What is Chronic Myeloid Leukemia (CML)?

Chronic myeloid leukemia is a type of blood cancer that begins in the bone marrow and enters into the blood. Also known as chronic myelogenous leukemia or CML, this type of blood cancer develops from the myeloid cells, the cells that normally develop into red blood cells, platelets, and white blood cells.1,2

CML is generally slower growing, and the cancerous cells of CML may accumulate over a long period of time. However, CML may change into a more quickly growing leukemia that can be more difficult to treat.1

How does chronic myeloid leukemia develop?

The cells that make up the blood, including white blood cells, red blood cells, and platelets, are made in the bone marrow. Each of the different blood cells has their own purpose. Red blood cells carry oxygen to the tissues, platelets help stop bleeding by creating clots, and white blood cells are the immune system’s primary defense and are critical for fighting infections.3,4

The bone marrow is the spongy center portion of bones. The bone marrow contains blood stem cells, which are immature cells that can become different types of cells in the body. The stem cells differentiate into myeloid cells or lymphoid cells. Myeloid cells can become white blood cells, red blood cells, and platelets.4,5 CML is a blood cancer that develops from myeloid cells.1,2

The Philadelphia chromosome

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

These errors can cause the cell to develop abnormally and may lead to the development of cancer through the activation of oncogenes or the suppression of tumor suppressor genes. Oncogenes can cause the cells to replicate more, which can result in cancer. Tumor suppressor genes are normally a stop mechanism that cause damaged cells to die or slow cell growth. When tumor suppressor genes are turned off, cancer cells can replicate more easily.1

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

The Philadelphia Chromosome

Who gets chronic myeloid leukemia?

CML mostly occurs in adults, with an average age at diagnosis of approximately 64 years. CML accounts for approximately 10% of all leukemias.1

What factors increase a person’s risk of developing chronic myeloid leukemia?

Risk factors are characteristics that can increase someone’s chances of developing 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 may be diagnosed before the person experiences any symptoms when undergoing blood tests for other reasons. When people do experience symptoms, they may develop gradually. Common symptoms of CML include:

The symptoms of CML can occur as the cancerous cells crowd out healthy blood cells, leading to conditions like anemia (low red blood cells), increased risk of infections (due to low white blood cells), and increased bleeding or bruising (due to low platelets).2

How is chronic myeloid leukemia staged?

CML is characterized by three phases:

  • Chronic phase is defined as the blood and bone marrow containing less than 10% blasts (immature white blood cells). Most people are diagnosed at this phase.
  • Accelerated phase is generally characterized by an increase in blasts (more than 10% but less than 20%), a high basophil (specific white blood cell) count in the blood (at least 20% of the white blood cells), new genetic changes in the leukemia cells, changes in the counts of white blood cells (high), and/or changes in the counts of platelets (very high or very low).
  • Blast phase, also called blast crisis, occurs when the bone marrow or blood has more than 20% blasts, and the blast cells typically have spread beyond the bone marrow to other organs or tissues in the body. At this phase, CML tends to behave more like an aggressive leukemia.1,5

What is the prognosis for chronic myeloid leukemia?

Over the past few decades, the survival rate for leukemia, including CML, has been steadily increasing. (Survival rates are based on previous outcomes of people who survive a set amount of time after diagnosis. In cancer estimates, experts use the “five-year survival rate” as a marker. However, it is important to keep in mind that many people live beyond five years after diagnosis and the statistics are not necessarily predictive for any one individual.) Based on data from 2007-2013 (the most recent data available), the National Cancer Institute has determined the five-year survival rate for CML is 66.9%.7

New, highly effective medications have become available to treat CML, and while the long-term data is still being collected, many people with CML who have been treated with these newer drugs have experienced longer survival rates.8,9

Written by: Emily Downward | Last reviewed: February 2018
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