A 45-year-old woman undergoes mastectomy for an infiltrating ductal carcinoma. Pathology report notes: ER+, PR+, HER2−, Ki-67 15%, grade 2 (Nottingham). Which molecular subtype does this tumor belong to?
- A Luminal B (intermediate prognosis, higher proliferation)
- B HER2-enriched (amplified HER2 without hormone receptor expression)
- C Luminal A (best prognosis, low proliferation) ✓
- D Triple-negative/basal-like (worst prognosis)
Explanation
Luminal A breast cancers are ER+, PR+, HER2−, and have low proliferative index (Ki-67 <20%), low histological grade, and the most favorable prognosis. They respond to endocrine therapy (tamoxifen/aromatase inhibitors) and typically do not require adjuvant chemotherapy. Luminal B tumors are ER+ but have HER2 amplification or high Ki-67 (≥20%) or higher grade, conferring intermediate prognosis. The clinical relevance of molecular subtyping lies in guiding treatment decisions: luminal A patients are often spared chemotherapy based on genomic assays like Oncotype DX recurrence scores.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.