10.4 Cooper Ligaments, Pectoral Muscle, Vascular/Lymphatic Systems, and Cytology

Support, vessels, nodes, and the meaning of invasion

ARRT groups Cooper ligaments (suspensory), pectoral muscle, vascular system, lymphatic system, and cytology (epithelial cells, myoepithelial cells, basement membrane) under physiology. Together they explain a retracted skin dimple, an inadequate MLO, a calcified artery that is not DCIS, a kidney-shaped UOQ density that is a node, and a pathology report that says in situ versus invasive.

Quick Answer: Cooper ligaments tether skin to deep fascia; shortening causes skin retraction and architectural distortion. The pectoral muscle on MLO should reach nipple / PNL with a convex border. Veins are the vessels you usually see; arterial tram-track calcifications are typically benign. Lymph drains chiefly to axillary nodes (Levels I–III relative to pectoralis minor) and to intramammary nodes. Invasion = malignant epithelial cells through the basement membrane; myoepithelial cells remain in in-situ disease.

Cooper ligaments (suspensory)

Cooper ligaments are fibrous suspensory bands that run from the deep (pectoral) fascia, through the breast, to insert on the dermis. They hold the gland against gravity. On a mammogram they appear as thin, gently curved lines, easiest to see in fatty regions.

When tumor, surgery, or fat necrosis shortens Cooper ligaments, they tether the skin and pull surrounding architecture:

• Skin retraction / dimpling
• Nipple retraction if central ligaments shorten
• Architectural distortion — spiculated tethering without a discrete mass

That is the anatomy behind a physical-exam dimple and behind mammographic distortion without a mass. Ligaments can also thicken in inflammation. Normal Cooper lines are gently curved and bilateral in pattern. Distortion is focal tethering toward a point. Do not call every ligament a spiculated mass, and do not ignore focal tethering as just ligaments.

Pectoral muscle

Pectoralis major is the muscle the breast rests on. The MLO exists in large part to show it, because posterior gland sits on it.

MLO critique:

• Convex anterior margin
• Inferior muscle visible to the PNL / nipple level (classic inclusion rule)
• Adequate width, not a thin triangle
• Relaxed arm and correct pec angle (section 10.2) so the muscle is in profile

Pectoralis minor is deeper. You do not need it on the mammogram, but axillary nodal levels are defined by it. Pec on the CC is not required if retroglandular fat is seen; a posterior strip of pec on CC is acceptable and more common in muscular patients.

If the pec is concave or stops well above the nipple line, posterior and deep UOQ tissue may be missing. Repeat the MLO. A sliver of muscle is not a pass if the PNL rule fails.

Vascular system

Arterial supply is mainly:

• Internal mammary (internal thoracic) artery — medial breast via intercostal perforators
• Lateral thoracic artery — lateral breast and axillary tail
• Anterior intercostal perforators — additional medial and central supply

Venous drainage generally follows the arteries into internal mammary and axillary veins. On mammograms you see veins far more often than arteries: veins are larger and more tortuous and can engorge with superior vena cava obstruction, pregnancy, or (rarely) portal hypertension.

Vascular calcifications you will meet again with other calcifications are typically arterial: parallel tram-track calcifications in the vessel wall. They are usually benign and must not be managed as DCIS casting calcifications. Veins rarely calcify. A dense tortuous vessel is still a vessel — not a dilated duct — if you can trace a branching vascular pattern.

For localization, a vessel is a landmark, not a quadrant. New unilateral venous engorgement is worth documenting clinically.

Lymphatic system

Most breast lymph (about three-quarters or more) drains to the ipsilateral axillary nodes. Medial lesions can also drain to internal mammary (parasternal) nodes. A small amount may cross the midline.

Axillary levels are named relative to pectoralis minor. You will not see painted zones on every MLO, but the concepts are testable:

Intramammary lymph nodes lie inside the breast, most often in the UOQ or axillary tail. Classic mammographic look: reniform (kidney-shaped) mass with a lucent fatty hilum and often a feeding vessel, typically under about 1 cm. That is normal anatomy. Growth, loss of hilum, or round replacement is how a node stops looking like a node — pathology, but you must know the normal first.

Do not label a level I axillary node as intramammary. Intramammary means surrounded by breast tissue, inside the breast margins. Axillary means in the axilla, lateral to the glandular disk.

Cytology: epithelial cells, myoepithelial cells, basement membrane

A normal duct or acinus is a three-layer construct:

1. Epithelial (luminal) cells — inner layer that can secrete; origin of most carcinomas
2. Myoepithelial cells — basal contractile layer that squeezes milk toward the nipple; they sit between epithelium and the basement membrane
3. Basement membrane — extracellular barrier around the TDLU; beyond it is stroma

flowchart TB
  Lumen["Duct / acinar lumen"] --> Epi["Epithelial cells"]
  Epi --> Myo["Myoepithelial cells"]
  Myo --> BM["Basement membrane"]
  BM --> Stroma["Stroma, fat, and vessels"]

In situ carcinoma (DCIS, LCIS): malignant epithelial cells remain inside the basement membrane. Myoepithelial cells are typically preserved. The lesion has not reached stromal lymphatics and vessels, which is why true in situ disease does not metastasize — management still matters because DCIS can progress.

Invasive carcinoma: malignant cells have broken through the basement membrane into the stroma. Myoepithelial cells are typically absent at the invasive front. That is the histologic definition of invasion. It is not defined by large, palpable, or spiculated, although those are imaging clues that stroma is already involved.

Technologist-level correlation:

• A cluster of calcifications without a mass can still be DCIS (in situ) or invasive — imaging does not replace the basement-membrane call
• An irregular mass with spiculation and Cooper-ligament retraction is suspicious for invasive disease because it is already in stroma pulling ligaments
• Pathology mentions of myoepithelial stains (p63, calponin, smooth muscle myosin) ask whether that layer is intact: intact favors in situ; lost favors invasive

You will not diagnose invasion from a mammogram alone. You will be asked which structure must be crossed for a cancer to be invasive (basement membrane), which cell lines the duct (epithelial), and which cell sits on that membrane (myoepithelial).

Put the four physiology topics on one clinical chain: a TDLU epithelial cancer that crosses the basement membrane can shorten Cooper ligaments (skin retraction, distortion), travel along vessels and lymphatics to an intramammary or level I node, and still be missed if the MLO pec never reached the PNL.