocy tochemical and ultrastructural studies are necessary. A specific cytodiagno sis is possible in recurrences, metastases and cases of NF1 or a preexistin g neurofibroma.