Breast Anatomy - Quadrants, Lymphatic Drainage, and Sentinel Node

Definition / Overview

The breast is a modified apocrine sweat gland situated within the superficial fascia of the anterior chest wall. A thorough understanding of breast anatomy is fundamental to interpreting clinical findings, planning operative approaches, and managing lymphatic spread of breast cancer. For the GSSE, emphasis falls on surface anatomy, internal architecture, lymphatic territories, and the basis of sentinel lymph node biopsy (SLNB).

Surface Anatomy and Quadrants

Boundaries

The breast occupies a consistent region of the anterior chest wall, with the following approximate borders:

• Superior: second or third rib
• Inferior: sixth or seventh costal cartilage (inframammary fold)
• Medial: lateral border of the sternum
• Lateral: mid-axillary line

The breast lies over the pectoralis major (central and lateral portions), serratus anterior (lateral portion), and external oblique (inferolateral portion).

The Axillary Tail of Spence

A projection of glandular tissue extends superolaterally through an opening in the deep fascia into the axilla, the axillary tail (tail of Spence). This is clinically important because:

• It may be mistaken for an axillary mass
• Breast cancer arising here can present as an apparent axillary lump
• It must be included in total mastectomy specimens

Quadrant Division

The breast is divided into four quadrants by vertical and horizontal lines through the nipple:

The UOQ contains the greatest volume of glandular tissue, which explains why the majority of breast carcinomas arise there. The UOQ also contains the axillary tail, further increasing its clinical significance.

Internal Architecture

Glandular Structure

The breast comprises 15-20 lobes arranged radially around the nipple, each draining via a lactiferous duct that converges on the nipple. Each duct dilates beneath the areola to form a lactiferous sinus before opening at the nipple apex.

Each lobe is subdivided into lobules composed of secretory acini (terminal ductal lobular units, TDLUs), the functional unit and the site of origin of most breast carcinomas and benign fibrocystic changes.

Supporting Framework, Cooper's Ligaments

Fibrous strands called suspensory ligaments of Cooper connect the overlying dermis to the deep fascia, maintaining the breast's structural position. These ligaments pass through the breast parenchyma and fat, dividing the breast into compartments. When infiltrated by malignancy:

• Tethering causes skin dimpling
• Contraction causes nipple retraction
• Lymphatic obstruction leads to peau d'orange (skin with an orange-peel texture due to dermal oedema and tethering at hair follicles)

Fascial Layers

• Superficial layer of superficial fascia: lies just deep to the dermis; the breast lies within and posterior to this layer
• Deep layer of superficial fascia (retromammary fascia): separates the breast from the deep pectoral fascia; the retromammary (submammary) space lies between these layers and contains loose areolar tissue, which is the surgical plane used during mastectomy

Blood Supply

Venous drainage mirrors arterial supply, ultimately reaching the internal thoracic vein, axillary vein, and azygos system, the azygos/vertebral venous connection explains haematogenous spread to the vertebrae.

Nerve Supply

Knowledge of the last three nerves is critical to safe axillary surgery; inadvertent injury during axillary lymph node dissection (ALND) causes recognised, examinable complications.

Lymphatic Drainage

Overview

Lymphatic drainage of the breast is the central question underpinning axillary staging in breast cancer. Approximately 85% of breast lymph drains to the axillary nodal basin; the remainder drains to the internal mammary chain and, to a lesser extent, to supraclavicular and infraclavicular nodes.

Axillary Lymph Node Levels

The axillary nodes are surgically classified into three levels relative to the pectoralis minor muscle:

• Standard ALND encompasses Levels I and II
• Rotter's nodes (interpectoral nodes) lie between pectoralis major and minor; they are clinically relevant because they are inaccessible without dividing or removing pectoralis minor
• Lymph flows sequentially from Level I → II → III, then to the subclavian trunk
• Skip metastases (Level III positive with Levels I-II negative) occur in roughly 2% of cases

Level I Node Groups (Haagensen Classification)

The named groups at Level I include:

• External mammary (anterior/pectoral) nodes, along the lateral thoracic vessel, receive drainage from the breast directly
• Subscapular (posterior/scapular) nodes, along the subscapular vessels
• Lateral (brachial/axillary vein) nodes, along the axillary vein; largest group at Level I

Central nodes (Level II) lie embedded in axillary fat and receive efferent vessels from Level I nodes.

Internal Mammary Nodes

Approximately four to five nodes per side, located in the intercostal spaces adjacent to the internal thoracic vessels. They receive drainage predominantly from:

• Medial and central breast quadrants
• Deep breast parenchyma

Their involvement has independent prognostic significance but they are not routinely dissected (unless detected on lymphoscintigraphy or imaging). Involvement of internal mammary nodes may alter radiation planning.

Supraclavicular and Infraclavicular Nodes

These receive drainage from the apical (Level III) axillary nodes. Supraclavicular involvement is classified as N3c in TNM staging and denotes stage IIIC disease, an important distinction from distant metastases (M1).

Sentinel Lymph Node, Concept and Anatomy

Definition

The sentinel lymph node (SLN) is the first lymph node (or nodes) in the regional lymphatic basin to receive direct lymphatic drainage from the primary tumour. The anatomical basis assumes an orderly, hierarchical pattern of lymphatic drainage, involvement of the SLN is a prerequisite for involvement of more distal nodes (with rare biologic exceptions).

