A 45-year-old woman with a 4 cm breast mass undergoes core biopsy showing invasive ductal carcinoma. Immunohistochemistry reveals ER-positive, PR-positive, HER2-negative, and Ki-67 index of 35%. According to the St. Gallen surrogate intrinsic subtype classification, what is the molecular subtype?

• A Luminal A
• B Triple negative
• C HER2-enriched
• D Luminal B (HER2-negative) ✓

Explanation

Luminal D (HER2-negative) is defined as ER-positive, HER2-negative, with either high Ki-67 (≥20%) or low PR (<20%). A Ki-67 of 35% indicates high proliferative activity, distinguishing it from Luminal A, which has low Ki-67. HER2-enriched is ER-negative and HER2-positive. Triple negative lacks ER, PR, and HER2.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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