Breast Cancer: A Comprehensive Overview

Epidemiology

• Breast cancer in men accounts for ~1% of total incidence
• ~75% of US cases occur in women over age 50
• • Goldman-Cecil Medicine, Chapter 183

Risk Factors

Genetic / Hereditary (5-8% of cases)

Hormonal / Reproductive

• Early menarche, late menopause, nulliparity, late first pregnancy
• Prolonged combined estrogen + progesterone HRT (but NOT estrogen alone)
• Ongoing oral contraceptive use
• Postmenopausal obesity (via estrogen conversion in adipose tissue)

Breast Pathology

• Atypical ductal or lobular hyperplasia
• Lobular carcinoma in situ (LCIS)
• High mammographic breast density

Environmental

• Ionizing radiation during adolescence
• Alcohol consumption
• No convincing evidence for estrogenic pesticides or high-fat diet

Pathology and Molecular Subtypes

1. Histologic type: Invasive ductal carcinoma (most common, ~80%), invasive lobular, mucinous, tubular, medullary, etc.
2. Hormone receptor status: Estrogen receptor (ER), progesterone receptor (PR)
3. HER2 overexpression/amplification
4. Histologic grade (Nottingham grading: I-III)

Molecular Subtypes (by gene expression profiling)

• Goldman-Cecil Medicine, p. 2083

Diagnosis and Staging

Diagnosis

• Screening mammography - primary modality; reduces mortality from advanced disease
• Ultrasound / MRI - used for inconclusive mammography, dense breasts, or high-risk patients
• Core needle biopsy - guided by mammography, US, or MRI for non-palpable lesions; required for all suspicious lesions
• Pathologic concordance: ~96% for invasive carcinoma, ~85% for DCIS, ~50% for atypia

Anatomic Staging (TNM)

• In stages I-II: CBC, chemistry panel, CXR are sufficient (low yield of CT/PET/bone scan)
• In stages III-IV: CT of chest/abdomen/pelvis + PET/bone scan for metastatic workup

Treatment

In Situ Carcinoma

• ~20-25% of newly diagnosed breast cancer in mammography-screened populations
• ~30% risk of progression to invasive cancer in the same breast if untreated
• Options:
  • Lumpectomy + radiotherapy (preferred breast-conserving approach)
  • Mastectomy (for large, multicentric, or high-risk DCIS)
  • Adjuvant tamoxifen for ER+ DCIS to reduce ipsilateral and contralateral recurrence
  • Sentinel lymph node biopsy only if invasive component is suspected

• A marker of increased bilateral breast cancer risk (not a direct precursor)
• Management: close surveillance (mammography + clinical exam ± MRI) or chemoprevention with tamoxifen/raloxifene/aromatase inhibitors

Early-Stage Invasive Breast Cancer (Stages I-III)

• Breast-conserving surgery (BCS/lumpectomy) + radiotherapy is equivalent to mastectomy in survival for most patients
• Sentinel lymph node biopsy (SLNB) is standard for clinically node-negative patients; full axillary dissection only if SLNB positive
• Recent NCCN update (2025): SLNB may be omitted in postmenopausal women >50 with cT1N0, HR+/HER2-, grade 1-2 tumors receiving whole-breast RT + endocrine therapy (based on SOUND and INSEMA trials)

• Indicated after BCS (whole-breast RT) to reduce local recurrence
• Post-mastectomy RT indicated for ≥4 positive nodes, T3-T4 tumors, or positive margins

• Endocrine therapy (for ER+ and/or PR+ tumors):
  • Premenopausal: tamoxifen 5-10 years ± ovarian suppression
  • Postmenopausal: aromatase inhibitors (anastrozole, letrozole, exemestane) 5-10 years; superior to tamoxifen in postmenopausal patients
• Chemotherapy: indicated for HER2+, TNBC, high-grade ER+, or multigene assay showing high recurrence risk
  • Common regimens: AC-T (doxorubicin + cyclophosphamide → paclitaxel); TC (docetaxel + cyclophosphamide)
  • Neoadjuvant (preoperative) chemotherapy for stage II-III to downstage tumors
• Anti-HER2 therapy:
  • Trastuzumab (Herceptin) + chemotherapy: standard adjuvant for HER2+ tumors >1 cm or node-positive
  • Pertuzumab added for high-risk HER2+ (neoadjuvant + adjuvant)
  • T-DM1 (trastuzumab emtansine): adjuvant therapy for HER2+ patients with residual disease after neoadjuvant chemotherapy
• PARP inhibitors (olaparib, talazoparib): adjuvant therapy for high-risk HER2-, BRCA1/2-mutated early breast cancer

