Extraneural perineurioma

Extraneural perineurioma is a rare, benign soft tissue tumor that arises from perineurial cells — the specialized cells that normally form the protective sheath (perineurium) surrounding peripheral nerve fascicles. Unlike intraneural perineurioma, which develops within a nerve, extraneural perineurioma (also called soft tissue perineurioma) occurs outside of nerves, typically presenting as a painless, slow-growing mass in the subcutaneous tissue or deeper soft tissues. It most commonly affects the limbs and trunk, though it can occur in various anatomical locations including the skin (cutaneous or sclerosing perineurioma variant), gastrointestinal tract, and other sites. Clinically, patients usually present with a well-circumscribed, firm, painless nodule or mass that grows slowly over months to years. The tumor is composed of spindle-shaped perineurial cells that characteristically express epithelial membrane antigen (EMA) and claudin-1 on immunohistochemistry, which helps distinguish it from other soft tissue tumors such as schwannomas, neurofibromas, and low-grade fibromyxoid sarcomas. The tumor is classified as benign by the World Health Organization, and local recurrence after excision is uncommon. Malignant transformation has not been convincingly documented. The standard treatment for extraneural perineurioma is complete surgical excision, which is generally curative. No adjuvant therapy such as chemotherapy or radiation is typically required. Prognosis is excellent, with very low recurrence rates following adequate resection. Because of its rarity, diagnosis can be challenging and often requires histopathological examination with immunohistochemical confirmation. Patients should be followed clinically after surgery, though long-term surveillance protocols are not well established due to the tumor's benign nature.

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