T-cell lymphoma with eosinophilic infiltration involving the intestinal tract in 11 dogs.

Among the intestinal tumors of hematopoietic cell origin, lymphoma is the most common in the dog. Herein, we characterized the clinical and pathologic features of 11 dogs (average age, 10.6 +/- 2.5 years) with T-cell lymphoma of the intestinal tract with eosinophil infiltrates. No sex predominance was apparent. All had localized tumor masses in the small intestine. Grossly, the intestinal wall was thickened, and the lumen of the affected intestine was usually narrowed. Microscopically, we observed transmural diffuse invasion of round to pleomorphic tumor cells. Tumor cells showed varying morphology, from scanty to abundant cytoplasm, and round to ovoid nuclei with scattered to dense chromatin. In seven of the dogs, tumor cells had infiltrated into the epithelium. All showed infiltration of eosinophils and all 11 tumors had a T-cell phenotype (CD3+, CD79-). Only one tumor stained positive for the mast cell marker c-kit and none was positive for mast cell tryptase. We did not observe ultrastructurally apparent granules in any of the tumor cells. These results suggest that, in dogs, T-cell lymphomas of intestinal origin resemble mast cell tumors of intestinal origin with respect to cell structure and eosinophil infiltration. Therefore, in the absence of epitheliotropism, it is difficult to confirm the differential diagnosis without immunostaining for mast cell and lymphocyte markers, including mast cell tryptase, c-kit, CD3, and CD79.