Metastatic (Stage IV) Breast Cancer

• Biopsy of metastatic lesion recommended to reassess receptor status (can change from primary)
• Surgery: limited role; useful for chest wall nodules, solitary brain metastasis, orthopedic stabilization
• Radiotherapy: effective for brain metastases, painful bony metastases, chest wall recurrence

• First-line: CDK4/6 inhibitor (palbociclib, ribociclib, abemaciclib) + aromatase inhibitor or fulvestrant - prolongs PFS significantly
• After CDK4/6 inhibitor failure: PI3K inhibitor (alpelisib for PIK3CA-mutated tumors) + fulvestrant; mTOR inhibitor (everolimus) + exemestane; oral SERDs (elacestrant for ESR1-mutated); AKT inhibitors (capivasertib)
• Chemotherapy when endocrine-refractory

• First-line: trastuzumab + pertuzumab + taxane (CLEOPATRA regimen) - OS >5 years in trials
• After first-line: T-DXd (trastuzumab deruxtecan / Enhertu) - DESTINY-Breast09 trial (2025) showed unprecedented ~40-month PFS, poised to shift first-line practice
• Tucatinib + trastuzumab + capecitabine (HER2CLIMB regimen): effective in brain metastases (HER2CLIMB-05 trial updates)
• Lapatinib + capecitabine for later lines

• T-DXd (trastuzumab deruxtecan): shown to benefit HER2-low patients (DESTINY-Breast04 - a new actionable category)
• HER2-ultra-low (IHC >0 but <1+): emerging data from 2025 suggests further benefit from T-DXd

• Immunotherapy: pembrolizumab + chemotherapy (nab-paclitaxel or gemcitabine/carboplatin) if PD-L1+ (first-line metastatic); pembrolizumab + chemo as neoadjuvant for early high-risk TNBC
• Sacituzumab govitecan (anti-Trop-2 ADC): active in metastatic TNBC
• Olaparib / talazoparib (PARP inhibitors): for BRCA1/2-mutated metastatic TNBC
• Sequential chemotherapy (capecitabine, eribulin, gemcitabine, vinorelbine)

Bone Metastases

• Bisphosphonates (zoledronic acid, pamidronate) or denosumab (RANK-L inhibitor): reduce skeletal-related events, recommended for all patients with bone metastases
• Radiotherapy for localized pain

Surveillance after Treatment (ASCO Guidelines)

Prevention and Risk Reduction

• Chemoprevention (high-risk women): tamoxifen, raloxifene (postmenopausal), or aromatase inhibitors reduce risk by ~50% in high-risk women
• Prophylactic mastectomy: considered for BRCA1/2 carriers; reduces breast cancer risk by >90%
• Prophylactic salpingo-oophorectomy: also reduces breast + ovarian cancer risk in BRCA carriers
• Lifestyle: maintain healthy weight, limit alcohol, avoid prolonged HRT

Recent Advances (2024-2026)

• DESTINY-Breast09 (SABCS 2025): T-DXd as first-line for HER2+ metastatic breast cancer showed ~40-month PFS, potentially replacing pertuzumab + trastuzumab + taxane as the standard
• HER2CLIMB-05: Updated tucatinib + trastuzumab + capecitabine data for HER2+ including brain metastases
• lidERA trial: New SERD data in HR+ disease
• Neoadjuvant immunotherapy (PMID: 39207778, JAMA Oncology 2024): Meta-analysis confirms immune checkpoint inhibitors + chemotherapy significantly improve pathologic complete response in early breast cancer, particularly TNBC
• CDK4/6 inhibitor sequencing (PMID: 39851969): Network meta-analysis 2025 evaluates optimal treatment after CDK4/6 inhibitor failure in HR+ metastatic disease
• NCCN Guidelines v5.2025: Supports SLNB omission in selected low-risk patients, and updates anthracycline-free HER2+ regimens

Key Takeaways

1. Breast cancer is the most common female cancer worldwide; early detection via mammography saves lives
2. Molecular subtyping (ER, PR, HER2, BRCA status) drives every treatment decision
3. Most patients present at early stage and achieve long-term survival with multimodality therapy
4. Metastatic breast cancer is increasingly managed as a chronic disease with targeted therapies
5. T-DXd (trastuzumab deruxtecan) is transforming HER2+ and HER2-low metastatic treatment
6. CDK4/6 inhibitors remain the cornerstone of HR+ metastatic therapy