A 28-year-old woman presents with a 2 cm breast lump that has been stable for 3 years. Imaging shows a well-circumscribed, homogeneous lesion with no internal vascularity. Which single feature best distinguishes fibroadenoma from phyllodes tumour?

Explanation

## Distinguishing Fibroadenoma from Phyllodes Tumour ### Key Histological Difference **Key Point:** The hallmark discriminating feature is the **leaf-like (phyllodes) stromal architecture** seen in phyllodes tumours but absent in fibroadenomas. This architectural pattern is pathognomonic and is the basis for the tumour's name. ### Comparative Table | Feature | Fibroadenoma | Phyllodes Tumour | | --- | --- | --- | | **Stromal architecture** | Uniform, non-leaf-like | Leaf-like (phyllodes) — **DISCRIMINATOR** | | **Stromal cellularity** | Low to moderate, uniform | High, variable; increased mitoses | | **Age of onset** | 20–40 years (peak 20–25) | 40–60 years (older) | | **Imaging margins** | Well-defined | Well-defined (overlap) | | **Growth pattern** | Slow, stable | Rapid, progressive | | **Mitotic rate** | < 5 per 10 HPF | Often > 10 per 10 HPF | | **Malignant potential** | None | Yes (5–10% phyllodes are malignant) | ### Clinical Pearl **Clinical Pearl:** While both lesions may appear well-circumscribed on imaging and present in reproductive-age women, **only phyllodes tumours show the characteristic leaf-like stromal folding pattern on histology**. This is the single most reliable microscopic discriminator and is the reason the WHO classification uses this feature as the defining criterion. **High-Yield:** Fibroadenomas have **uniform, non-leaf-like stroma**, whereas phyllodes tumours have **prominent leaf-like stromal projections with increased cellularity and mitotic activity**. This distinction is essential for prognosis and management (fibroadenoma = benign; phyllodes = potentially malignant). ### Why Other Options Are Misleading - **Age of presentation (option 1):** Both can occur in young women; phyllodes typically occurs in older women (40–60), but overlap exists. - **Well-defined margins (option 2):** Both lesions typically have well-defined imaging margins; this is not discriminatory. - **Absence of mitotic activity (option 3):** Fibroadenomas have low mitotic rates, but this is a matter of degree, not a sharp cutoff. Phyllodes can also show low mitotic activity in benign variants.