Concurrent multiple myeloma and mast cell neoplasia in a 13‐year‐old castrated male Maine Coon cat

A 13‐year‐old, castrated male Maine Coon cat was presented to Oklahoma State University Boren Veterinary Medical Teaching Hospital for yearly echocardiographic examination monitoring hypertrophic cardiomyopathy (HCM) diagnosed in 2003. Physical examination revealed a heart murmur and premature beats, likely related to HCM, but was otherwise unremarkable. A biochemistry profile revealed a hyperglobulinemia (6.3 g/dL). Cytologic examination of fine‐needle aspirates from liver and spleen revealed increased numbers of plasma cells and mast cells, confirmed with subsequent histologic examination. Immunohistochemistry (IHC) for c‐kit in the spleen and liver showed mast cells predominantly exhibiting type I staining pattern, with moderate numbers exhibiting type II pattern in spleen, and scattered cells exhibiting type II and III patterns in liver. Bone marrow cytology and core biopsy documented approximately 22% plasma cells. Cutaneous masses on the cat's left shoulder and right carpus were cytologically confirmed mast cell tumors. Serum protein electrophoresis with immunofixation confirmed an IgG monoclonal gammopathy. This is an example of 2 hematologic neoplasms occurring simultaneously in a cat. Concurrent pathologies may be overlooked if a single disease is diagnosed and suspected of causing all clinical signs. Both neoplasms were well differentiated, and neoplastic cells could have easily been interpreted as a reactive population had a full workup not been performed. Missing either diagnosis could result in a potentially lethal outcome. Eleven months after diagnoses, the cat was clinically doing well following a splenectomy and oral prednisolone and chlorambucil chemotherapy. Globulins decreased to 4.9 g/dL.