Forty five lymphomas, 14 of T cell origin, 28 of B cell and 3 with null-cell phenotype, were included in this study. Tumors were classified according to the updated Kiel classification system adapted to canine lymphomas. The percentage of Ki67+ cells and mitotic index (MI) were estimated in each specimen. Most of lymphomas (39 of 45) had high proliferation activity. Among them in 27 cases 50-70% of lymphoma cells expressed Ki67, the highest Ki67 expression (>70% Ki67+ cells) was identified less frequently, in 12 cases. Moderate Ki67 expression (20-50% positive cells) was observed in 5 cases, only one tumor had low Ki67 expression (<20% positive cells). Lower percentage of Ki67+ cells was usually accompanied with lower MI. The mean MI values in discussed groups differed significantly. Mean MI value was also significantly higher in T cell than in B cell lymphomas (4.30 vs. 3.33). Moreover, high positive correlation between the expression of Ki67 and MI was found (r = 0.668; P ≤ 0.001). In T-cell tumors the correlation was very high (r = 0.83; P ≤ 0.001) and in B-cell lymphomas the correlation was high (r = 0.61; P ≤ 0.001). There were also differences between mean MI values in the lymphomas of different morphological subtypes, but in some of them high variations in the range of MI values were identified and wide overlaps of MI between individual cases from different subtypes were observed. Because of differences in the proliferation activity in single cases of the same subtype of lymphoma, the proliferation activity assessment may be helpful to chose appropriate scheme of treatment and should be commonly performed during routine histopathological diagnosis of canine lymphomas.
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