Chronic myeloid leukemia (CML) is also known as chronic myelogenous leukemia. It is a type of cancer that starts in the blood-forming cells of the bone marrow. It then invades the blood and can spread to other parts of the body. In CML, leukemia cells tend to build up in the body over time. In many cases people don't have any symptoms for at least a few years. CML can also change into a fast-growing, acute leukemia that invades almost any organ in the body. Most cases of CML occur in adults, but it is rarely found in children, too. Their treatment is the same as for adults.

Leukemia is different from other types of cancer that start in organs such as the lungs, colon, or breast and then spread to the bone marrow. Cancers that start elsewhere and then spread to the bone marrow are not leukemia.

Normal bone marrow, blood and lymph tissue

To understand the different types of leukemia, it is helpful to know a little about the blood and lymph systems. This information is quite complex. It may be helpful, but you do not need to understand all of it in order to learn more about your leukemia.

Bone marrow

Bone marrow is the soft inner part of some bones, like the skull, shoulder blades, ribs, pelvis, and backbones. Bone marrow is made up of blood-forming cells, fat cells, and tissues that help cells grow.

The cells that make blood are called blood stem cells. These stem cells only make new blood-forming cells. They cannot make any other kinds of cells.

Stem cells normally divide to make new stem cells. But some of the stem cells go through a series of changes. During this process they can become red blood cells, white blood cells, or platelets.

Red blood cells

Red blood cells carry oxygen from the lungs to all other cells in the body. They also carry away carbon dioxide, a cell waste product. A shortage of red blood cells (anemia) makes a person feel weak, short of breath, and tired.

Platelets

Platelets help prevent bleeding by plugging up holes of blood vessels caused by cuts or bruises. A person with a shortage of platelets can bruise or bleed easily.

White blood cells

White blood cells help defend the body against infections. Lymphocytes are one type of white blood cell, but there are other types too. Lymphocytes are the main cells that make up lymphoid tissue, an important part of the immune system. Lymphoid tissue is found in lymph nodes, the thymus gland, the spleen, the tonsils and adenoids, and is scattered throughout the digestive and respiratory systems and the bone marrow.

There are 2 types of lymphocytes:

• B lymphocytes protect the body from invading germs.
• T lymphocytes destroy cells that are infected with viruses.

How leukemia starts

Any of the blood-forming or lymphoid cells from the bone marrow can turn into leukemia cells. Once this happens, the cells don't go through normal growth. And in many cases, these cells do not die as they should. This allows them to build up, spill into the bloodstream, and spread to other organs. Having too many leukemia cells can keep other cells in the body from working the way they should.

Types of leukemia

Not all leukemias are the same. Leukemias are divided into 4 main types. Knowing the exact type of leukemia can help doctors better predict each patient's outlook (prognosis) and select the best treatment.

Acute leukemia versus chronic leukemia

In acute leukemia, the cells grow fast but do not mature the way they should. The cells keep on reproducing and build up in the blood. Without treatment, most patients with acute leukemia would live only a few months. Some types of acute leukemia respond well to treatment, and many patients can be cured.

In chronic leukemia the cells look mature but they are not really normal. The cells live too long, build up, and crowd out normal cells. Chronic leukemias tend to progress over a longer period of time, and most patients can live for many years. But chronic leukemias are often harder to cure than acute leukemias.

Myeloid leukemia versus lymphocytic leukemia

Myeloid leukemia mostly starts in white blood cells (other than lymphocytes), but can also start in the cells that mature into platelets or red blood cells.

Lymphocytic leukemias start in lymphocytes (a type of white blood cell) in the bone marrow.

By looking at whether a leukemia is acute or chronic and whether it is myeloid or lymphocytic, most cases of leukemia can be sorted into 1 of the 4 main types shown in the table here:

Here we are only covering chronic myeloid leukemia (CML) of adults. The other types of adult leukemia are covered in different American Cancer Society documents. For information about leukemia in children, please see the document, Childhood Leukemias.

Last Medical Review: 10/08/2008
Last Revised: 05/06/2009