Breast Cancer
Breast cancer is the most common cancer in women worldwide (~30% of all female cancers). Molecular subtyping — driven by hormone receptor (ER/PR) and HER2 status — defines treatment strategy and genomic testing priorities. Comprehensive biomarker profiling is required for all invasive breast cancers, with additional tests indicated by stage and subtype.
Select Subtype — 3 subtypes with recommendations
HR+/HER2− Breast Cancer
→Hormone receptor positive (ER+ and/or PR+), HER2 negative. The most common breast cancer subtype (~70%). Genomic expression profiling (Oncotype DX, MammaPrint) guides adjuvant chemotherapy decisions in early-stage disease. In metastatic setting, PIK3CA mutation, ESR1 mutation, BRCA1/2 status, and HER2-low expression drive targeted therapy eligibility.
HER2+ Breast Cancer
→HER2 overexpressed or amplified (IHC 3+ or FISH amplified). Accounts for ~15–20% of breast cancers. HER2-targeted therapy (trastuzumab, pertuzumab, T-DM1, T-DXd) has dramatically improved outcomes. HER2 testing by IHC and FISH is required for all newly diagnosed invasive breast cancer. PI3K pathway (PIK3CA) and BRCA1/2 testing are relevant in the metastatic setting.
Triple-Negative Breast Cancer (TNBC)
→ER−, PR−, HER2− breast cancer. Accounts for ~15% of breast cancers. More aggressive, higher rates of early relapse. PD-L1 expression (CPS ≥10) guides pembrolizumab eligibility in early and metastatic TNBC. BRCA1/2 germline mutation drives PARP inhibitor therapy (olaparib, talazoparib). NTRK fusions are rare but actionable. TMB and MSI are emerging biomarkers for second-line immunotherapy.