Malignant peripheral nerve sheath tumour with divergent epithelioid differentiation in a cat

Divergent differentiation, mainly towards various subsets of mesenchymal cells, is encountered sporadically in human malignant peripheral nerve sheath tumours (MPNSTs) but this is the first report of epithelioid components within this neoplasm in a cat. An 8-year-old, spayed female Domestic Shorthaired cat was presented for surgical removal of a subcutaneous mass on the right flank. Morphological and immunohistochemical analysis revealed a malignant neoplasm with spindloid cells intermixed with an epithelioid component that had squamous differentiation. There was intense immunolabelling of vimentin, S100 protein, neuron-specific enolase, laminin and glial fibrillary acidic protein in the spindloid cell component and for cytokeratin (CK) AE1/AE3 and CK5/6 in the epithelial elements. Melanoma-associated antigen, desmin, α-smooth muscle actin, CD18, CD31, ionized calcium binding adapter molecule-1 and CK8/18 were not expressed, which helped differentiate the tumour from other feline spindloid cell neoplasms. These features are characteristic of divergent epithelioid differentiation of MPNST.