Cancer starts when cells start to grow out of control. Cells in nearly any part of the body can become cancer. To learn more about how cancers start and spread, see What Is Cancer? For information about the differences between childhood cancers and adult cancers, see Cancer in Children.
Leukemia is a cancer that starts in early blood-forming cells found in the bone marrow, the soft inner part of certain bones. Most often, leukemia is a cancer of the white blood cells, but some leukemias start in other blood cell types.
Any of the blood-forming cells from the bone marrow can turn into a leukemia cell. Once this change takes place, the leukemia cells no longer mature in a normal way. Leukemia cells might reproduce quickly, and not die when they should. These cells build up in the bone marrow, crowding out normal cells. In most cases, the leukemia cells spill into the bloodstream fairly quickly. From there they can go to other parts of the body such as the lymph nodes, spleen, liver, central nervous system (the brain and spinal cord), testicles, or other organs, where they can keep other cells in the body from doing their jobs.
Some other childhood cancers, such as neuroblastoma or Wilms tumor, start in other organs and can spread to bone marrow, but these cancers are not leukemia.