Gastrointestinal round cell tumours in cats and dogs: a retrospective and comparative analysis.

Introduction: Alimentary lymphoma (AL) in cats is the most common type of gastrointestinal (GI) neoplasm and also the most frequent form of lymphoma. On the contrary, in dogs primary GI tumours are mostly epithelial in origin, and AL accounts for approximately only 5-7% of all canine lymphomas. Other primary GI round cell tumours are rare in both species, almost exclusively represented by plasma cell and mast cell tumours (in dogs and cats, respectively). The purpose of this study was to classify round cell tumours of the GI tract in dogs and cats, to determine their immunophenotype and to compare the results between the two species. Materials and Methods: Haematoxylin and eosin stained sections from formalin fixed paraffin embedded samples of GI tumours archived in our Department from 12 year period (1992-2004) have been reviewed, and round cell tumours extrapolated. In the selected 82 cases (68 cats, 18 dogs), CD3 and CD79 immunohistochemistry was performed, and these were classified histologically according to the recent WHO classification system for the hematopoietic tumours. Results: In our series of 109 feline GI neoplasms, round cell tumours accounted for 62.3% (30.4% epithelial, 7.3% mesenchymal tumours), and mainly consisted of AL (65/68, 96%) located in the intestine in 59 out of 65 cases. The only other primary non-lymphoid neoplasms were two malignant histiocytomas and one mast cell tumour. In the 96 canine GI neoplasms, round cell tumours accounted for 19% (59.3% epithelial, 21.9% mesenchymal tumours), all represented by AL (17 out of 18 cases located in the intestine). Most of feline GI lymphomas showed a CD3+ T cell phenotype, while a clear prevalence was not observed in dogs. In both dogs and cats, T cell lymphomas were mainly represented by intestinal T cell lymphoma, although some cases of peripheral T cell lymphoma and angiotropic lymphoma were also present. As regard the B cell lymphomas, B large cell lymphomas (diffuse large cell lymphomas and large cell immunoblastic lymphoma) were the most frequent in dogs and cats, followed by follicular (centre cell and MALT lymphoma). Conclusion: Our results showed in the feline GI neoplasms the uppermost prevalence of round cell tumours, whereas in the dog epithelial tumours were most common. Within round cell tumours, almost all were GI lymphomas in both species. The absence of prevalence of T out of B cell lymphomas in the dog and the marked prevalence of T cell lymphomas in the cat observed in our study are not aligned with the most recent literature data. These preliminary results lay the basis to further studies including a wider number of cases