Background: It is important to differentiate between schwannomas and neurofibromas for the cases in which the histopathologic features overlap. Depending on the tumor type, surgeons can decide on a treatment method and whether to preserve or sacrifice the nerve; the possibility of malignant transformation in the case of neurofibromas also needs to be considered. Methods:We studied 101 cases of schwannoma and 103 cases of neurofibroma. All the hematoxylin and eosin slides for these cases were reviewed, and tissue microarrays were prepared from the representative areas. Immunohistochemical analysis was performed using antibodies for S-100 protein, calretinin, CD56 and CD34. Results: All the tumors except 3 neurofibromas were positive for the S-100 protein. Calretinin was found in 26.7% of the schwannomas (27/101), but it was not found in any of the neurofibromas. CD56 was positive in 77.2% of the schwannomas (78/101) and in 9.8% of the neurofibromas (10/102). CD34 was positive in 42.5% of the schwannomas (43/101) and in 80.2% of the neurofibromas (81/101). Statistically, calretinin was significantly specific for schwannomas (p< 0.001) and CD56 was also sensitive for these tumors (p < 0.001). On the other hand, a CD34 expression seemed highly sensitive (p < 0.001) for neurofibromas. Conclusions: We concluded that combined immunohistochemical analysis for calretinin, CD56, and CD34 may be very useful for differentiating schwannomas from neurofibromas.

Perineuriomas are composed of differentiated perineurial cells. Perineuriomas have been recently recognized by the immunoreactivity for epithelial membrane antigen (EMA). Microscopically, perineuriomas show proliferation of spindle cells with wavy nuclei and delicate elongated bipolar cytoplasmic processes. The tumor cells are usually negative for the S-100 protein. Ultrastructurally, perineurial cells reveal slender, nontapered processes containing pinocytic vesicles and discontinuous basal lamina. Interestingly, hybrid tumors of benign peripheral nerve sheath tumor (PNST) have been recently reported by using immunohistochemical and ultrastructural investigations. Herein, we report a case of soft tissue perineurioma arising in the skin of a 56-year-old female; another case of a hybrid tumor of perineurioma and schwannoma in the posterior mediastinum occurred in a 53-year-old male, which is the first case of the hybrid PNST tumor reported in Korea.