Histologic classification and immunophenotyping of canine non-Hodgkin's lymphomas: unexpected high frequency of T cell lymphomas with B cell morphology.

Using three different classification schemes (Rappaport, the Working Formulation, and the Kiel classification), 116 canine malignant lymphomas were classified histologically. The number of lymphomas with a completely follicular growth pattern was low (14.9%). The majority of the lymphomas (50.8%) had a diffuse type of architecture, while 34.3% were diffuse with some nodularity. In the Rappaport scheme, 69.3% of the canine lymphomas were classified as histiocytic lymphomas, but these consisted of a group of tumors with different morphologic and immunologic cell types. The Working Formulation and the Kiel classification could be applied to differentiate the canine lymphomas cytomorphologically. In both the Working Formulation and the Kiel classification, only a minority of lymphomas (16.4 and 12.0%, respectively) were low-grade malignant lymphomas. Large cell or centroblastic lymphomas were the most frequently encountered in the Working Formulation or the Kiel classification, respectively. Immunophenotyping of 95 lymphomas was performed with the aid of a panel of monoclonal and polyclonal antibodies. The majority of the lymphomas were of B cell origin (58.9%) while three were classified as non-B/non-T cell lymphomas. Contrary to the distribution pattern of human non-Hodgkin's lymphoma (NHL) in western countries, there was a high percentage of T cell lymphomas (37.9%) in the canine. However, the phenotype could not be predicted by the morphologic characteristics alone.