Chronic myeloid leukemia (CML) is a type of leukemia—a cancer of the bone marrow and blood. This disease is sometimes referred to as chronic myelogenous leukemia.
The American Cancer Society estimates that about 6,700 people are diagnosed with CML each year in the U.S. About one out of every 10 new cases of leukemia is CML.
What Is CML?
Leukemia begins in the bone marrow. Myeloid, or myelogenous, leukemias occur in bone marrow cells that normally become red blood cells, some types of white blood cells, and platelets.
How quickly leukemia progresses and how the leukemia cells replace normal blood and marrow cells differ with each type of leukemia. When it’s acute, leukemia quickly prevents a large proportion of early blood cells from maturing. In chronic leukemia, more healthy blood cells get a chance to mature, so symptoms usually come on more gradually, and they are less severe at first.
In CML, too many blood stem cells become granulocytes—a type of white blood cell. These abnormal granulocytes, also called CML or leukemia cells, multiply just like other cells. But the CML cells grow and survive better than normal cells. Over time they crowd out healthy cells the body needs.
Understanding the Bone Marrow and Blood
To understand leukemia, it helps to have basic knowledge of the bone marrow and how healthy blood cells form and what they do.
Stem cells are cells in the body that have the potential to turn into any kind of cell, such as a skin cell, a liver cell, a brain cell, or a blood cell. Stem cells that turn into blood cells are called hematopoietic stem cells, or blood stem cells. Blood stem cells are mainly found in bone marrow (the soft, spongy tissue inside your bones), but some are also found in circulating blood. Normal blasts (immature blood cells) are found only in the bone marrow, not in circulating blood. When blood cells become old or damaged, they die, and blood stem cells produce new blood cells to replace them.
Blood stem cells produce myeloid stem cells and lymphoid stem cells. Myeloid stem cells produce myeloblasts, which turn into white blood cells known as granulocytes, as well as red blood cells and platelets. Lymphoid stem cells produce lymphoblasts, which in turn produce several types of white blood cells, including lymphocytes and natural killer cells.
- White blood cells (leukocytes) fight infection.
- Red blood cells (erythrocytes) carry oxygen from the lungs to the other parts of the body and take carbon dioxide back to the lungs to be removed.
- Platelets (thrombocytes) make the blood clot and slow or stop bleeding.
What Causes CML?
Doctors do not know what causes CML. In almost all cases, CML cells have a chromosomal abnormality known as BCR-ABL, or the Philadelphia chromosome.
CML typically affects older people—the average age of a person with the disease is 64—but it can affect people of any age, including children, although this is rare. The disease is slightly more common in males than in females. In addition to age and sex, the only other factor that may increase your odds for developing CML is exposure to high levels of radiation. However, no link has been shown between dental or medical X-rays and CML risk.
Many of the risk factors for other cancers, including smoking, diet, exposure to chemicals, infections, and family history of the disease, do not seem to increase risk for CML.
Keep in mind that most people who develop CML have none of the risk factors and most people with the risk factors do not develop the disease.