Comparison between Ki-67 index and mitotic index for predicting outcome in canine mast cell tumours

To assess correlation between Ki67 index and mitotic index and determine which more accurately predicts survival in canine mast cell tumours.

Retrospective analysis of cases from three UK referral hospitals. Correlation between Ki67 index and mitotic index was assessed and survival analysis performed.

A total of 162 dogs were included: 57 dogs died with 37 due to mast cell tumour. Correlation between Ki67 index and mitotic index was moderate, while the agreement was poor. A high Ki67 index was considered sensitive (86 · 5%) at predicting mast cell tumour-related death, but poorly specific (57 · 9%). Mitotic index(>5) was poorly sensitive (32 · 4%), but highly specific (96%). A mitotic index of ê2 had a 75 · 7% sensitivity and an 80 · 0% specificity. Ki67 index showed a statistically significant survival difference within the mitotic index <2 (P = 0 · 009) group. Ki67 index did not predict survival rate in tumours with mitotic index of ê2.

Correlation between Ki67 and mitotic index is moderate. High mitotic index accurately predicted death, but many dogs with low mitotic index also died. Low Ki67 accurately predicted survival, but high Ki67 should not be considered a poor prognostic indicator. A three-tier mitotic index assessment may more accurately predict death due to mast cell tumour.