Phyllodes tumour of the breast is correctly classified into borderline category based on which histological feature?
- A 5-9 mitoses per 10 high-power fields, infiltrative margins, mild stromal overgrowth ✓
- B 0-4 mitoses per 10 HPF, pushing margins, mild atypia, no stromal overgrowth
- C >10 mitoses per 10 HPF, infiltrative margins, marked stromal overgrowth
- D Heterologous stromal elements (liposarcoma, rhabdomyosarcoma) regardless of mitotic count
Correct answer: A. 5-9 mitoses per 10 high-power fields, infiltrative margins, mild stromal overgrowth
Explanation
Phyllodes tumours are classified as benign (0-4 mitoses/10 HPF, pushing margins, mild atypia), borderline (5-9 mitoses/10 HPF, infiltrative or pushing margins, mild-moderate atypia), or malignant (≥10 mitoses/10 HPF, infiltrative margins, marked stromal overgrowth or heterologous elements). Borderline tumours require wide local excision with 1-cm clear margins to minimize recurrence risk. Heterologous elements (option D) characterize malignant phyllodes.
Reference: Bailey & Love's Short Practice of Surgery, 27th ed.
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