Abstract. Mitotic index (MI) is an indirect measure of cell proliferation that has been demonstrated to be a strong predictor of outcome for several human and canine cancers. The purpose of this study was to evaluate the utility of MI as a predictor of biologic behavior and survival in dogs with cutaneous mast cell tumors (MCTs). Medical records from 148 dogs with histologically confirmed MCTs were reviewed. Information regarding tumor grade, local recurrence, metastatic disease, date of death/last follow-up, and outcome was obtained. The region of the tumor with the highest overall mitotic activity was chosen for evaluation, and the MI value was defined as the number of mitotic figures/10 high-power fields (4003, 2.7 mm 2 ). A Cox proportional hazards regression model was used to compare MI with survival data. A Mann-Whitney test was used to compare MI on the basis of the development of local recurrence and metastatic disease. The MI correlated directly with tumor grade (P , .0001). The median survival time for dogs with an MI #5 was significantly longer (70 months) than for those with an MI .5 (2 months), regardless of grade (P , .001). For grade II tumors with an MI #5, the median survival time (MST) was 70 months, compared with 5 months for those with an MI .5 (P , .001). For grade III tumors with an MI #5, the MST was not reached, compared with ,2 months for those with an MI .5 (P , .001). In conclusion, MI is a strong predictor of overall survival for dogs with cutaneous MCTs and should be included as a prognostic indicator when determining therapeutic options.

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