Prognostic value of histopathological grading in canine extramedullary plasmacytomas.

The canine extramedullary plasmacytoma (cEMP) has recently been the subject of numerous investigations, indicating that the histomorphologic diagnosis is often difficult because of the variety of morphologic features. Therefore, the objective of this study was to establish a subclassification scheme for cEMPs and to evaluate correlations between the types and malignancy. Retrospectively, 117 cEMPs, all immunohistochemically characterized by a monoclonal immunoglobulin light-chain expression, were collected and assigned to morphologic types. These types were compared using data from a follow-up study on metastasis and tumor recurrence, then compared by proliferation rate, determined by immunohistochemical detection of the antigen Ki-67. Histopathologic typing revealed five different types of cEMPs, ranging from the mature type with typical plasma cells to the polymorphous-blastic type. Between these two forms, three additional types were established: hyaline, cleaved, and asynchronous. Most of the cEMPs were of the cleaved and asynchronous types. In all cEMPs, mononuclear and multinuclear giant cells were present to varying degrees. Although the results of cell proliferation and the follow-up study indicated less benign behavior by the polymorphous-blastic type, the proliferation rate revealed no statistically significant differences among the cEMP types. The clinical data therefore confirmed previous findings that the risk of tumor recurrence and metastasis in general can be classed as low. The established cEMP typing system is probably a very helpful diagnostic tool, although the types cannot be used for a tumor grading system.