Worldwide, there are approximately 1 million new cases annually. Breast cancer is the most frequently diagnosed cancer in women in the United States accounting for 175,000 new cases and 43,300 deaths in 1999. Breast cancer accounts for approximately 30 percent of all cancer diagnosed and approximately 16 percent of all cancer deaths in American women. In the USA, the incidence of breast cancer increases with age, and a woman's lifetime risk of developing breast cancer are estimated at 1 in 8.

Breast cancer surgery at the beginning of the twentieth century was one procedure, Radical Mastectomy (removal of the entire breast, underlying chest muscle and lymph nodes in the axilla). In the 1970's, it became apparent that the same results could be obtained with Modified Radical Mastectomy (entire breast and axillary lymph nodes removed). In the 1980's, breast conservation surgery evolved when it became apparent that the same results or better could be obtained with lumpectomy (limited breast removal), axillary dissection and radiation to remainder of the breast. Today, clinical trials involving sentinel node biopsy are being carried out on breast cancer patients to determine if axillary dissection is necessary in all breast cancer patients.

Anatomy

• Each breast is composed of fifteen to twenty lobes. Each lobe has a single lactiferous (milk) duct, which opens in a depression at the tip of the nipple (Figure 1)
  
• The ducts are parallel to one another in the nipple but then diverge toward the periphery of the gland. Distally the ducts divide and end in a spherical alveolus (Figure 2). A number of alveoli open into a common duct and constitute a lobule. All the lobules draining through the same duct make up a lobe
  
• Each alveolus (gland) and duct are composed of a single layer of epithelial cells that lay against a basement membrane
  
• The stroma is the connective tissue that holds the ducts and lobes together
  
• Lymph channels within the breast spread outward from the nipple along the major lactiferous ducts and along draining veins to beds of lymph nodes. The major beds (regional nodes) are the external mammary group, axillary vein group, and central axillary group (Figure 3). Scapular and subclavicular nodes drain into the axillary nodes. Lymphatics also drain directly into the internal mammary chain

Figure 1 - Anatomy of the breast in a cross-section through the nipple. ©R. Walls	Figure 2 - Diagram of the microscopic structure of breast tissue. Each lobule is composed of several alveoli that drain into a common duct. ©R. Walls