Morphology and immunoreactivity of canine and feline extramedullary plasmacytomas

The aim of the study was the evaluation of morphology and immunophenotype of canine (19 cases) and feline (7 cases) extramedullary plasmacytomas. Tumours, located in skin, oral cavity and spleen were surgically excised, fixed and processed for histopathology and immunohistochemistry (CD79α, CD18, proliferating cell nuclear antigen, metallothionein). Histologically, tumours were classified into mature, cleaved, asynchronous, polymorphous blastic, hyalin, or monomorphous blastic type. All evaluated tumours showed cytoplasmic expression of CD79α antigen. The expression of CD18 was observed in canine cutaneous and splenic tumours. In canine tumours expression of metallothionein was low to moderate, while in feline plasmacytomas - absent or low. In canine tumours, the mitotic index and proliferating cell nuclear antigen index were positively correlated with the expression of metallothionein. In feline tumours no correlation between mitotic index, proliferating cell nuclear antigen and metallothionein was found. This is the first study describing expression of metallothionein in canine and feline extramedullary plasmacytoma.