What Is Cancer?

Cancer is a group of diseases in which abnormal body cells in one part of the body start to grow out of control. Normal body cells grow, divide, and die in an orderly fashion. Cancer cells grow and divide without stopping. As a result, they live longer than normal cells and they keep forming new abnormal cells.

Cancer cells may join together to create a lump, mass, or tumor. Some cancers, such as leukemia, start in the blood and blood-forming organs and do not form tumors. Instead, these cancer cells circulate through other tissues where they grow.

Eventually, this uncontrolled growth damages normal cells and interferes with normal body functions. Treatments for cancer focus on stopping this growth by killing cancer cells while causing as little damage as possible to surrounding normal cells.

The many different kinds of cancer start in different areas of the body. These illnesses grow at different rates and respond differently to treatment. The human body is made of more than 200 different types of living cells. Cancer can start in any of these cell types. For this reason, scientists believe that there are more than 200 types of cancer.

The Different Types of Cancer

Cancers are categorized according to their primary site, or the part of the body where the cancer started, and by the type of tissue in which the cancer began (histological type). There are four major types:

Carcinomas—Carcinomas start in the internal or external lining of the body. They are the most common type of cancer, accounting for about 80 to 90 percent of all cancer cases. Examples of this type of cancer are breast, colon, and lung cancer.

Leukemias—Leukemias start in the bone marrow, or soft, spongy center of the bone where blood cells are made. In most cases of leukemia, the body makes large number of white blood cells (leukocytes) that do not work properly.

Lymphomas—Lymphomas start in the glands or nodes of the lymphatic system. The lymphatic system is a network of vessels, nodes, and organs that purify body fluids and make lymphocytes, a type of white blood cell that fights infection.

Sarcomas—Sarcomas start in supportive or connective tissue, such as the bones, tendons, cartilage, muscle, and fat.