Chronic myeloid leukemia (CML), also known as chronic myelogenous leukemia, is a type of cancer that starts in the blood-forming cells of the bone marrow and invades the blood. It is usually associated with a chromosome abnormality called the Philadelphia chromosome. In CML, leukemia cells tend to build up in the body over time, but in many cases people don't have any symptoms for at least a few years. In time, the cells can also invade other parts of the body, including the spleen. 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 very rarely it occurs in children, too. In general, 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 lymphoid tissue

To understand the different types of leukemia, it helps to have some basic knowledge about the blood and lymph systems. The information which follows is quite complex. It may prove helpful, but you don't need to understand all of it to learn more about your leukemia.

Bone marrow

Bone marrow is the soft inner part of some bones such as the skull, shoulder blades, ribs, pelvis, and backbones. The bone marrow is made up of a small number of blood stem cells, more mature blood-forming cells, fat cells, and supporting tissues that help cells grow.

The blood-forming cells come from blood stem cells. These stem cells only make new blood-forming cells, not other kinds of cells. (This makes them different from embryonic stem cells, which form in a developing fetus and can develop into most other cells in the body.)

Stem cells go through a series of changes. During this process, the cells develop into cells that become lymphocytes (a kind of white blood cell) or into cells that form other cells found in blood. These other cells include red blood cells, white blood cells (other than lymphocytes), or platelets.

Red blood cells

Red blood cells carry oxygen from the lungs to all other tissues in the body and take carbon dioxide back to the lungs to be removed. Not having enough red blood cells in the body is called anemia. Symptoms of anemia include weakness, fatigue, and shortness of breath because the body tissues are not getting enough oxygen.

Platelets

Platelets aren't actually whole cells, but instead are pieces of large cells found in the bone marrow called megakaryocytes. Platelets are important in plugging up holes in blood vessels caused by trauma. A shortage of platelets is called thrombocytopenia, and can lead to problems with easy bruising and bleeding.

White blood cells

White blood cells are important in defending the body against infections. Lymphocytes are one type of white blood cell. The other types of white blood cells are granulocytes and monocytes.

Lymphocytes are the main cells that make up lymphoid tissue, a major part of the immune system. Lymphoid tissue is found in lymph nodes, the thymus gland, the spleen, the tonsils, and the adenoids. It is also scattered throughout the digestive and respiratory systems and the bone marrow.

Lymphocytes develop from cells called lymphoblasts to become mature, infection-fighting cells. The 2 major types of lymphocytes are known as B lymphocytes (B cells) and T lymphocytes (T cells).

• B lymphocytes protect the body from invading germs by developing (maturing) into plasma cells, which make antibodies. These antibodies attach to the germs, such as bacteria, viruses, and fungi. Once the germ has been coated in this way, it can be targeted by other parts of the immune system and be destroyed.

• T lymphocytes can recognize cells infected by viruses and directly destroy these cells.

Granulocytes develop from blood-forming cells called myeloblasts to become mature, infection-fighting cells. These white blood cells are called granulocytes because they contain granules, which can be seen as spots in the cells when they are looked at under the microscope. These granules contain enzymes and other substances that can destroy germs such as bacteria. The 3 types of granulocytes -- neutrophils, basophils, and eosinophils -- are distinguished by the size and color of their granules. Neutrophils are the most common type of granulocyte in the blood. They are essential in destroying bacteria that have invaded the blood.

Monocytes are related to granulocytes and are important because they protect the body against bacteria. They start in the bone marrow as blood-forming monoblasts and develop into mature monocytes. Monocytes circulate in the bloodstream for about a day and then they enter body tissues to become macrophages. These microphages can destroy some germs by surrounding and digesting them. Macrophages are also important because they help lymphocytes recognize germs and start making antibodies to fight them.

How leukemia starts

Any of the blood-forming or lymphoid cells from the bone marrow can turn into a leukemia cell. Once this change takes place, the leukemia cells fail to go through their normal process of maturing. Leukemia cells may reproduce too quickly, but in most cases the problem is that they don't die when they should. They survive and build up, often crowding out normal bone marrow cells. This can lead to low counts of normal blood cells. Over time, leukemia cells spill into the bloodstream and spread to other organs, where they can prevent 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 specific type of leukemia can help doctors better predict each patient's prognosis (outlook) and select the best treatment.

Acute leukemia versus chronic leukemia

The first factor to consider in classifying a patient's leukemia is whether most of the abnormal cells are mature (look like normal white blood cells) or immature (look more like stem cells).

Acute leukemia: In acute leukemia, the bone marrow cells cannot mature the way they should. Immature leukemia cells continue to reproduce and build up. Without treatment, most patients with acute leukemia would only live a few months. Some types of acute leukemia respond well to treatment, and many patients can be cured. Other types of acute leukemia have a less favorable outlook.

Chronic leukemia: In chronic leukemia, the cells can mature partly but not completely. These cells are not really normal, but they look more normal than the immature cells of acute leukemia. They generally do not fight infection as well as normal white blood cells do. And, of course, they survive longer, build up, and crowd out normal cells. Chronic leukemias tend to develop over a longer period of time, and most patients can live for many years. However, chronic leukemias are generally harder to cure than acute leukemias.

Myeloid leukemia versus lymphocytic leukemia

The second factor to consider in classifying leukemia is the type of bone marrow cells that are affected.

Leukemias that start in early myeloid cells -- the cells that become white blood cells (other than lymphocytes), red blood cells, or platelet-making cells (megakaryocytes) -- are called myeloid leukemias. These are also known as myelocytic, myelogenous, or non-lymphocytic leukemias.

Leukemias that start in the cells that become lymphocytes are called lymphocytic leukemia. These are also known as lymphoblastic or lymphoid leukemias.

A doctor can look at the leukemia cells under a microscope and do certain tests to see if the leukemia is acute or chronic and whether the cells are myeloid or lymphocytic. This allows leukemias to be divided into 4 main types:

• acute myeloid (or myelogenous) leukemia (AML)
• chronic myeloid (or myelogenous) leukemia (CML)
• acute lymphocytic (or lymphoblastic) leukemia (ALL)
• chronic lymphocytic leukemia (CLL)

The rest of this document contains information on chronic myeloid leukemia (CML) only. Separate American Cancer Society documents on other forms of acute and chronic leukemias are available. This document does not contain information on chronic myelomonocytic leukemia (CMML), which is covered in our separate document, Chronic Myelomonocytic Leukemia.

Last Medical Review: 11/05/2009
Last Revised: 11/05/2009