Cancer is a disease in which the cells in part of a person’s body grow uncontrollably. Although there a many different types of cancer, they all develop as a result of uncontrolled growth of abnormal malignant cells.

Normally, cells grow, divide and die in an orderly way. The rates of new cell growth and old cell death are kept in balance. In a healthy person, the cells divide at a rate to repair injuries and to replace depleted or dying cells, while the old and damaged cells self-destruct in a process known as apoptosis.

This normal balance is disrupted during the development of cancer. Cancer cells continue to grow and divide uncontrollably, outliving normal cells and continuing to form new abnormal cells. This increase in cell division generates a growing mass of tissue known as a tumor or neoplasm. As new cells are being produced in greater numbers than needed by the body, the tumor will increase in size. The growth rate of a tumor depends on the rate of cell division. When cells divide rapidly, the tumor will grow quickly and vice versa.

Typically, cancer forms as a tumor. However, not all tumors are malignant. Noncancerous or benign tumors do not spread and are normally not life threatening. Some types of cancer do not involve tumor masses in one place, but are diffused throughout the body. An example is the leukemias, which involve the blood and blood-forming organs.

Frequently, cancer cells travel from the initial tumor to other areas of the body where they begin to grow and replace healthy tissue. There are two main terms used to describe the spread of cancer from its original location:

• Invasion. Cancer cells invade neighboring tissues. For example, invasive cancer cells in the breast may penetrate into tissues within or near the breast.
  
  
• Metastasis. Cancer spreads beyond the immediate neighboring area to other body areas. It may penetrate the lymph vessels and blood vessels, enter the bloodstream and invade normal tissue in adjacent and distant areas of the body. The likelihood that cancer will metastasize depends on the type of cancer and its stage. Although each patient’s case is unique, some types of cancer have a tendency to spread to certain areas, including the lungs, bones and brain.
  
   

  

Cancer can develop almost anywhere in the body. Although it may spread to other parts of the body, it is always named for the area where it originated. For example, melanoma (cancer of the cells that produce skin coloring) that spreads to the liver is classified as metastatic melanoma, not liver cancer.

According to the American Cancer Society (ACS), approximately one out of every two men and one out of every three women will develop some type of cancer at some point during their lifetime. Although cancer is the second leading cause of death in the United States, millions of people are living with cancer or have had cancer. The National Cancer Institute estimates that 10.5 million Americans with a history of cancer were alive in January 2003.

More than one million people develop cancer each year, according to the ACS. Although anyone can develop cancer, it is more common in certain age groups and ethnic populations. Approximately 77 percent of all cancers are diagnosed in people age 55 and older, and the rate of occurrence (incidence rate) varies among ethnic groups. According to the ACS, the cancers most commonly diagnosed in the United States include: