Phyllodes tumor of the breast: which feature most reliably distinguishes a borderline from a malignant phyllodes tumor on histology?
- A Mitotic count ≥10 per 10 high-power fields with stromal overgrowth ✓
- B Stromal hypercellularity
- C Leaf-like intracanalicular architecture
- D Epithelial hyperplasia within the fronds
Correct answer: A. Mitotic count ≥10 per 10 high-power fields with stromal overgrowth
Explanation
Malignant phyllodes tumor is characterized by ≥10 mitoses per 10 high-power fields, marked stromal overgrowth (stroma replacing all epithelium in a 4× field), severe stromal atypia, and infiltrative margins. Borderline tumors show intermediate features (5–9 mitoses/10 HPF, moderate atypia, pushing margins). Stromal hypercellularity alone is present across all grades. Leaf-like architecture is a feature of phyllodes (vs. fibroadenoma) regardless of grade. Epithelial hyperplasia is an incidental finding.
Reference: Bailey & Love's Short Practice of Surgery, 27th ed.
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