If a canine mast cell tumor shows Pattern I KIT staining, what is the expected relative prognosis compared with Pattern II/III?

• A. Pattern I indicates worse prognosis
• B. Pattern I indicates no prognostic value
• C. Pattern I indicates better prognosis
• D. Pattern I and Pattern II are equivalent

Answer: (C)

KIT staining pattern in canine mast cell tumors reflects how the KIT receptor is localized and signaling. When staining shows a membranous, outward-facing (pattern I) distribution, this suggests normal KIT trafficking with less disruption of the receptor, typically associated with less aggressive tumor biology. In contrast, cytoplasmic or perinuclear (patterns II and III) staining indicates abnormal KIT localization often driven by activating KIT mutations, which is linked to more aggressive behavior, higher mitotic activity, and poorer prognosis. Therefore, a tumor with pattern I staining generally has a better prognosis than tumors with patterns II or III.