Which of the following is NOT listed as a current prognostic indicator for mast cell tumors?

• A. Size of tumor
• B. KIT
• C. Ki67
• D. BAX

Answer: (A)

Prognosis in canine mast cell tumors hinges on tumor biology rather than how large the tumor is. The most informative predictors are histologic grade and mitotic index, which reflect how aggressively the cells are dividing. Proliferation markers like Ki-67 provide a quantitative measure of cellular turnover and consistently correlate with outcomes. Abnormal KIT signaling—seen as distinctive KIT staining patterns and specific KIT mutations—also tracks with more aggressive behavior and helps refine prognosis and guide potential targeted therapy. BAX, a pro-apoptotic protein, has been explored as part of the apoptotic balance within the tumor and can be associated with prognosis in some studies, though it is not as universally applied as Ki-67 or KIT status. In contrast, the physical size of the tumor alone does not reliably predict behavior or outcome across cases; small tumors can be high-grade and aggressive, while large ones can be less so. Therefore, size is not considered a current prognostic indicator.