Breast cancer is a growth of abnormal cells, usually within the ducts (which carry the milk to the nipple) or lobules (glands for milk production) of the breast. In more advanced stages of the disease, these out-of-control cells invade nearby tissues or travel throughout the body to other tissues or organs. More than 178,000 women and 2,000 men are diagnosed each year with breast cancer, according to the American Cancer Society (ACS). For women, this rate is second only to non-melanoma skin cancer.

In women, breasts are glands that are capable of producing milk. Each breast is made up of 15 to 20 sections known as lobes. Each lobe contains a number of smaller lobules which contain the milk secreting cells. The milk is then transported to the nipple by ducts. The lobular cells and the ductal cells can both be affected by cancer.

The breasts also contain lymph vessels that transport a clear fluid called lymph through the body to the lymph nodes. Lymph nodes near the breast are found under the arm, above the collarbone and behind the breastbone.

The body’s organs and glands (including breasts) are made up of tissues, which are made up of cells. Normal cell function requires these building blocks to divide and also to die when they grow old – allowing for new cells to take their place in an organized manner. When old cells do not die and the body continues to create new cells it does not need, a mass of cells form a growth or tumor. These tumors do not always signal cancer, particularly in the breast. They can be benign (noncancerous) or malignant (cancerous).

Benign breast lumps are common and may be due to fibrocystic changes in the breast tissue. Most benign breast lumps are fibroadenomas or papillomas. In addition, benign tumors:

• Are rarely life-threatening
• Can be removed and seldom grow back
• Do not spread to tissue around them or to other parts of the body

Malignant tumors have significant differences from benign tumors. Malignant tumors:

• Are generally more serious than benign tumors
  
• May be life-threatening
  
• Can often be removed but they can grow back
  
• Can invade nearby tissues and organs (local invasion)
  
• Can break away in the form of cells that enter the bloodstream or lymphatic system and travel to other areas (distant metastasis)

Breast cancer begins with a growth of abnormal cells within the breast tissues. The type of breast cancer is determined by where the cancer began – in the ducts, the lobules or other areas, such as the connective tissue or in the blood vessels. It is also important to determine if the cancer has spread beyond the ducts or lobules and invaded nearby lymph nodes.

The lymph nodes are bean-shaped groupings of immune system cells that help the body fight off infections and other threats. A woman’s breast contains both blood vessels and lymph vessels. Within the lymph vessels is a clear fluid called lymph. Fluid from the breast tissue drains through the lymph vessels to the lymph nodes under the armpit, near the breastbone and above the collar bone. 

Thus, when breast cancer starts to spread, the most common first location is the nearby lymph nodes. If breast cancer has spread to the axillary lymph nodes (located in the underarm region of the body), it can cause swelling of these nodes. After the cancer cells have spread to the lymph nodes it is more likely that the cancer will spread to other areas as well, such as the lungs, bones or brain.

According to the National Cancer Institute  (NCI), breast cancer is the most common cancer among women, excluding non-melanoma skin cancers. It accounts for nearly 1 in 3 cancers diagnosed in women in the United States. Since 1990, the death rate from breast cancer in women has declined. The decreased number of deaths has been attributed to both earlier detection and advances in treatment of the disease.

A study released in 2006 indicated that the incidence, or new cases, of invasive breast cancer declined in the period from mid-2002 to 2003. The ACS statistics for 2001 to 2003 indicate that new breast cancer cases leveled off after 20 years of increases. Many factors may contribute to this change, including changes in early detection or a major decrease in the use of hormone replacement therapy by women after menopause. However, the cause and effect of such changes are difficult to establish, especially in the short term.