Chronic Myeloid Leukemia (CML)

What It Is

Chronic myeloid leukemia (CML) is a slow growing cancer in which the bone marrow- the soft, spongy tissue in the center of bones- makes too many white blood cells. Although normal white blood cells help the body fight infection and disease, leukemia cells look different than normal blood cells and do not function correctly. Large numbers of leukemia cells also interfere with other blood cells, usually red blood cells and platelets.

CML is called a chronic leukemia because the leukemia cells typically grow more slowly at first and it takes longer for the disease to get worse. Chronic myeloid leukemia is also called chronic myelogenous leukemia, chronic myelocytic leukemia, or chronic granulocytic leukemia.

 

What Causes It

In about 95% of patients with CML, a defect in a chromosome (the genetic material) in the leukemia cells occurs. This defect is called the Philadelphia chromosome, named for the city where it was discovered. This abnormal chromosome forms when two chromosomes in the cell swap their genetic material. It is not clear what causes this to happen. Because of this event, a protein that normally helps to regulate the production of new white blood cells in the bone marrow—the Abl protein--becomes stuck in the "on" position, telling the body to keep making more abnormal blood cells.

 

The Phases of CML

CML progresses through three phases. The number of immature leukemia cells (blasts) in the blood and bone marrow and the severity of symptoms help doctors determine which phase of CML a patient is in.

Chronic phase. In this phase there are few blasts (fewer than 5%) in the blood and bone marrow. There may be no symptoms of CML or symptoms may be mild. This phase can last for a few months to several years. Most cases of CML are diagnosed at this phase.

Accelerated phase. The number of blast cells increases to about 15%. This phase can last weeks to months.

Blastic phase, or blast crisis. More than 30% of cells are blasts. Sometimes blast cells will form tumors outside of the bone marrow in the bone or lymph nodes. At this point, chronic leukemia has become an aggressive acute (fast-growing) leukemia.

 

Who Is More At Risk

CML is an uncommon type of leukemia, making up about 15% of all cases of leukemia among adults. About 4,000 to 5,000 new cases of the disease are diagnosed each year in the United States. People of any age can develop CML, but it is most common in adults 50 years of age and older.

Children of parents with CML do not have a greater risk of developing the disease. There is a question as to whether radiation used for treatment raises the risk of CML. No infectious agents are linked to the development of this leukemia.

 

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