Histopathologic classification of 171 cases of canine and feline non-Hodkgin lymphoma according to the WHO classification.

Introduction: A retrospective study on 123 canine and 48 feline lymphomas was carried out in order to confirm the suitability of the classification system for animal lymphomas, proposed in 2002 by the WHO, which is derived from the REAL classification for human Non-Hodgkin lymphoma. Materials and Methods: All cases were classified according to the WHO classification, the Kiel classification and the National Cancer Institute Working Formulation. Microscopic examination was performed after standard staining (haematoxylin and eosin) and immunohistochemical labelling for B (CD79) or T (CD3) cell phenotypes. Results: A high prevalence of B cell lymphomas in dogs (97/123; 79.9%) and of T cell lymphomas in cats (31/48; 64.6%) was observed. Particularly, in dogs the most frequent B cell lymphomas were B large cell lymphomas (B-LCL: 56/97; 57.7%) and plasmacytic tumours (PCT: 20/97; 20.6%). Among T cell lymphomas the T lymphoblastic type was the most frequent in dogs (T-LBL: 8/26; 30.8%) as intestinal (ITCL: 16/31; 51.6%) and peripheral (PTCL: 11/31; 35.5%) in cats; B-LCL (8/17; 47.1%) was the commonest feline B cell lymphoma. Discussion: The main difficulties using the WHO classification have been the presence, among B-LCL, apart from the diffuse large B cell lymphoma (DLBCL) type and the large cell immunoblastic lymphoma (LCIBL) type often present in mixed type, of 13 cases (12 dogs and 1 cat) with a plasmacytoid differentiation which is not considered as a distinct subtype. In addition, two cases of PCT in dogs expressed both CD79 and CD3 positivities. Furthermore, we met a certain difficulty in differentiating B-LBL (3 dogs) from Burkitt-type lymphoma. ITCL (5 dogs and 16 cats) exhibited a huge morphologic variability. Finally, there were 4 cases of multicentric mature small T cell lymphomas and a thymic T cell lymphoma in dogs, which are not codified in the REAL classification. Conclusion: The WHO classification has been adapted from human to veterinary medicine, but since in some cases no category fits to animal lymphomas and some categories seem to be not represented in veterinary medicine, further studies in cooperation between clinicians and pathologists should be performed to improve its effectiveness; a further separation into specific classifications for canine and feline lymphoma seems to be required.