Immunohistochemical Labeling of Multiple Myeloma Oncogene 1/Interferon Regulatory Factor 4 (MUM1/IRF-4) in Canine Cutaneous Histiocytoma.

Multiple myeloma oncogene 1/interferon regulatory factor 4 (MUM1/IRF-4) immunohistochemistry (IHC) is mainly used for diagnostic confirmation of plasma cell tumors (PCTs) in dogs and cats. This article describes MUM1/IRF-4 IHC expression in 20 cases of canine cutaneous histiocytoma (CH) and compares it with 10 cutaneous or mucocutaneous PCTs and 5 cutaneous histiocytic sarcomas (HSs) submitted to the same IHC protocol. All histiocytomas had strong nuclear and variable cytoplasmic immunolabeling for MUM1/IRF-4, whereas all PCTs had strong nuclear and moderate cytoplasmic immunolabeling for MUM1/IRF-4. No MUM1/IRF-4 immunolabeling was detected in the HSs. Although not typically a diagnostic challenge, MUM1/IRF-4 expression may have to be used with caution or in conjunction with additional immunomarkers to differentiate among poorly differentiated round cell tumors, especially when a histiocytic or plasma cell origin is suspected.