What is Chronic Myeloid Leukemia (CML)?

Reviewed by: HU Medical Review Board | Last reviewed: November 2019

Chronic myeloid leukemia (CML) is a type of blood cancer that begins in the bone marrow and moves into the blood. It is also called chronic myelogenous leukemia. This type of blood cancer develops from the myeloid cells, which are the cells that normally develop into red blood cells, platelets, and white blood cells.1,2

CML tends to grow slowly. However, CML can change into a rapidly growing leukemia that can be more difficult to treat.1

How does chronic myeloid leukemia (CML) develop?

The cells that make up the blood include white blood cells, red blood cells, and platelets. These cells are made in the bone marrow, the spongy center of bones. Each of the different blood cells has a job. Red blood cells carry oxygen to the tissues. Platelets create blood clots to help stop bleeding. White blood cells fight infections.3,4

The bone marrow also contains blood stem cells. These immature cells can become different types of cells in the body, including myeloid cells or lymphoid cells. Myeloid cells turn into white blood cells, red blood cells, and platelets.4,5 CML is a blood cancer that develops from myeloid cells.1,2

This or That

Are you or a loved on living with leukemia?

The Philadelphia chromosome

A gene is a segment of DNA that is passed from parent to child. Genes are arranged into chromosomes, and each person has 23 pairs of chromosomes.6 Each time any cell divides, it makes a new copy of its DNA. Sometimes errors occur during this division.1

These DNA errors can cause cancer when a cell grows out of control and crowds out healthy cells. Or, a DNA error can cause healthy cells to turn off rather than fight as they should.1

Many people with CML have a genetic abnormality called the Philadelphia chromosome, which occurs between chromosomes 9 and 22. The switching of the pieces of chromosomes 9 and 22 result in the creation of an oncogene called BCR-ABL, which can lead to the development of CML.1,2,5

Figure 1. Philadelphia chromosome

Normal chromosomes 9 and 22 break apart and switch pieces, forming the Philadelphia Chromosome

CML mostly occurs in adults and men, with an average age at diagnosis of 65. CML accounts for about 15 percent of all leukemias in adults.1

What factors increase a person’s risk of developing it?

Risk factors are things that can increase the chance someone will develop a disease. Only a few risk factors for CML are known, including:

  • Exposure to high doses of radiation (like a nuclear reactor accident or atomic bomb blast)
  • Age (the risk increases as people get older)
  • Gender (the risk is slightly higher for men than women)1

What are the common symptoms of chronic myeloid leukemia (CML)?

The symptoms of CML may be vague and easy to attribute to another illness. That is why many people are diagnosed when they have a blood test for other reasons. Symptoms tend to come on slowly.

Common symptoms of CML include:

CML can cause other problems as the cancerous cells crowd out healthy blood cells, including:

  • Anemia (low levels of red blood cells)
  • Increased risk of infections (due to a low white blood cell count
  • Increased bleeding or bruising (due to a low platelet count)1-2

How is it staged?

CML is grouped into 3 phases:

  • In the chronic phase, the blood and bone marrow contain less than 10 percent blasts (immature white blood cells). Most people are diagnosed at this phase.
  • In the accelerated phase, more changes are seen in the blood. The blood will have more than 15 percent but less than 30 percent blasts. Basophils (a type of white blood cell) will make up 20 percent or more of the blood and bone marrow. Platelet counts will be low, and the spleen will grow in size. Anemia will get worse. New genetic changes will be found in the CML cells.
  • The blast phase is also called a blast crisis. It occurs when the bone marrow or blood has more than 20 percent blasts. The blast cells usually will have spread beyond the bone marrow to other tissue. At this stage, CML tends to behave more like an acute leukemia.1,2,5

What is the prognosis?

Over the past few decades, the survival rate for leukemia, including CML, has improved. Based on data from 2009-2015, the National Cancer Institute has determined the 5-year survival rate for CML is 69.2 percent.7

Survival rates are based on earlier outcomes of people who survive a set amount of time after diagnosis. With cancer, experts talk about the “5-year survival rate.” It is important to keep in mind that many people live more than 5 years after diagnosis. Plus, these statistics do not always take into account people treated with newer, more effective drugs.

Recently, highly effective medications have become available to treat CML. Many people with CML who have used these newer drugs have experienced longer survival rates.8,9

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