Phyllodes tumour of the breast is classified as benign, borderline, or malignant. Which histological feature is MOST important in distinguishing borderline from malignant phyllodes tumour?

• A Presence of any stromal overgrowth
• B Epithelial atypia
• C Tumour size >5 cm
• D Number of mitoses per 10 high-power fields (≥10 = malignant) ✓

Explanation

The WHO classification distinguishes phyllodes tumour grades primarily by stromal mitotic activity: benign <5/10 HPF, borderline 5–9/10 HPF, and malignant ≥10/10 HPF. Stromal overgrowth (stroma obliterating epithelial elements), marked stromal atypia, and infiltrative margins also contribute, but mitotic count is the single most important criterion. Tumour size does not define grade. Epithelial atypia is a feature of the epithelial component and does not drive phyllodes grading.

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

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