Nerve Sheath Tumor in a Cynomolgus Macaque (Macaca fascicularis) and Diagnostic Differentiation from Amelanotic Melanoma.

There are few reports describing spontaneous neoplasms in cynomolgus macaques, despite the frequent use of this species in laboratory research. This report describes cytologic, histologic, and immunohistochemical findings of a cutaneous to subcutaneous nerve sheath tumor located within the haired skin of the abdomen of a 2.5-y-old, intact, female, captive Mauritius cynomolgus macaque. The nerve sheath tumor was well demarcated, partially encapsulated, densely cellular, and extended from the subcutis to the most superficial dermis, abutting the epidermis. Neoplastic cells formed intersecting streams and had a high mitotic count (18 per 2.37 mm2). Due to the substantial morphologic overlap of this neoplasm with amelanotic melanoma, particularly the close association with the epidermis, immunohistochemistry was required for definitive diagnosis. Neoplastic cells were immunoreactive to vimentin, S-100, SOX10, laminin, collagen IV, and CD56, and negative for melan-A, tyrosinase, MITF, and HMB45. This immunohistochemical profile is diagnostic for nerve sheath tumor based on human and canine criteria and rules out amelanotic melanoma. Despite incomplete excision, the nerve sheath tumor had not grossly recurred after 1 mo, at which point the animal was euthanized for unrelated reasons. This report underscores the importance of using an immunohistochemical panel in cases of cutaneous and subcutaneous spindle cell neoplasms, as there is substantial morphologic and immunohistochemical overlap between nerve sheath tumors and melanocytic neoplasms due to their shared neuroectodermal origin. To our knowledge, this is the first report of a nerve sheath tumor in a cynomolgus macaque, and one of the few reports of spontaneous neoplasia in this species.