The Breast

General Anatomy

The breast ( Fig. 9.1 ) overlies the second to sixth ribs on the anterior chest wall. It is hemispherical with an axillary tail (of Spence) and consists of fat and a variable amount of glandular tissue. It is entirely invested by the fascia of the chest wall, which splits into anterior and posterior layers to envelop it. The fascia forms septa called Cooper’s ligaments , which attach the breast to the skin anteriorly and to the fascia of pectoralis posteriorly. They also run through the breast, providing a supportive framework between the two fascial layers. The pigmented nipple projects from the anterior surface of the breast. It is surrounded by the pigmented areola and its position is variable, but it usually lies over the fourth intercostal space in the nonpendulous breast.

Lobular Structure ( Fig. 9.2 )

The internal architecture of the breast is arranged into 15–20 lobes , each of which is drained by a single major lactiferous duct that opens on to the nipple. Each lobe is made up of several lobules , each of which drains several acini . The lobules drain via a branching arrangement of ducts to the single lobar duct. Each lobule drains several acini – these are blind saccules into which milk is secreted during lactation. The glandular tissue of the acini and the ductal tissue draining them comprise the breast parenchyma . The fat surrounding the parenchymal structures and the fibrotic framework of the breast constitute the stroma . The relative abundance of parenchyma and stroma varies according to age, parity and other factors.

The breast may be described as having three anatomical zones – premammary, mammary and retromammary – consisting of fatty tissue, fibroglandular tissue, and fat and muscular tissue from superficial to deep.

Blood Supply ( Fig. 9.3 )

The blood supply to the breast is composed of the following:

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Branches of the internal mammary (thoracic) artery pierce the intercostal spaces and traverse pectoralis muscle to supply approximately 60% of the breast – mainly medial and central.
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The lateral thoracic branch of the axillary artery supplies 30%, mainly the upper outer quadrant.
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Perforating branches of the anterior intercostal arteries.
Venous drainage accompanies the arteries to the axillary and subclavian veins and the azygos system.

Lymphatic Drainage ( Figs. 9.3 , 9.4 )

Lymphatics originate in the walls of the lactiferous ducts in the interlobular tissue of the breast. Lymph drains from the breast lobules to a rich subareolar plexus (Sappey’s plexus). Lymphatic vessels are elastic in structure with valves preventing backflow, and have small nodes along their paths, including within the breast tissue. The lymph drainage of the breast in complex with approximately:

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75% drainage to axillary nodes (mostly outer breast)

Fig. 9.4

Axillary lymph nodes groups.
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20% drainage to internal mammary nodes (mostly inner breast)
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5% drainage to deep channels leading to subclavicular plexus (mostly deep tissues)

RADIOLOGY PEARLs

Whilst the axilla receives most of its lymph from the outer breast, and the internal mammary chain receives most of its lymph from the inner breast, all parts of the breast may drain to any nodal station.Internal mammary nodes may receive lymph from the contralateral breast. Lymph from the inferior breast may communicate with subdiaphragmatic or subperitoneal lymph channels.The significance of identifying the nodal groups is that breast cancer is thought to spread in a sequential fashion, initially to the level I nodes. If the level I nodes are not involved, then it is unlikely that other ‘higher level’ nodes will be involved. This forms the basis for sentinel node mapping techniques (see following).

If lymph node channels are blocked by disease, surgery or radiotherapy, lymph is more likely to flow through alternative channels to the contralateral breast, the neck, the liver and peritoneal space and the groin via the rectus sheath.

Axillary Lymph Nodes

The axillary nodes are the most important to consider in detail as they are of major significance in breast cancer and may be removed surgically.

There are 20–30 axillary nodes arranged into five groups:

Anterior: deep to pectoralis major muscle at its lower border
Posterior: along the subscapular vessels at the posterior axilla
Lateral: along the axillary vein
Central: in the axillary fat
Apical: at the apex of the axilla medial to the axillary vein and above pectoralis minor.
All groups drain to the apical group.

The axillary nodes may also be divided into three levels with respect to pectolaris minor.

Level I: below pectoralis minor (includes anterior, lateral and central groups)
Level II: deep to pectoralis minor (posterior group)
Level III: above pectoralis minor (apical group)

Lymph Node Imaging

Axillary lymph nodes are best seen on MRI with its wide field of view. Axillary, intramammary and internal mammary nodes may be identified. Normal nodes enhance with contrast and have an ovoid or ‘bean’ shape, with a fatty hilum. They vary in length mm to several cm. On mammograms, lymph nodes are often seen in the axilla on oblique views. Intramammary nodes may be seen if outlined by fat. Ultrasound may also identify lymph nodes by their smooth ovoid appearance and fatty hilum. The axilla is assessed along with the breast tissue in breast ultrasound.

LOCALIZATION OF BREAST LESIONS USING THE CLOCK FACE ( Fig. 9.5 )

The breast is frequently described as a clock face with respect to the nipple, with the site of any finding localized by its position on the clock face and its distance from the nipple, for reproducibility. Thus the outer right breast in line with the nipple is the 9 o’clock position; the outer left breast at the level of the nipple is described as the 3 o’clock position. It is important to localize lesions accurately so that palpable lesions can be correlated with imaging findings.