Breast Anatomy - Quadrants, Lymphatic Drainage, and Sentinel Node

Anatomical Position and Structural Overview

• The breast lies on the anterior thoracic wall, positioned between the skin and subdermal adipose layer above, and the superficial pectoral fascia overlying the pectoralis major muscle below
• The anatomically relevant regions for clinical and oncological purposes include: the breast parenchyma itself, the underlying chest wall and musculature, and the axillary, internal mammary, and supraclavicular nodal areas
• The breast parenchyma contains fat, stroma, lactiferous ducts, and lobular units

Nipple-Areola Complex: Relevant Anatomy

• Within and around the areola, bundles of smooth muscle fibres are arranged in three orientations: radially, circumferentially, and longitudinally, allowing nipple erection in response to stimuli
• Along the margin of the areola are sebaceous glands and sweat glands
• Montgomery glands are accessory glands with nodular elevations called tubercles of Morgagni, which open onto the areolar surface
• The nipple is highly innervated, containing:
  • Free sensory nerve endings
  • Meissner's corpuscles
• The areola contains:
  • Ruffini-like endings
  • Krause end bulbs
• Sensory innervation of the breast is primarily derived from the lateral and anterior cutaneous branches of the intercostal nerves

Breast Quadrants

• The breast is divided into four quadrants by two perpendicular lines intersecting at the nipple:

• The axillary tail of Spence is a projection of breast tissue extending into the axilla from the upper outer quadrant, through an opening in the deep fascia
• Breast tissue also overlies the anterior chest wall and can extend toward the clavicle superiorly, the sternum medially, the costal margin inferiorly, and into the axilla laterally

Lymphatic Drainage: Overview and Clinical Importance

• Lymphatic drainage of the breast is central to understanding the metastatic spread of breast cancer and to the rationale for surgical staging of the axilla
• The breast's lymphatic system provides the route by which metastatic cells travel from the primary tumour, through intramammary lymphatics, to the first few regional nodes most likely to have received those metastases (the basis of sentinel lymph node biopsy, SLNB)
• Blue dyes and/or technetium-labelled sulfur colloid injected into the breast are taken up by the lymphatic system and allow identification of these sentinel nodes:
  • Blue-stained nodes are identified by visual inspection
  • Radioactive nodes are identified using an intraoperative gamma detector
• Early SLNB mapping used peritumoral injections of these agents

Axillary Lymph Nodes: Primary Drainage Pathway

• The axillary lymph nodes are the dominant drainage pathway, receiving the majority of lymph from the breast
• The axilla is the principal nodal basin addressed in surgical staging and management of invasive breast cancer
• Axillary lymph node dissection (ALND) targets this nodal basin; its impact on outcomes may be limited to specific subsets of breast cancer patients
• Once the breast has been removed (e.g. at mastectomy), the opportunity to reliably perform lymphatic mapping via breast injection is lost, because the intramammary conduits are no longer intact
• For patients undergoing lumpectomy for DCIS where preoperative biopsy may have understaged disease, a concomitant SLNB may be offered to avoid a second surgical procedure for staging

Axillary Node Levels

• Lymph generally flows from Level I to Level II to Level III in sequential fashion
• Skip metastases (involvement of higher levels without lower level involvement) do occur but are less common

Internal Mammary Lymph Nodes

• The internal mammary (parasternal) nodes represent the second most important drainage pathway
• They are located along the internal mammary vessels within the intercostal spaces, adjacent to the sternum
• Inner quadrant tumours (upper and lower inner) have higher rates of internal mammary node drainage compared with outer quadrant tumours, although outer quadrant tumours can also drain to this basin
• Internal mammary nodes are clinically significant in the context of nodal irradiation planning: the Alliance 11202 trial (investigating omission of complete ALND after neoadjuvant chemotherapy) specifies that both arms receive radiation to the infraclavicular, supraclavicular, and internal mammary nodal regions
• Historically, internal mammary nodes were accessed surgically; in modern practice they are addressed primarily through radiation therapy fields

Supraclavicular Lymph Nodes

• The supraclavicular nodes represent a higher-echelon (Level III apex extension) nodal station
• Involvement of supraclavicular nodes historically classified disease as distant metastatic (M1) in older staging systems, though contemporary staging reclassifies ipsilateral supraclavicular disease as regional nodal disease (N3c)
• Supraclavicular fields are routinely included in regional nodal irradiation for high-risk breast cancer
• These nodes also receive lymph from the breast via the apical axillary (Level III) nodes

Summary Table: Lymphatic Drainage Pathways by Quadrant

• In practice, any quadrant can drain to either the axillary or internal mammary basin; the proportions differ by location but are not absolute
• The axillary pathway carries the majority (approximately 75% or more) of breast lymphatic drainage regardless of quadrant

Sentinel Lymph Node Biopsy: Anatomical Principles

• The sentinel node concept relies on the orderly, predictable flow of lymph from the breast through intramammary channels to the first draining regional node
• Mapping agents (blue dye, radiocolloid, or both) are injected into the breast to trace this pathway in real time at operation
• The identified sentinel node(s) are the nodes most likely to harbour micrometastases if nodal spread has occurred; if these are negative, the remainder of the nodal basin is highly likely to be negative
• Reoperative axillary surgery: SLNB can still be attempted in patients who have had prior axillary surgery (prior ALND or prior SLNB), though the lymphatic anatomy may be disrupted and success rates are lower
• Clip-node localisation (targeted axillary dissection, TAD) after neoadjuvant chemotherapy: the biopsied positive node is clipped at diagnosis, and this specific node is localised intraoperatively (e.g. using fluoroscopic intraoperative neoplasia and node detection, the FIND technique, or wire/tattoo localisation) so it can be excised and confirmed as part of the surgical specimen

Physiological Changes Affecting Lymphatic Anatomy

• During pregnancy and lactation, the breast undergoes substantial reorganisation driven by gestational hormones, enlarging and altering the parenchymal architecture
• During the late luteal (premenstrual) phase of the menstrual cycle, intralobular oedema and fluid accumulation occur, which can increase nodularity and may be mistaken clinically for a malignant mass
• These physiological changes do not fundamentally alter lymphatic drainage pathways but can complicate clinical and imaging assessment of the axilla

Key Clinical Applications for Registrars

• Tumour location by quadrant informs the surgeon which nodal basin is most at risk and guides the approach to SLNB or ALND
• Outer quadrant tumours are the most common; virtually all will be staged via the axillary nodal basin
• Inner quadrant tumours have a clinically meaningful incidence of internal mammary nodal involvement, which may not be captured by axillary surgery alone and must be considered when planning adjuvant radiation fields
• Phyllodes tumours are a notable exception: lymph node involvement is very rare (in large registry data, only 8 of 498 women had involved nodes), so routine SLNB is generally not performed for phyllodes tumours, even though the same lymphatic pathways exist
• Negative surgical margin for invasive breast cancer is now defined as no tumour at the ink margin (not a specific millimetre clearance for invasive disease), which is distinct from the goal of 1 cm margins recommended for phyllodes tumours
• Understanding the intramammary lymphatic conduit is critical: once the breast is removed (mastectomy), injection-based lymphatic mapping cannot reliably identify sentinel nodes, making preoperative planning essential