In phyllodes tumor of the breast, which histological feature most reliably distinguishes malignant from borderline phyllodes?

• A Leaf-like intracanalicular architecture
• B Stromal cellularity of 5–10 mitoses per 10 HPF with permeative margins
• C Stromal overgrowth defined as stroma replacing epithelium in ≥1 low-power field
• D Presence of heterologous elements such as liposarcomatous or chondrosarcomatous differentiation ✓

Explanation

The presence of malignant heterologous stromal elements (liposarcoma, chondrosarcoma, rhabdomyosarcoma, osteosarcoma) is pathognomonic of malignant phyllodes tumor and is not seen in benign or borderline variants. Stromal overgrowth and increased mitoses are features used in WHO grading but are gradients rather than absolute classifiers. Leaf-like architecture is a defining feature of phyllodes tumor in general, not the malignant variant specifically. Heterologous differentiation is the single most discriminating histological feature for malignancy.

Reference: Bailey & Love's Short Practice of Surgery, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.