Malignant nerve sheath tumor infiltrating the epididymis of an inguinal cryptorchid testis in a domestic shorthair cat.

Case summary: An 8-year-old, male domestic shorthair cat presented with a firm mass in the right inguinal region. Microscopic evaluation of the surgically excised mass revealed a soft tissue sarcoma (STS), with features most consistent with a malignant nerve sheath tumor (MNST), infiltrating the epididymis of an atrophic intra-inguinal cryptorchid testis. The tumor showed classical Antoni A and B patterns on histopathology, with spindle cells arranged in short interlacing fascicles, and a mitotic count of 14 per 10 high-power fields, supportive of a malignant form. Immunohistochemistry revealed positive labeling of the neoplastic cells for the markers S-100 and vimentin, while labeling for glial fibrillary acidic protein was negative; these findings supported the diagnosis of MNST. Relevance and novel information: Nerve sheath tumors (NSTs) in cats are not uncommon neoplasms, typically arising from cutaneous or subcutaneous nerves of the head, neck or limbs. NSTs arising from visceral sites appear relatively rare, and there are no previously published reports describing such a tumor associated with a cryptorchid testis. This case expands the documented anatomical distribution of feline NSTs and underlines the fact that they may occur at visceral sites as well as within the skin and subcutis. It also highlights the need to include NST in the list of tumors that may arise within or be associated with the testes, cryptorchid or otherwise, and to consider neoplasia arising within a cryptorchid testis as a potential differential for an inguinal swelling in an intact male cat.