In dermal MCTs, what was the HR for recurrence/metastasis in the high mitotic count group compared with the low mitotic count group?

• A. HR 7.9
• B. HR 6.0
• C. HR 11.1
• D. HR 3.0

Answer: (C)

High mitotic activity in dermal mast cell tumors signals a more aggressive tumor biology, which translates into a higher risk of adverse outcomes over time. In survival analysis, the hazard ratio compares the instantaneous risk of an event between two groups across the follow-up period. An HR of 11.1 means that at any point in time, dogs with a high mitotic count have about 11 times the risk of recurrence or metastasis compared with dogs with a low mitotic count, assuming the model’s proportional hazards hold. This large value indicates a strong prognostic impact of mitotic count on recurrence/metastasis. Of course, the exact interpretation depends on the study’s confidence interval and design, but the point estimate shows a markedly higher risk associated with high mitotic count.