Phyllodes tumour of the breast is classified by the WHO into benign, borderline, and malignant. Which histological feature best distinguishes malignant phyllodes from borderline phyllodes?

• A Stromal hypercellularity
• B Presence of a leaf-like architecture
• C Stromal mitotic count >10 per 10 high-power fields with stromal overgrowth ✓
• D Epithelial atypia

Explanation

Malignant phyllodes tumours are characterised by stromal mitoses >10/10 HPF, marked stromal atypia, stromal overgrowth (defined as absence of epithelium in at least one low-power field), and infiltrative borders. Stromal hypercellularity alone is seen in all grades. Leaf-like architecture is the hallmark of all phyllodes tumours regardless of grade. Epithelial atypia is not the defining criterion; the malignant behaviour resides in the stromal component.

Reference: Bailey & Love's Short Practice of Surgery, 27th ed.

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