Which feature best discriminates triple-negative breast carcinoma (TNBC) from hormone receptor-positive, HER2-negative breast carcinoma?

## Triple-Negative vs. Hormone Receptor-Positive Breast Carcinoma ### Immunohistochemical Profile **Key Point:** Triple-negative breast carcinoma (TNBC) is defined by the absence of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) expression. This immunophenotype is the single best discriminator from hormone receptor-positive, HER2-negative tumors. **High-Yield:** TNBC lacks the three major therapeutic targets (ER, PR, HER2), making it inherently resistant to endocrine therapy and HER2-targeted therapy. This results in limited treatment options and worse prognosis compared to hormone receptor-positive tumors. ### Comparative Table | Feature | TNBC | HR+/HER2− | |---------|------|----------| | **ER expression** | Negative | Positive | | **PR expression** | Negative | Positive | | **HER2 expression** | Negative | Negative | | **Ki-67 index** | High (>30%) | Low to intermediate | | **Molecular subtype** | Basal-like (majority) | Luminal A or B | | **Age at diagnosis** | Younger (premenopausal) | Older (postmenopausal) | | **Grade** | High grade (G3) | Low to intermediate | | **Prognosis** | Poor (5-yr OS ~65%) | Good (5-yr OS >85%) | | **Treatment response** | Chemotherapy, immunotherapy | Endocrine therapy, chemotherapy | | **BRCA1 mutation** | Enriched (20–30%) | Less common | ### Clinical Significance **Clinical Pearl:** TNBC is enriched for BRCA1 mutations and basal-like molecular features, including high mitotic rate, central necrosis, and lymphocytic infiltration. These tumors are chemotherapy-responsive but lack endocrine and HER2-targeted options. **Mnemonic:** **TNT** = Triple Negative, Therapy-resistant (to endocrine/HER2), Tough prognosis. ### Therapeutic Implications **Warning:** Do not confuse "triple-negative" with "no treatment options." TNBC responds well to chemotherapy and increasingly to immunotherapy (PD-L1 inhibitors like atezolizumab + nab-paclitaxel). However, the absence of ER/PR/HER2 eliminates endocrine and trastuzumab-based strategies. **High-Yield:** In India, TNBC prevalence is ~20–25% of all breast cancers (higher than in Western cohorts), making recognition of this phenotype critical for treatment planning. [cite:Robbins 10e Ch 24]