Immunohistochemical and clonality analyses of CD30 ⁺ MUM1 ⁺ nonepitheliotropic T/NK-cell lymphomas in digits of dogs

The diagnosis of T-cell/natural killer (T/NK)-cell lymphoma in dogs may be challenging when neoplastic cells lack CD3 but express multiple myeloma oncogene-1 (MUM1), raising concerns for its misdiagnosis as a plasma cell tumor. We performed immunohistochemical and clonality analyses of 8 cases of canine digital CD30 ⁺ MUM1 ⁺ nonepitheliotropic lymphoma. Histologically, the diffuse or angiocentric infiltration of pleomorphic, large neoplastic lymphocytes was observed in the skin, often accompanied by granulation tissue and/or eosinophilic infiltration. Immunohistochemically, neoplastic lymphocytes expressed CD3 in 3 cases, but not in 5; however, all cases expressed at least 1 T/NK-cell marker, including CD8 or cytotoxic granules (granzyme B, perforin, or T-cell intracellular antigen-1). MUM1 was expressed in all cases, whereas immunoglobulin markers were negative. T-cell receptor gamma ( TRG ) clonality was detected in 5 cases and immunoglobulin heavy chain ( IGH ) clonality in 4 cases, limiting their diagnostic utility. These results demonstrate that canine digital, CD30 ⁺ MUM1 ⁺ nonepitheliotropic lymphomas retain T/NK-cell immunophenotypes and underscore the importance of comprehensive immunophenotyping for an accurate diagnosis.