Hybrid Schwannoma/Perineurioma: Morphologic Variations and Genetic Profiles

To clarify the morphologic spectrum and molecular profiles of hybrid schwannoma/perineurioma (HSP), we investigated 15 tumors clinicopathologically and cytogenetically. HSP was classified into 2 morphologic types: mixed cellular and combined tumor types. The former comprising of 14 tumors mostly arose in the subcutaneous tissue of the extremities and the trunk of middle-aged adults. They were well-circumscribed and composed of elongated spindle-shaped tumor cells arranged in storiform and whorl patterns. Immunostaining revealed a mixed cellular proliferation of S-100 protein-positive and SOX10-positive Schwann cells and epithelial membrane antigen-positive, claudin 1-positive, and GLUT1-positive perineurial cells. During follow-up, no tumors were found to have recurred in any cases. In contrast, in the combined tumor type arising in the mediastinum of a young male with neurofibromatosis type 2, the intraneural perineurioma-like areas, characterized by small whorl-like structures, were present in plexiform schwannoma-like areas. No recurrence was noted in the case. Molecular analyses (array comparative genomic hybridization and fluorescence in situ hybridization) revealed LOH 22q in 2 tumors of 5 studied: one each of the mixed cellular and combined tumor types. Although the same diagnostic term, HSP, has been applied to both mixed and combined types, they should be separated from each other.