Phyllodes tumor of the breast is a fibroepithelial lesion. Which histological feature MOST reliably distinguishes malignant phyllodes tumor from a cellular fibroadenoma?

• A Marked stromal hypercellularity and atypia, with stromal mitoses ≥10/10 HPF and infiltrative margins ✓
• B Presence of leaf-like fronds covered by a double layer of epithelium
• C Dense fibrous stroma with hyalinization and calcification
• D Cleft-like spaces compressed by stromal proliferation with intracanalicular pattern

Explanation

Phyllodes tumors are classified into benign, borderline, and malignant categories based on stromal features rather than epithelial changes. Malignant phyllodes tumor is distinguished by marked stromal cellular atypia, high mitotic count (≥10 mitoses per 10 HPF), infiltrative tumor margins, and often stromal overgrowth (areas devoid of epithelium in ≥1 low-power field). Malignant phyllodes can undergo divergent sarcomatous differentiation (liposarcoma, rhabdomyosarcoma). Leaf-like fronds covered by epithelium define phyllodes tumors generally (vs. fibroadenoma) but do not distinguish malignant from benign. Fibroadenomas have bland, paucicellular stroma with low mitotic activity.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.