Anatomical Rationale

• Lymphatic channels from the breast parenchyma converge in the subareolar plexus (Sappey's plexus) before draining to the axilla
• The subareolar plexus communicates with deeper parenchymal lymphatics, this anatomical convergence explains why injection into either the periareolar region or peritumoral/intradermal sites reliably identifies the same SLN
• In most cases, the SLN is found at Level I of the axilla

Sentinel Node Mapping Techniques

Injection sites used in practice:

• Periareolar (intradermal or subdermal), exploits Sappey's plexus
• Peritumoral (subcutaneous or parenchymal)
• Subdermal over tumour

Intraoperative Identification

1. Inject tracer ± dye at the chosen site (typically 5-20 min before incision for blue dye; 2-4 hours for radiotracer)
2. Use gamma probe to localise the "hot" node transcutaneously, then intraoperatively
3. Identify blue-stained lymphatic channels leading to a blue node
4. The SLN is defined as: (a) any node with radioactive counts ≥ 10% of the hottest node, or (b) any visually blue node, or (c) any palpably suspicious node
5. Manual palpation of the axillary bed is performed to detect suspicious nodes not captured by tracer/dye

Histological Evaluation of the SLN

Metastasis size classification:

• Macrometastasis: $> 2\,\text{mm}$, standard positive node (pN1)
• Micrometastasis: $0.2-2\,\text{mm}$, pN1mi
• Isolated tumour cells (ITCs): $\leq 0.2\,\text{mm}$ or $< 200$ cells, pN0(i+); generally not considered node-positive for treatment decisions

False-Negative Rate

The false-negative rate for SLNB is reported between 0-11% in series data, with the majority of experienced centres achieving <5%. Causes include:

• Incorrect node identified (technical error)
• Aberrant lymphatic drainage bypassing the SLN
• Incomplete histological sampling
• Afferent lymphatic obstruction by tumour causing rerouting

The false-negative rate decreases with increasing surgeon experience and with combined dual-mapping techniques.

Management: Axillary Staging Strategy

Standard Indications for SLNB

• Clinically node-negative (cN0) invasive breast cancer, T1-T2 tumours
• Planned as part of breast-conserving surgery or mastectomy
• SLNB can also be considered for large tumours, multicentric disease, and some DCIS cases (particularly where mastectomy is planned)

When to Proceed to ALND

Post-Neoadjuvant Chemotherapy Setting

• SLNB after neoadjuvant chemotherapy (NAC) is technically feasible but the false-negative rate is higher (~12-14% with single mapping; reduced with dual mapping and removal of ≥3 SLNs)
• Targeted axillary dissection (TAD), clip the positive node at diagnosis, confirm its removal at surgery alongside SLNB, improves accuracy
• Patients who are cN1 converting to ycN0 after NAC are candidates for SLNB in experienced centres

Complications and Special Considerations

Complications of SLNB

• Seroma, most common; occurs in the axillary dead space
• Wound infection
• Paraesthesia/numbness, particularly in the medial upper arm (intercostobrachial nerve territory)
• Allergic or anaphylactic reaction, blue dyes carry a ~1% anaphylaxis risk; methylene blue carries its own risks of skin necrosis and interference with pulse oximetry (SpO₂ drops transiently due to optical interference)
• Lymphoedema, much lower risk than ALND (~5% vs ~20-30%)
• Shoulder dysfunction, uncommon compared with full ALND

Complications of ALND (Comparative Context)

Axillary Reverse Mapping (ARM)

A technique to identify and preserve lymphatics draining the arm during ALND or SLNB, thereby reducing lymphoedema. Blue dye or fluorescent tracer is injected into the arm; arm lymphatics are identified and preserved separately from breast lymphatics. LYMPHA (lymphatic microsurgical preventive healing approach) involves anastomosing transected arm lymphatics to a nearby vein at the time of ALND, prospective data support significant reduction in lymphoedema rates.

Perioperative and Examination Considerations

Key Anatomical Points for the GSSE Viva

• The long thoracic nerve (C5-7) runs along the medial axillary wall on the surface of serratus anterior, injury causes winging of the scapula
• The thoracodorsal nerve runs with the thoracodorsal vessels, injury weakens shoulder internal rotation and adduction (latissimus dorsi)
• The intercostobrachial nerve (lateral cutaneous branch of T2) crosses the axilla horizontally, sacrifice causes numbness in the medial upper arm, a near-universal finding after formal ALND
• The axillary vein forms the superolateral boundary of the axillary dissection, must be preserved
• Rotter's nodes are inaccessible without sacrificing pectoralis minor and are not included in standard ALND

High-Yield Summary Points

• 85% of breast lymph drains to axillary nodes; 15% drains to internal mammary chain
• UOQ contains the most glandular tissue and gives rise to the most breast cancers
• The SLN is the first node receiving tumour lymphatic drainage; a negative SLN predicts negative axillary basin with high probability
• Dual mapping (blue dye + radiotracer) achieves the highest SLN identification rate
• SLNB is the standard of care for cN0 invasive breast cancer
• ALND is standard for cN1 disease; may be omitted after positive SLNB in selected patients undergoing lumpectomy + whole-breast radiation (≤2 positive SLNs)
• Lymphoedema is the most clinically significant long-term complication distinguishing ALND from SLNB

Sources

• RACS position statements & curriculum (Royal Australasian College of Surgeons)
• NCCN clinical practice guidelines in oncology