title: Benign peripheral nerve sheath tumor of the perianal region in a young pony.
abstract: A 20 x 10 cm, lobulated mass was present in the perianal region of a 4-year-old Dales pony. Histopathology revealed an unencapsulated, loose arrangement of sheets and whorls of narrow mesenchymal cells, situated in the deep dermis. Intervening areas had a myxomatous appearance. The whorls were often arranged around a central structure resembling an axon or a vascular structure. Immunohistochemistry revealed that the elongated mesenchymal cells and central axon-like structures expressed vimentin, S-100, and neuron-specific enolase, but not pancytokeratin, glial fibrillary acid protein, and the neurofilament markers, NR4 and 2F11. On the basis of the histopathology and immunohistochemistry, a diagnosis of benign peripheral nerve sheath tumor (schwannoma type) was made. This case was unusual in that the concentric laminations of Schwann cells were very loosely arranged, with an intervening myxomatous stroma (Antoni type B appearance) and despite its benign histological appearance, the mass extended deeply to the proximal sacral vertebrae. Its exact origin was unclear; it may have arisen from cutaneous nerves with deep extension or from neural structures in the sacral region. Due to the incomplete surgical removal, regrowth of the mass occurred.
authors.affiliation: Veterinary Pathology Unit, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Veterinary Centre, Roslin, Midlothian, EH25 9RG, United Kingdom.
authors.forename: Ben P R
authors.initials: BP
authors.lastname: Sturgeon
authors.forename: Elspeth M
authors.initials: EM
authors.lastname: Milne
authors.forename: Kenneth C
authors.initials: KC
authors.lastname: Smith
mesh_terms: D000818:Animals; D001005:Anus Neoplasms / Q000473:pathology / Q000601:surgery / Q000662:veterinary*; D006734:Horse Diseases / Q000473:pathology* / Q000601:surgery; D006736:Horses; D007150:Immunohistochemistry / Q000662:veterinary; D008297:Male; D009364:Neoplasm Recurrence, Local / Q000473:pathology / Q000662:veterinary; D018317:Nerve Sheath Neoplasms / Q000473:pathology / Q000601:surgery / Q000662:veterinary*