Which statement best describes the relationship between histologic grading and KIT pattern information in prognosis for canine mast cell tumors?

• A. KIT pattern provides the most reliable prognostic information, and histologic grade is secondary.
• B. Histologic grading is the only prognostic indicator; KIT pattern is not informative.
• C. Histologic grading remains the most reliable prognostic indicator, with KIT pattern offering additional, but complementary, information.
• D. Only Ki67 has prognostic value.

Answer: (C)

Histologic grading remains the strongest predictor of prognosis in canine mast cell tumors, reflecting the tumor’s inherent aggressiveness. KIT pattern information from immunohistochemistry adds valuable, complementary insight that helps refine prognosis within a given grade. Specifically, a pattern showing diffuse cytoplasmic (KIT pattern III) tends to be associated with more aggressive behavior, while a membranous pattern (KIT pattern I) generally aligns with less aggressive disease, with the intermediate pattern II falling in between. So, while KIT pattern can sharpen risk assessment, it does not replace the primary importance of histologic grade. Ki67 can also provide prognostic information, but it does not stand alone as the sole predictor.