LAUSR

A 5-year-old male neutered Bernese Mountain Dog was presented for cutaneous plasmacytoma, which was treated by surgical excision. Four months later, the dog developed multiple skin masses, hyphema, pericardial and mild bicavitary effusions, myocardial masses, and marked plasmacytosis in the peripheral blood. Circulating plasma cells expressed CD34 and MHC class II by flow cytometry. Immunocytochemistry demonstrated that these cells were strongly positive for multiple myeloma oncogene 1/interferon regulatory factor 4 (MUM-1) and weakly to moderately positive for Pax5. The dog was hypoglobulinemic but had a monoclonal IgA gammopathy detected by serum immunofixation electrophoresis. The PCR analysis of antigen receptor gene rearrangements (PARR) by fragment analysis using GeneScan methodology revealed that plasmacytoid cells in the original cutaneous plasmacytoma and peripheral blood had an identical immunoglobulin heavy chain gene (IgH) rearrangement, indicating that both populations were derived from the same neoplastic clone. Canine cutaneous plasmacytoma rarely progresses to a malignant form and plasma cell leukemia is rarely diagnosed in the dog. This report describes a case of cutaneous plasmacytoma progressing to plasma cell leukemia with a rapid and aggressive clinical course. This report also highlights the utility of flow cytometry, immunocytochemistry, immunofixation electrophoresis, and PARR by fragment analysis using GeneScan methodology in the diagnosis of this hematopoietic neoplasm.