Histologic spectrum of canine extramedullary plasmacytomas: Diagnostic challenges and immunohistochemical differentiation from cutaneous histiocytomas

Here we assess the diagnostic features of 73 canine extramedullary plasmacytomas (EMPs) and their differentiation from canine cutaneous histiocytomas (CHs) using histology and immunohistochemistry for MUM1, CD3, CD79a, immunoglobulin (Ig) light chains (λ and κ), and IBA1. Most EMPs were classified as cleaved and mature subtypes. Almost 92% of EMPs were positive for MUM1, whereas 77% were positive for CD79a and 74% for Ig light chains λ and κ. The use of MUM1 and Ig light chains resulted in a sensitivity of 100% in diagnosing EMPs, surpassing the combination of MUM1 and CD79a (94%). In 8% of the EMPs, there was pseudofollicular arrangement of neoplastic cells, whereas 7% had amyloid deposition and 3% had mineralization. CHs were positive for IBA1 but negative for all other IHCs.