Which statement best describes Patnaik Grade III MCTs?

• A. They are well-differentiated and confined to the dermis
• B. They are highly cellular, poorly differentiated, replacing subcutaneous and deep tissues, with 3 to 6 mitotic figures per HPF
• C. They have 0–2 mitotic figures per 400 HPF
• D. They show only limited invasion and low mitotic rate

Answer: (B)

Patnaik Grade III MCTs are the most aggressive canine mast cell tumors, characterized by high cellularity, poor differentiation, and invasion into subcutaneous and deeper tissues, accompanied by a brisk mitotic rate. The statement describing a tumor that is highly cellular, poorly differentiated, replacing subcutaneous and deep tissues, with 3 to 6 mitotic figures per HPF matches this profile, highlighting both the malignant morphology and the frequent cell division that define a high-grade tumor. In contrast, lower-grade tumors are more well differentiated, often confined to the dermis or superficial tissues, and exhibit fewer mitotic figures, which is why those descriptions do not fit Grade III.