B cell lymphoma with mott cell differentiation in a dog

Abstract

Background: B cell lymphoma with Mott cell differentiation is rarely reported in dogs. This neoplasms has been diagnosed in the gastrointestinal tract and therefore is considered the differential diagnosis of obstruction by foreign body, enteritis and other intestinal neoplasms. The objective of the present study was to describe macroscopic, microscopic and immunohistochemical findings of a B cell lymphoma with Mott cell differentiation in the gastrointestinal tract of a dog. Case: A 7-year-old male German Shepherd dog was referred to necropsy with a mass 12 cm in diameter in the ileum-cecocolic junction, compromising mesenteric lymph nodes. Cytology showed two types of lymphoid cells. Approximately 80-90% of cells were round, with large cytoplasm containing numerous basophilic inclusion corpuscles, round nuclei, condensed chromatin, inconspicuous nucleoli, moderate anisokaryosis and marked anisocytosis. The other cells were round, with scarce basophilic cytoplasm, round nuclei, condensed chromatin and inconspicuous nucleoli. Mild anisokaryosis and anisocytosis and mitotic figures were sometimes identified. Histology revealed the predominance of round cells, with large cytoplasm containing numerous eosinophilic corpuscles (Russel bodies). Neoplastic cells were PAS and PTAH-positive, and toluidine blue-negative. Immunohistochemistry revealed positive immunoreactivity to antibodies against the CD-79α antigen, though no immunolabeling was observed to anti-CD3 and anti-lysozyme antibodies, which defined the diagnosis of canine B cell lymphoma with Mott cell differentiation. Discussion: B cell lymphoma in the gastrointestinal tract of dogs with Mott cell differentiation is a rare neoplasia that affects mainly the small intestine and the stomach. Cytological examination is a useful tool in the diagnosis of B cell lymphoma. In the disease, the punction of the neoplastic mass or of compromised lymph nodes reveals numerous neoplastic cells with large, slightly basophilic cytoplasm containing multiple inclusions of different sizes, indicative of Russell bodies. The histological examination of the neoplasm analyzed in this report underlines the similarities with gastrointestinal B cell lymphoma with Mott cell differentiation and confirms the heterogeneous cellular character of the tumor, with the predominance of lymphocytes with scarce homogeneous basophilic cytoplasm, round nucleus, condensed chromatin and high mitotic index. The cytoplasm inclusions present in Mott cells were PAS-positive, similarly to previous reports and PTAH-positive, which is common in this cell type. The intense and diffuse immunoreactivity of neoplastic cells to the anti-CD79α antibody demonstrates the predominance of B lymphocytes, which agrees with most previous reports. Although immunohistochemistry reveals the origin of this lymphoma, the differential diagnosis between B cell neoplasias like multiple myeloma and extracellular plasmocytoma is essential. B cell lymphoma with Mott cell differentiation in the gastrointestinal tract of dogs is rare, but it should be investigated in the differential diagnosis of proliferative or obstructive lesions in the small intestine. In the present report, histology indicated the diagnosis of round cell neoplasia, with the predominance of Mott cells, containing a large amount of Russell bodies that were intensely stained with PAS and PTAH. Immunohistochemistry revealed the strain of these lymphocytes, confirming the diagnosis.