Aim: Canine lymphoma is one of the most common canine neoplasms, but little is known regarding the effects of exposure to tobacco smoke on their biologic behavior. As cytology is the most frequent diagnostic method of canine lymphoma, the aims of this study were to perform an immunocytochemical study of canine lymphomas, including subtyping and cell proliferation analysis, and to establish their correlation with tobacco smoke exposure.

Materials and Methods: A total of 23 dogs diagnosed with lymphoma were subjected to careful fine-needle biopsies of enlarged lymph nodes. The smears were air-dried, fixed with cold acetone, and immunocytochemically stained using CD3, PAX5, and Ki-67. Owners were requested to complete an epidemiologic questionnaire.

Results: According to the updated Kiel classification, 65% were B-cell lymphomas - three low grade (LG) and 12 high grade (HG) and 35% were T-cell - two LG and six HG. Thirteen tumors presented high Ki67 indexes (>40%) (11 HG and 2 LG), two revealed moderate ones (20-40%) (1 HG and 1 LG), and three had low indexes (≤20%) (1 HG and 2 LG). Both a significant positive correlation and a significant linear-by-linear association (p=0.018) were observed between high Ki67 indexes and smoking owners (r=0.753, p=0.002) as well as with the number of smokers in the household (r=0.641, p=0.001). Moreover, the mean percentage of Ki67⁺ cells from the group of "smoker owners" was statically higher (p=0.011) than that from the "non-smoker owners."

Conclusion: The results suggest that cytological diagnosis of canine lymphomas benefits from being complemented with immunocytochemical studies that include subtyping and assessment of proliferative activity, both contributing for the prognosis and therapeutic planning. Furthermore, exposure to tobacco smoke seems to be related to the biological behavior of canine lymphomas.

Keywords: canine lymphoma, immunocytochemistry, proliferation, tobacco smoke.

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