PROGNOSTIC VALUE OF KI67 IN CANINE LYMPHOMA DETERMINED BY FLOW CYTOMETRY

Background: Percentage of Ki67-positive cells (%Ki67) is related to malignancy and survival in human nonHodgkin’s lymphoma. Little data from immunohistochemical studies are available for canine lymphoma (LSA). Objective: To investigate the prognostic significance of flow cytometric %Ki67 in canine LSA. Methods: 47 canine LSAs were classified by flow cytometry and cytology, according to the updated Kiehl classification as: 33 high-grade (HG) and 1 low-grade (LG) B-LSA; 10 HG and 3 LG T-LSA. Lymph node aspirates were labeled with Ki67-FITC (clone MIB1) using a methanol-based method. Differences in %Ki67 were assessed by a t-test. Kaplan-Meier curves were used to evaluate prognostic significance of %Ki67 for complete remission and survival in 16 dogs treated with the University of Wisconsin- Madison protocol. Results: %Ki67 was significantly higher in HG compared with LG LSA (p<0.001). No significant difference between B- and T-LSA was detected. Treated LSA with Ki67>40% showed a significantly shorter time to complete remission (CR) and a significantly lower survival at the end of chemotherapy (day 175) and overall survival compared to LSA with Ki67<40%, regardless of grade of malignancy and lineage. Ki67>40% was also associated with a shorter time to CR among HG LSA, B-LSA and HG B-LSA and with a lower overall survival among HG LSA. Conclusion: Flow cytometric %Ki67 is useful in differentiating HG vs LG LSA using a cutoff value of 12%. Regardless of cell lineage, %Ki67 can also be considered as a prognostic marker for time to remission and survival using a cutoff value of 40%.