Cancer is a group of many related diseases that begin in cells, the body's basic unit of life. To understand cancer, it is helpful to know what happens when normal cells become cancerous.

The body is made up of many types of cells. Normally, cells grow and divide to produce more cells only when the body needs them. This orderly process helps keep the body healthy. Sometimes, however, cells keep dividing when new cells are not needed. These extra cells form a mass of tissue called a growth or tumor.

Tumors can be benign or malignant.

• Benign tumors are not cancer. They can often be removed and, in most cases, they do not come back. Cells from benign tumors do not spread to other parts of the body. Most important, benign tumors are rarely a threat to life.
• Malignant tumors are cancer. Cells in these tumors are abnormal and divide without control or order. They can invade and damage nearby tissues and organs. Also, cancer cells can break away from a malignant tumor and enter the bloodstream or the lymphatic system. That is how cancer spreads from the original cancer site to form new tumors in other organs. Cancer that has spread is called metastatic cancer.

Most cancers are named for the organ or type of cell in which they begin. When cancer spreads (metastasizes), cancer cells are often found in nearby or regional lymph nodes (sometimes called lymph glands). If the cancer has reached these nodes, it means that cancer cells may have spread to other organs, such as the liver, bones, or brain. When cancer spreads from its original location to another part of the body, the new tumor has the same kind of abnormal cells and the same name as the primary tumor. For example, if lung cancer spreads to the brain, the cancer cells in the brain are actually lung cancer cells. The disease is called metastatic lung cancer (not brain cancer).

Children can get cancer in the same parts of the body as adults do, but some types of cancer are more common in children. The most common form of childhood cancer is leukemia. Leukemia is cancer of the blood. It develops in the bone marrow, which is a spongy substance that fills the inside of the bones and makes blood cells. Other cancers often found in children are brain tumors, childhood lymphomas, Hodgkin's disease, Wilms' tumors, neuroblastomas, osteogenic sarcomas, Ewing's sarcomas, retino-blastomas, rhabdomyosarcomas and hepatoblastomas. The Appendix contains information on the major types of childhood cancer.

Children's cancers do not always act like, get treated like, or respond like adult cancers. Avoid reading about adult cancer to learn about your child's prognosis. Childhood cancers can occur suddenly, without early symptoms, and have a high rate of cure. You can find more details about these types of cancer in other National Cancer Institute (NCI) booklets. NCI's What You Need to Know About... brochures have information about specific types of cancer. (See page 96 for more information on available booklets.) To receive copies from the NCI-supported Cancer Information Service (CIS), call 1-800-4-CANCER (1-800-422-6237) or TTY at 1-800-332-8615. Also, many NCI publications may be viewed or ordered on the Internet at http://cancer.gov/publications.

Glossary Terms

Not cancerous. Benign tumors may grow larger but do not spread to other parts of the body. Also called nonmalignant.

The soft, sponge-like tissue in the center of most bones. It produces white blood cells, red blood cells, and platelets.

Cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of blood cells to be produced and enter the bloodstream.

Cancer that begins in cells of the immune system. There are two basic categories of lymphomas. One kind is Hodgkin lymphoma, which is marked by the presence of a type of cell called the Reed-Sternberg cell. The other category is non-Hodgkin lymphomas, which includes a large, diverse group of cancers of immune system cells. Non-Hodgkin lymphomas can be further divided into cancers that have an indolent (slow-growing) course and those that have an aggressive (fast-growing) course. These subtypes behave and respond to treatment differently. Both Hodgkin and non-Hodgkin lymphomas can occur in children and adults, and prognosis and treatment depend on the stage and the type of cancer.

Cancerous. Malignant tumors can invade and destroy nearby tissue and spread to other parts of the body.

An abnormal mass of tissue that results when cells divide more than they should or do not die when they should. Tumors may be benign (not cancer), or malignant (cancer). Also called neoplasm.