Immunohistochemical Demonstration of Factor XIIIa Expression in Neurofibromas: A Practical Means of Differentiating These Tumors From Neurotized Melanocytic Nevi and Schwannomas

• Neurofibromas, schwannomas, and neurotized melanocytic nevi may closely resemble one another at the light microscopic level. We studied 10 neurofibromas, 10 schwannomas, and 10 partially neurotized melanocytic nevi immunohistochemically using an antibody directed against factor XIIIa to determine if this antibody might provide a useful method of differentiating these lesions. The cases were also stained with S100 protein. All of the neurofibromas stained intensely for factor XIIIa. The proportion of cells staining within the tumors varied from 30% to 70%. In contrast, none of the schwannomas and neurotized nevi studied demonstrated staining of tumor cells with this antibody. S100 protein was expressed by 100% of neurofibromas, schwannomas, and melanocytic nevi. Our findings suggest that factor XIIIa may provide a reliable and practical means of differentiating cutaneous neurofibromas from neurotized nevi and cutaneous schwannomas. Distinguishing between these different tumor types may be important in some clinical situations, particularly with respect to rendering a diagnosis of von Recklinghausen's neurofibromatosis. The differences in the immunohistochemical profiles of neurofibromas and neurotized nevi support the concept that these tumors are histogenetically distinct, despite their similar histologic appearances.(Arch Dermatol. 1990;126:472-476)