Both dermal and subcutaneous MCTs were categorized into low and high grade using what subcutaneous mitotic count threshold?

• A. Low mitotic count: ≤2 per 10 HPF; High mitotic count: ≥8 per 10 HPF
• B. Low mitotic count: ≤6 per 10 HPF; High mitotic count: ≥12 per 10 HPF
• C. Low mitotic count: ≤4 per 10 HPF at 400×; High mitotic count: ≥5 per 10 HPF
• D. Low mitotic count: ≤3 per 10 HPF; High mitotic count: ≥7 per 10 HPF

Answer: (C)

Mitotic activity is a key measure of tumor proliferation and directly influences prognosis in canine mast cell tumors. In the two-tier grading approach used for both dermal and subcutaneous MCTs, tumors are stratified by how many mitotic figures appear across 10 high-power fields when viewed at 400×. The cutoff is five mitotic figures per 10 HPF: four or fewer in those 10 HPFs is considered low-grade, while five or more is high-grade. This threshold provides the best discrimination of prognosis, with higher counts associated with worse outcomes, including greater metastatic risk and shorter survival. When counting, examine the most mitotically active area and tally mitotic figures in 10 consecutive HPFs at 400×, focusing on mitotic figures in viable tumor tissue.