Plexiform perineurioma of the lip: a case report and review of literature.

We describe a plexiform perineurioma, which is an extremely rare variant of soft tissue perineurioma. A 43-year-old Japanese man presented with a painless, well-demarcated, elastic, soft tumor measuring 2.0 × 1.9 cm on the upper lip that had persisted for three years. Microscopically, a plexiform tumor composed of minimally atypical spindle cells with wavy nuclei was located in the lamina propria of the oral mucosa. Tumor cells were concentrically arranged around small vessels and aligned in parallel with delicate collagen fibers on a fibromyxoid background in the hypocellular area. Tumor cells were immunohistochemically positive for EMA, GLUT-1, claudin-1, and CD34 and negative for S-100 protein. The histopathological and immunohistochemical profiles of the excised tumor indicated a diagnosis of plexiform perineurioma. The patient has remained free of recurrence for sixteen months. Intraoral soft tissue perineurioma is rare and two of the four reported plexiform perineuriomas, including that described herein, notably involved the intraoral area. According to previous reports, plexiform perineuriomas largely develop in middle-aged women without a history of type 1 or type 2 neurofibromatosis. The clinical courses of all reported plexiform perineuriomas have been benign.