Phyllodes tumor of the breast is classified as benign, borderline, or malignant based on histological criteria. Which single histological feature most reliably distinguishes malignant from borderline phyllodes tumor?

• A Presence of leaf-like clefts in the stroma
• B Stromal cellularity alone
• C Stromal overgrowth (absence of epithelial component in any single 10x field) ✓
• D Epithelial hyperplasia within the leaf-like fronds

Explanation

Stromal overgrowth — defined as the absence of any epithelial component in at least one low-power field (10x or 4x) — is the single most important criterion distinguishing malignant phyllodes tumor and is associated with the highest risk of distant metastasis. Malignant phyllodes also shows marked stromal cellularity (>10 mitoses/10 HPF), permeative/infiltrative margins, and overt sarcomatous stromal differentiation (heterologous elements like liposarcoma, rhabdomyosarcoma). Leaf-like clefts and stromal cellularity are present in all grades. Epithelial hyperplasia is not a grading criterion.

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

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Written and medically reviewed by the StethoPrep medical team.