Breast anatomy

Extent and bed

• The breast is a modified apocrine sweat gland in the superficial fascia. It extends from the 2nd to the 6th rib vertically and from the sternal edge to the mid-axillary line. The axillary tail (of Spence) passes through an opening in the deep fascia into the axilla, so a lump in the axilla can be breast tissue.
• Bed of the breast: about two-thirds lies on the pectoral fascia over pectoralis major, the outer third on serratus anterior, and the lower inner part on the aponeurosis of the external oblique.
• The retromammary space (loose areolar tissue) between the breast and pectoral fascia lets the breast move. Fixation of a lump when pectoralis major is contracted means invasion of the pectoral fascia or muscle.

Internal structure

• 15–20 lobes, each drained by a lactiferous duct that widens into a lactiferous sinus beneath the areola and opens on the nipple. Lobes are made of lobules, and lobules of secretory alveoli (acini).
• Suspensory ligaments (of Cooper): fibrous bands from the dermis through the gland to the deep fascia. A cancer that infiltrates and shortens them pulls the skin in, causing dimpling.
• Nipple retraction: a carcinoma beneath the areola provokes fibrosis that shortens the lactiferous ducts, drawing the nipple in over weeks to months. Long-standing bilateral inversion since puberty is developmental. Duct ectasia in perimenopausal women causes slit-like retraction with thick discharge.
• Peau d'orange: cancer cells block dermal lymphatics, so the skin swells with lymph while the hair follicles and sweat ducts stay tethered and appear as pits.
• The nipple lies over the 4th intercostal space in men and young women; its skin is supplied by the T4 dermatome.

Development and hormones

• The breast arises from the ectodermal mammary ridge (milk line) running from the axilla to the groin. Accessory nipples (polythelia) or breast tissue (polymastia) may appear along it, most often just below the normal breast or in the axilla.

• After delivery, oestrogen and progesterone fall and prolactin starts lactation; suckling maintains prolactin and triggers oxytocin release.
• Failure to lactate after a major postpartum haemorrhage, with later tiredness, cold intolerance and hypotension, suggests pituitary infarction (Sheehan syndrome).

Blood supply

• Internal thoracic (mammary) artery perforating branches, mainly through the 2nd–4th intercostal spaces: the medial part, about 60% of the supply.
• Lateral thoracic and thoracoacromial branches of the axillary artery: the lateral part.
• Lateral branches of the posterior intercostal arteries.
• Veins follow the arteries. The posterior intercostal veins connect with the vertebral venous plexus (Batson), a valveless network through which cancer cells reach the vertebrae, pelvis and skull without passing through the lungs.
• Mondor disease: thrombophlebitis of the thoracoepigastric vein, a superficial vein running from the outer breast towards the costal margin. A tender cord under the skin becomes obvious when the arm is raised. It settles by itself; anticoagulation is not needed for the typical form.

Lymph drainage

• About 75% of lymph passes to the axillary nodes, mainly from the lateral quadrants, reaching the anterior (pectoral) group first. Upper outer quadrant tumours therefore drain chiefly to the pectoral group at level I, then to central and apical nodes.
• About 20% passes to the internal thoracic (parasternal) nodes, especially from the medial quadrants.
• The rest reaches posterior intercostal, supraclavicular and contralateral nodes, and from the lower quadrants the subdiaphragmatic nodes and liver through the rectus sheath.
• The subareolar plexus (of Sappey) collects lymph from the nipple and areola.

• Sentinel lymph node biopsy: blue dye and radioisotope identify the first node receiving lymph; if it is free of tumour, axillary clearance is avoided. This has spared most women the nerve injuries and lymphoedema of clearance.
• Clinical staging counts the number and fixity of involved nodes; involvement of four or more axillary nodes indicates a higher nodal stage (N2).

Nerves at risk in breast and axillary surgery

Reconstruction flaps

• Latissimus dorsi flap: based on the thoracodorsal artery, so the thoracodorsal pedicle must be preserved at clearance.
• TRAM flap (transverse rectus abdominis myocutaneous, lower abdomen): a pedicled TRAM survives on the superior epigastric artery (the continuation of the internal thoracic) tunnelled up under the skin. A free TRAM or DIEP flap uses the deep inferior epigastric artery, which is joined to the internal thoracic or thoracodorsal vessels.

Remember Lateral quadrants drain to the axilla, medial quadrants to the internal thoracic chain.

Common conditions and their anatomy

Exam trap Skin dimpling is from Cooper ligaments; peau d'orange is from blocked dermal lymphatics.

Exam trap Axillary node levels are defined by pectoralis minor, not pectoralis major.

How FCPS asks about this

• Proportion of breast lymph to the axilla: about 75%; medial tumours also reach the internal thoracic nodes.
• Cause of recent nipple retraction over a subareolar mass: fibrosis shortening the lactiferous ducts.
• Tender cord from the outer breast towards the costal margin: Mondor disease, thoracoepigastric vein thrombophlebitis.
• Vertebral metastases from breast cancer: through the valveless vertebral venous plexus (Batson).
• Ducts and fat versus lobules: oestrogen develops ducts and fat; progesterone develops lobules and alveoli; oxytocin ejects milk.
• Cannot raise arm overhead after mastectomy: long thoracic nerve (serratus anterior).
• Artery of a pedicled TRAM flap: superior epigastric; free TRAM or DIEP uses the deep inferior epigastric.