A 55-year-old postmenopausal woman has a breast mass. Core biopsy shows infiltrating malignant cells arranged in a single-file (Indian file) pattern with no gland formation. Cells are small and discohesive, with target-like intracytoplasmic mucin. E-cadherin immunostaining is negative. This is most consistent with:

• A Invasive ductal carcinoma, NOS
• B Invasive lobular carcinoma ✓
• C Mucinous (colloid) carcinoma
• D Medullary carcinoma

Explanation

Invasive lobular carcinoma (ILC) is the second most common breast cancer type and is characterized by infiltrating single-file (Indian file) cords of small, uniform, discohesive cells, often arranged around ducts (targetoid pattern). Loss of E-cadherin expression (due to CDH1 mutation/methylation) is the defining molecular feature and explains the discohesive growth pattern. Signet-ring cells with intracytoplasmic mucin are a variant. Invasive ductal carcinoma (NST) forms irregular glands or nests; mucinous carcinoma shows abundant extracellular mucin; medullary carcinoma has large pleomorphic cells with syncytial growth and lymphoid stroma.

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

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Written and medically reviewed by the StethoPrep medical team.