PA30.{3,5} | Phyllodes Tumor & Breast Morphology — Summary & Reflection

REFLECT

Go back to the hook scenario — the 35-year-old woman with rapidly enlarging breast lump, final diagnosis borderline phyllodes tumor. You now know what that report means:

• 'Leaf-like epithelial-lined clefts' — the phyllodes architecture you can now visualise at low power
• 'Hypercellular stroma' — crowded spindle cells unlike the pale hypocellular stroma of fibroadenoma
• 'Mitoses 4 per 10 HPF' — below the malignant threshold (≥10) but above benign (<5) → borderline
• 'Margins: infiltrative' — borderline/malignant feature, not the pushing margins of benign
• 'No heterologous elements' — no sarcomatous differentiation → remains borderline

Borderline phyllodes = 2 cm wide margin needed. Not enucleation. Not mastectomy. The pathology report determined the surgical margin. This is why morphology matters.

Now take 5 minutes and draw from memory: the phyllodes leaf-like architecture, the Indian-file of ILC, and a comedo duct with central necrosis. Sketching histology is the fastest way to hard-wire the patterns.

KEY TAKEAWAYS

SDL3 Summary — Phyllodes Tumor & Breast Morphology Practical

Phyllodes tumor:
- Fibroepithelial tumor; older age, rapid growth, leaf-like architecture, hypercellular stroma — these 4 features distinguish it from fibroadenoma at first glance
- Grade benign/borderline/malignant using 5 WHO parameters: stromal cellularity, atypia, mitoses (< 5 / 5–9 / ≥10), margins (pushing/variable/infiltrative), stromal overgrowth (absent/absent/present)
- Metastasizes haematogenously to lung — NOT lymphatically. No axillary dissection.
- Treatment: wide local excision with 1/2/mastectomy margin for benign/borderline/malignant

Practical identification essentials:
- Fibroadenoma: hypocellular stroma, intracanalicular + pericanalicular patterns
- Fibrocystic change: cysts, apocrine metaplasia, fibrosis ± ADH
- Intraductal papilloma: fibrovascular core with bilayer (myoepithelial preserved)
- DCIS (comedo): comedonecrosis + central calcification; cells confined within intact duct
- LCIS: distended acini, discohesive cells, E-cadherin negative, bilateral risk marker
- IDC NST: desmoplasia + infiltrating nests — the most common invasive type
- ILC: Indian-file pattern, E-cadherin negative
- Mucinous: cells floating in mucin pools; gelatinous gross; good prognosis
- Medullary: syncytial sheets + lymphocytic infiltrate + pushing margins; BRCA1
- Tubular: angulated glands, single layer, no myoepithelium; best prognosis
- Paget disease: Paget cells in nipple epidermis; always associated with underlying DCIS or invasive carcinoma
- Inflammatory: dermal lymphatic tumor emboli; peau d'orange; T4d stage

Two critical exam distinctions:
1. Fibroadenoma (hypocellular stroma) vs Phyllodes (hypercellular stroma + leaf-like fronds)
2. DCIS (intact duct contour + myoepithelial rim) vs Invasive carcinoma (infiltrating nests, no duct contour, no myoepithelium)