Cancer Institute A national cancer institute
designated cancer center

What is cancer?

Cancer is an abnormal growth of cells. Cancer cells rapidly reproduce despite restriction of space, nutrients shared by other cells, or signals sent from the body to stop reproduction. Cancer cells are often shaped differently from healthy cells, they do not function properly, and they can spread to many areas of the body. Tumors, abnormal growth of tissue, are clusters of cells that are capable of growing and dividing uncontrollably; their growth is not regulated.

Oncology is the study of cancer and tumors. The term "cancer" is used when a tumor is malignant, which is to say it has the potential to cause harm, including death.

What do the terms benign and malignant mean?

Tumors can be benign (noncancerous) or malignant (cancerous). Benign tumors tend to grow slowly and do not spread. Malignant tumors can grow rapidly, invade and destroy nearby normal tissues, and spread throughout the body.

What do the terms "locally invasive" and "metastatic" mean?

Cancer is malignant because it can be "locally invasive" and "metastatic":

What are primary tumors?

The original tumor is called the "primary tumor." Its cells, which travel through the body, can begin the formation of new tumors in other organs. These new tumors are referred to as "secondary tumors."

The cancerous cells travel through the blood (circulatory system) or lymphatic system to form secondary tumors. The lymphatic system is a series of small vessels that collect waste from cells, carrying it into larger vessels, and finally into lymph nodes. Lymph fluid eventually drains into the bloodstream.

How is each cancer type named?

Cancer is named after the part of the body where it originated. When cancer spreads, it keeps this same name. For example, if kidney cancer spreads to the lungs, it is still kidney cancer, not lung cancer. (The lung cancer would be an example of a secondary tumor.)

Staging is the process of determining whether cancer has spread and, if so, how far. There is more than one system used for staging cancer.

What are the different types of cancer?

Cancer is not just one disease but rather a group of diseases, all of which cause cells in the body to change and grow out of control. Cancers are classified either according to the kind of fluid or tissue from which they originate, or according to the location in the body where they first developed. In addition, some cancers are of mixed types.

The following five broad categories indicate the tissue and blood classifications of cancer:

Stanford Medicine Resources:

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