What is Cancer?

Approved by the Cancer.Net Editorial Board, 07/2012

Cancer is a group of more than 100 different diseases that can begin almost anywhere in the body. It happens when normal cells in the body change and grow uncontrollably. These cells may form a mass called a tumor. A tumor can be benign (noncancerous) or malignant (cancerous, meaning it can spread to other parts of the body). However, some cancers do not form solid tumors. These include leukemias (see below), most types of lymphoma (see below), and myeloma (cancer of the plasma cells in the bone marrow, the spongy tissue inside of bones).

The naming of cancer

Cancer is named for the area of the body and the type of cell in which it started:

Learn more about specific types of cancer.

When cancer spreads

Sometimes, cancer will spread to the lymph nodes, which are tiny, bean-shaped organs that help fight infection. Lymph nodes are located in clusters in different parts of the body, such as the neck, the groin area, and the area under the arms. Cancer may also enter the bloodstream and spread to other parts of the body, such as the bones, liver, lungs, or brain. This process is called metastasis. However, even if the cancer has spread, it is still named for the area where it began. For example, if breast cancer spreads to the lungs, it is called metastatic breast cancer, not lung cancer.

Watch a brief video on how cancer begins and spreads to other areas of the body.

Video used with permission from BioDigital Systems.

The diagnosis of cancer

Often, a diagnosis begins when a person tells a doctor about any unusual symptoms. After discussing a person’s medical history and his or her symptoms, the doctor will perform various tests to find out the cause of a person’s symptoms. However, many times a person with cancer has no symptoms.

Sometimes a doctor diagnoses cancer after a cancer screening test in an otherwise healthy person. Examples of screening tests include a colonoscopy, a mammogram, and a Pap test. The results of these tests may mean a person has additional tests to confirm or disprove the result of the screening test.

Less often, a cancer is diagnosed when a person has a medical test for another reason.

For most cancers, a biopsy is the only way to make a definitive diagnosis. A biopsy is the removal of a small amount of tissue for further study. Learn more about making a diagnosis after a biopsy.

More Information

Risk Factors and Prevention

Tests and Procedures

When the Doctor Says Cancer

Newly Diagnosed

Last Updated: August 01, 2012