What are the three recognized KIT IHC staining patterns in canine mast cell tumors?

• A. Pattern II (focal/stippled cytoplasmic); Pattern III (diffuse cytoplasmic); Pattern I (membranous)
• B. Pattern I (membranous); Pattern II (diffuse cytoplasmic); Pattern III (nuclear)
• C. Pattern I (membranous); Pattern II (focal/stippled cytoplasmic); Pattern III (diffuse cytoplasmic)
• D. Pattern I (membranous); Pattern II (diffuse cytoplasmic); Pattern III (cytoplasmic)

Answer: (C)

In canine mast cell tumors, KIT immunohistochemistry reveals how the receptor tyrosine kinase KIT (CD117) is distributed inside tumor cells, and this distribution reflects underlying mutations and tumor behavior. The three recognized patterns are: membranous staining, where KIT is localized mainly at the cell membrane; focal or stippled cytoplasmic staining, where KIT is seen in spots or patches within the cytoplasm; and diffuse cytoplasmic staining, where KIT is distributed throughout the cytoplasm. Pattern I (membranous) typically corresponds to normal or less altered KIT localization and is associated with a more favorable prognosis. Pattern II (focal/stippled cytoplasmic) indicates partial mislocalization with an intermediate prognosis. Pattern III (diffuse cytoplasmic) reflects marked cytoplasmic accumulation and is often linked to activating KIT mutations, associated with more aggressive behavior and poorer prognosis. Recognizing these patterns helps in prognostication and can influence considerations for targeted therapies.