Benign peripheral nerve sheath tumor of the perianal region in a young pony.

The study presents a case of benign peripheral nerve sheath tumor found in the perianal region of a 4-year-old horse, demonstrating the complexity and unconventional nature of the tumor’s origin and growth.

Case Introduction and Diagnosis

• A massive (20 x 10 cm) and lobulated mass was discovered in the perianal region of a young Dales pony. The mass was unencapsulated, featuring a disorganized distribution of thin mesenchymal cells located in the deep dermis.
• Characteristics of the tumor included areas that possessed a myxomatous, or gelatinous, appearance and swirls of mesenchymal cells organized around a central structure, suggestive of either an axon or a vascular formation.
• The mesenchymal cells and the central structures similar to axons were positive for vimentin, S-100, and neuron-specific enolase – proteins produced by cells of neuroectodermal and mesenchymal origin. In contrast, these cells did not express pancytokeratin, glial fibrillary acid protein, and two neurofilament markers, NR4 and 2F11.
• The specific arrangement and immunohistochemical profile led to the diagnosis of a benign peripheral nerve sheath tumor, more specifically a schwannoma type of tumor. Schwannomas are benign tumors that originate from Schwann cells, which produce the myelin sheath around peripheral nerves.

Unusual Features of the Case

• This case was deemed unusual due to several factors. Firstly, the concentric arrangement of Schwann cells was very loose, interspersed with a myxomatous, or mucoid, stroma. This is referred to as an Antoni type B appearance.
• Secondly, despite its benign histological appearance, the growth extended deep into the nearby proximal sacral vertebrae.
• The exact origin of the tumor was difficult to determine; it could have developed from cutaneous nerves with deep extension or from neural structures situated in the sacral region.

Regrowth After Removal

• The tumor was not completely removed in surgery, leading to its regrowth. This points out the necessity of thorough surgical removal of such tumors to prevent recurrence.