Abstract
Background: Perineurioma is a benign mesenchymal neoplasm commonly occurring in superficial soft tissues of extremities or trunk. They express EMA, Claudin-1 and GLUT-1 but are negative for S100, cytokeratins, SMA, desmin and MUC4. We present an unusual case of perineurioma with metaplastic ossification and aberrant MUC4 expression. Case Description: A 52-year-old female presented with a mass near right knee at an outside hospital. Surgical excision of the mass was performed; we received the case in consultation. Microscopic examination showed a 2.5 cm subcutaneous circumscribed mass composed of bland spindle cells having elongated thin streamers of cytoplasm on both ends of the nuclei arranged in short fascicles and whorls in a background of dense sclerotic collagen. Mitotic figures were infrequent. Necrosis was not identified. Large islands of woven bone were present around the periphery of the lesion, forming an incomplete shell of bone. The lesional cells were positive for EMA, GLUT-1, Claudin-1 and MUC4 (focal scattered granular staining) and were negative for S100, CD34, SOX-10, desmin, neurofilament protein, SSX, and SS18-SSX. FISH for rearrangement of FUS and EWSR1 loci was negative. A diagnosis of benign soft tissue perineurioma was made. Conclusion: Metaplastic bone and MUC4 expression are both extremely rare findings in perineurioma; we are only aware of one prior report of MUC4 expression in perineurioma in the literature. Metaplastic bone could cause confusion with ossifying fibromyxoid tumor, and MUC4 expression could cause confusion with low grade fibromyxoid sarcoma. Pathologists should remain cautious of these diagnostic pitfalls.
