Intraneural perineurioma
An intraneural perineurioma is a rare benign tumor within the sheath of a single nerve that grows but typically does not recur or metastasize.  These lesions are only composed of perineurial cells, cloned from a single cell. They are distinct from schwannoma and neurofibroma.
Intraneural perineurioma is a neoplastic proliferation of perineurial cells with unique immunohistochemistry and ultrastructural features, and it is distinct from other onion bulb Schwann cell-derived entities. Despite harboring molecular abnormalities of the long arm of chromosome 22, intraneural perineurioma has not been associated with neurofibromatosis.Due to the involvement of one or more nerve fascicles, intraneural perineuriomas are distinguished by a localized, solitary expansion of peripheral nerves. These tumors develop slowly or stay stable over time.

References
Further reading
Macarenco RS, Ellinger F, Oliveira AM (2007). "Perineurioma: a distinctive and underrecognized peripheral nerve sheath neoplasm". Archives of Pathology & Laboratory Medicine. 131 (4): 625–36. doi:10.1043/1543-2165(2007)131[625:PADAUP]2.0.CO;2. PMID 17425397.
Kum YS, Kim JK, Cho CH, Kim HK (2009). "Intraneural reticular perineurioma of the hypoglossal nerve". Head Neck. 31 (6): 833–7. doi:10.1002/hed.20965. PMID 18972430.
Chung JH, Jeong SH, Dhong ES, Han SK (2014). "Surgical removal of intraneural perineurioma arising in the brachial plexus using an interfascicular dissection technique". Arch Plast Surg. 41 (3): 296–9. doi:10.5999/aps.2014.41.3.296. PMC 4037780. PMID 24883285.
Emory TS, Scheithauer BW, Hirose T, Wood M, Onofrio BM, Jenkins RB (1995). "Intraneural perineurioma. A clonal neoplasm associated with abnormalities of chromosome 22". Am J Clin Pathol. 103 (6): 696–704. doi:10.1093/ajcp/103.6.696. PMID 7785653.