Phyllodes tumour of the breast differs from fibroadenoma in several important aspects. Which feature MOST reliably distinguishes a borderline/malignant phyllodes tumour from a cellular fibroadenoma?
- A Biphasic epithelial and stromal composition
- B Presence of leaf-like projections on gross pathology
- C Stromal overgrowth defined as absence of epithelium in at least one low-power field ✓
- D Size greater than 5 cm
Correct answer: C. Stromal overgrowth defined as absence of epithelium in at least one low-power field
Explanation
Stromal overgrowth — defined as a low-power field (4x objective, 40 mm diameter) devoid of epithelium — is the most significant histological feature indicating malignant potential in phyllodes tumour and correlates with metastatic risk. Other grading criteria include stromal cellularity, mitotic rate (>10/10 HPF in malignant), nuclear pleomorphism, and infiltrative margins. Leaf-like projections and biphasic composition are present in all phyllodes grades. Size alone is not a reliable discriminator.
Reference: Bailey & Love's Short Practice of Surgery, 27th ed.
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