Breast Cancer

Incidence, genetic risk, clinical presentation, and stage-by-stage treatment of the most common cancer in women

Key Points

• Breast cancer is the most common cancer in women worldwide; ~300,000 new cases are diagnosed in the US annually.
• BRCA1/2 mutations confer a 50–70% lifetime risk; other high-risk genes include PALB2, CHEK2, and ATM.
• The most common presentation is a painless, palpable breast mass; inflammatory breast cancer is an aggressive subtype presenting with skin erythema and peau d'orange.
• Molecular subtypes (HR+/HER2−, HER2+, Triple-negative) drive systemic therapy decisions regardless of stage.
• Early-stage (I–II) breast cancer is potentially curable with surgery ± radiation ± adjuvant systemic therapy.
• Neoadjuvant chemotherapy is standard for locally advanced (stage III) and HER2+ or TNBC stage II disease.
• Metastatic (stage IV) breast cancer is treated with intent to control disease and maintain quality of life; median survival has improved markedly with CDK4/6 inhibitors and HER2-directed therapy.

Epidemiology & Incidence

Breast cancer is the most frequently diagnosed cancer in women globally and the second leading cause of cancer death in American women, after lung cancer. In 2024, an estimated 310,720 new cases of invasive breast cancer and 42,250 deaths were projected in the US. The lifetime risk for an average American woman is approximately 1 in 8 (12.5%). Incidence increases sharply with age: fewer than 5% of cases occur before age 40. Incidence rates are highest among non-Hispanic White and Black women, though Black women have disproportionately higher mortality due to later-stage presentation and…

Genetic Predispositions & Risk Factors

Approximately 5–10% of breast cancers are attributable to inherited high-penetrance gene mutations. BRCA1 and BRCA2 are the most well-characterized: BRCA1 mutations confer a 55–72% lifetime breast cancer risk and a 40–44% ovarian cancer risk; BRCA2 mutations confer a 45–69% breast cancer risk and a 11–17% ovarian cancer risk. Both follow autosomal dominant inheritance. Other moderate-to-high penetrance genes include: • PALB2 — 33–58% lifetime risk; interacts with BRCA1/2 in DNA repair • CHEK2 — 1.5–2× relative risk; particularly the 1100delC variant • ATM — biallelic mutations cause…

Clinical Presentation

The most common presentation of breast cancer is a painless, hard, irregular, non-mobile palpable mass discovered by the patient or on clinical examination. Most masses are in the upper outer quadrant, which contains the greatest volume of breast tissue. Other presenting features include: • Nipple discharge — particularly unilateral, spontaneous, and bloody or serous • Skin changes — dimpling, puckering (caused by Cooper's ligament involvement), or peau d'orange (thickened, pitted skin resembling an orange peel — a hallmark of lymphatic obstruction in inflammatory breast cancer) • Nipple…

Staging Overview

Breast cancer is staged using the AJCC 8th Edition TNM system, which integrates anatomic T (tumor size), N (nodal status), and M (metastasis) with biomarker modifiers: ER/PR status (Allred score), HER2 status (IHC/FISH), and grade. This "prognostic stage" can upstage or downstage tumors relative to anatomic stage alone. Stage I: T1 (≤2 cm) N0 or N1mi (micrometastasis ≤2mm), M0 Stage II: T0–2 N1, T2–3 N0, or T3 N0, M0 Stage III: T3 N1–2, T4 any N, any T N2–3, M0 (includes IBC as IIIC minimum) Stage IV: Any T, any N, M1 (distant metastasis) Workup: bilateral diagnostic mammography ±…

Stage I Treatment

Stage I breast cancer (tumors ≤2 cm, node-negative or micrometastases) is highly curable, with 5-year survival rates exceeding 99% for stage IA. Surgery: Breast-conserving surgery (lumpectomy) followed by whole-breast radiation is the standard for eligible patients and is equivalent to mastectomy in survival outcomes. Mastectomy (simple or modified radical) is preferred for multicentric disease, large tumor-to-breast size ratio, prior chest RT, or patient preference. Contralateral prophylactic mastectomy is discussed in high-risk individuals (BRCA carriers) but does not improve survival in…

Stage II Treatment

Stage II (T0–2 N1, T2–3 N0) disease remains highly curable with multimodality treatment. The approach depends heavily on molecular subtype. For HR+/HER2− Stage II: Surgery first is acceptable, followed by adjuvant chemotherapy (if Oncotype DX RS ≥26 or high-risk features) plus endocrine therapy ± ovarian suppression (premenopausal). Extended adjuvant therapy with abemaciclib (MonarchE trial) for high-risk node-positive HR+ disease reduces distant recurrence. For HER2+ Stage II: Neoadjuvant chemotherapy with pertuzumab + trastuzumab + taxane-based regimen (e.g., TCHP) is preferred to assess…

Stage III Treatment

Stage III (locally advanced breast cancer, LABC) includes large primary tumors with nodal involvement, skin/chest wall involvement (T4), and inflammatory breast cancer. The standard approach is neoadjuvant systemic therapy followed by surgery, then radiation. Neoadjuvant chemotherapy allows in vivo assessment of treatment response, may convert inoperable to operable disease, and guides adjuvant therapy selection based on pCR status. Standard regimens: AC (doxorubicin/cyclophosphamide) × 4 cycles → weekly paclitaxel × 12; with trastuzumab + pertuzumab added for HER2+ disease; with…

Stage IV (Metastatic) Treatment

Metastatic breast cancer (MBC) is currently considered incurable in most cases, with treatment goals of disease control, symptom palliation, and quality of life preservation. Median overall survival has improved substantially: ~3–5 years for HR+/HER2− with modern CDK4/6 inhibition; >5 years for HER2+ with current targeted therapy sequences. HR+/HER2− MBC (70% of MBC): First-line CDK4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) + aromatase inhibitor (postmenopausal) or fulvestrant is standard of care. Ribociclib + AI demonstrated OS benefit in multiple trials. Upon progression,…

HER2-Directed Therapy Sequencing

HER2-positive metastatic breast cancer (20–25% of all MBC) is managed with a planned sequence of HER2-targeted regimens. Each successive line exploits different mechanisms to overcome resistance, yielding median OS now exceeding 5 years from metastatic diagnosis.