A 55-year-old postmenopausal woman has a breast mass. Core biopsy shows a tumor with sheets of large cells with abundant eosinophilic cytoplasm; IHC reveals ER+, PR+, HER2−, and Ki-67 of 5%. Oncotype DX recurrence score is 12. According to the 2021 St. Gallen consensus, which category does this tumor most likely fall into?
- A Luminal A — hormone receptor-positive, HER2-negative, low proliferation, likely benefits from endocrine therapy alone ✓
- B Luminal B — HER2-negative, hormone receptor-positive, but high Ki-67 requiring chemotherapy
- C HER2-enriched — HER2 overexpression/amplification driving proliferation
- D Triple-negative — lacks ER, PR, HER2; treated with chemotherapy/immunotherapy
Correct answer: A. Luminal A — hormone receptor-positive, HER2-negative, low proliferation, likely benefits from endocrine therapy alone
Explanation
Luminal A breast cancer is defined by ER+, PR+ (high expression), HER2−, and low proliferation (Ki-67 <20%, Oncotype DX RS <18 for T1-T2, N0-N1). This tumor profile (ER+, PR+, HER2−, Ki-67 5%, RS 12) perfectly fits Luminal A, which has the best prognosis and is treated with endocrine therapy alone in most cases without chemotherapy. Luminal B has high Ki-67 or intermediate/high Oncotype RS. HER2-enriched and triple-negative lack ER/PR or overexpress HER2.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.