RESUMO
Background: Canine T-zone lymphoma (TZL) is recognized as an indolent CD45-T cell lymphoma, with low aggressiveness and high overall survival. The diagnosis is obtained by histopathology and immunohistochemistry, but also by cytological examination of the lymph node associated with immunophenotyping. Lymphocytosis is commonly identified as around 10,000 cells/µl and may reach 30,760 cells/µl. Case Description: The present report describes a case of a female Golden Retriever, nine years old, with generalized lymphadenopathy. In the cytological examination of the superficial cervical lymph node, a monomorphic population of small, "clear cells" and "hand mirror" lymphocyte shape was suggestive of TZL. The leukogram showed intense leukocytosis (160,050 cells/µl) due to small clear cell lymphocytosis (152,048 cells/µl). The myelogram showed a myeloid:erythroid ratio of 2:3; with a pyramidal distribution of cell types and the presence of 22.8% of lymphocytes in the differential count. Bone marrow, peripheral blood, and lymph node immunophenotyping resulted in lymphocyte gates with 97.3% to 99.5% CD5+, predominantly CD4-, CD8-, and CD45- confirming the diagnosis of TZL with associated leukemia. Treatment with chlorambucil and prednisolone was started. During the first month, the lymphocytosis remained above 200,000 cells/uL. After four months of treatment, there was a decrease in lymphocytes, which progressively reached a count of 10,800 cells/ul in the eleventh month. Conclusion: In the literature, lymphocytosis above 30,760 cells/µl has not been observed in TZLs. Thus, it is believed that this is the first report of extreme lymphocytosis with a slow response to chemotherapy.