Breast Anatomy

I. The Breast

Breast tissue is located between the subcutaneous fat and the fascia of the pectoralis major and serratus anterior muscles. Posterior to the breast and anterior to the pectoralis fascia is the retromammary space, which contains small lymphatics and vessels. Breast tissue can extend to the clavicle, into the axilla (axillary tail of Spence), to the latissimus dorsi, and to the top of the rectus muscle. Running through the breasts from the deep fascia to the skin are suspensory ligaments (Cooper’s ligaments); involvement of these ligaments by cancer may cause skin dimpling.

A. Vasculature

The arterial supply is from the internal thoracic artery, via perforating branches, and the axillary artery, via the long thoracic and thoracoacromial branches. Venous drainage is mainly to the axillary vein, as well as the internal thoracic, lateral thoracic, and intercostals veins.

B. Lymphatic drainage

Lymphatic drainage of the breasts occurs via interlobular lymphatic vessels into a subareolar plexus (Sappey’s plexus). From this plexus, the majority (75%) of the lymph drains into the axillary lymph nodes. Lymph from the medial breast may drain into the internal mammary nodes or the axillary nodes.

C. Innervation

Lateral and anterior cutaneous branches of the second to sixth intercostals nerves innervate the breasts.

I. The Axilla

The borders of the axilla are defined as the axillary vein superiorly, latissimus dorsi laterally, and the serratus anterior muscle medially.

A. Axillary lymph nodes

Axillary lymph nodes are classified according to their anatomic location relative to the pectoralis minor muscle.

  • Level I nodes. Lateral to the pectoralis minor muscle
  • Level II nodes. Posterior to the pectoralis minor muscle
  • Level III nodes. Medial to the pectoralis minor muscle and most accessible with division of the muscle
  • Rotter’s nodes. Between the pectoralis major and the minor muscles

B. Axillary nerves

Three motor and several sensory nerves are located in the axilla. Preservation of all is preferred during an axillary lymph node dissection; however, direct tumor invasion may require resection along with the specimen.

  • Long thoracic nerve: travels from superiorly to inferiorly along the chest wall at the medial aspect of the axilla and innervates the serratus anterior muscle. Injury to this nerve causes a “winged” scapula in which the medial and inferior angle of the scapula abduct away from the chest wall with arm extension.
  • Thoracodorsal nerve: courses along the posterior border of the axilla from superiorly to inferiorly on the subscapularis muscle and innervates the latissimus dorsi. Injury to this nerve causes weakness in arm abduction and external rotation.
  • Medial pectoral nerve: travels from the posterior aspect of the pectoralis minor muscle around the lateral border of the pectoralis minor to the posterior aspect of the pectoralis major muscle. It innervates the lateral third of the pectoralis major; injury to this nerve results in atrophy of the lateral pectoralis major muscle.
  • Intercostal brachial sensory nerves: travel laterally in the axilla from the second intercostal space to the medial upper arm. Transection causes numbness in the posterior and medial surface of the upper arm.


Clinical Assessment

I. History

Patients seek medical attention most commonly for an abnormal mammogram, a breast mass, breast pain, nipple discharge, or skin changes. History should include the following:

  • Duration of symptoms, change over time, associated pain or skin changes, relationship to pregnancy or the menstrual cycle, previous trauma.
  • Date of last menstrual period and regularity of the menstrual cycle.
  • Age of menarche.
  • Number of pregnancies and age at first full-term pregnancy.
  • Lactational history.
  • Age at menopause or surgical menopause (i.e., oophorectomy).
  • Prior history of breast biopsies or breast cancer.
  • Mammogram history.
  • Oral contraceptive and hormonal replacement therapy.
  • Family history of breast and gynecologic cancer, including the age at diagnosis. This should include at least two generations as well as any associated cancers, such as ovary, colon, prostate, gastric, or pancreatic.

A. Assessment of cancer risks

1.Hormonal, environmental exposure, and genetics are correlated to an increased risk for breast cancer. A family history of breast cancer in a first-degree relative is associated with a doubling of risk. If two first-degree relatives (e.g., a mother and a sister) have breast cancer, the risk is further elevated. These familial effects are enhanced if the relative had either early-onset cancer or bilateral disease. Breast-feeding may exert a protective effect against the development of breast cancer. Overall, factors that increase a patient’s risk by 1.5- to 4-fold include the following:

a.Increased estrogen or progesterone exposure due to early menarche (before age 12 years) or late menopause (age >55 years).

b.Late age at first full-term pregnancy: Women with a first birth after age 30 years have twice the risk of those with a first birth before age 18 years.

c.High body-mass index after menopause.

d.Exposure to ionizing radiation.

2.BRCA1 and BRCA2 are breast cancer susceptibility genes associated with 80% of hereditary breast cancers but account for only 5% of all breast cancers. Women with BRCA1 mutations have an estimated risk of 85% for breast cancer by age 70 years, a 50% chance of developing a second primary breast cancer, and a 20% chance of developing ovarian cancer. BRCA2 mutations carry a lower risk for breast cancer and account for 4% to 6% of all male breast cancers. Screening for BRCA gene mutations should be reserved for women who have a strong family history of breast or ovarian cancer.

3.Prior breast biopsies. Some pathologic features are associated with increased cancer risk.

a.No increased risk is associated with adenosis, cysts, duct ectasia, or apocrine metaplasia.

b.There is a slightly increased risk with moderate or florid hyperplasia, papillomatosis, and complex fibroadenomas.

c.Atypical ductal (ADH) or lobular hyperplasia (ALH) carries a 4- to 5-fold increased risk of developing cancer; the risk increases to 10-fold if there is a positive family history. Patients with increased risk should be counseled appropriately and should be followed with semiannual physical examinations and yearly mammograms.

4.Models for breast cancer risk. The original Gail model estimates the absolute risk (probability) that a woman in a program of annual screening will develop breast cancer over a defined age interval. The risk factors in this model include current age, age at menarche, age at first full-term pregnancy, previous breast biopsies, presence of ADH on prior biopsy, and number of affected first-degree relatives. The National Surgical Adjuvant Breast and Bowel Project (NSABP) modified this model to project the absolute risk of developing only invasive breast cancer. This modified Gail model has been used to define eligibility criteria for entry into chemoprevention trials. The NSABP and the National Cancer Institute offer an interactive online risk assessment tool, which is available at http://www.