Doctors divide chronic myeloid leukemia into three phases: the chronic phase, accelerated phase, and blast phase. The chronic phase is the first phase and the easiest to treat.

Chronic myeloid leukemia (CML) is a type of leukemia that tends to grow slowly and usually has a positive outlook with standard treatment. It makes up about 30% of leukemias in adults.

Unlike many other cancers, doctors don’t divide CML into stages. Instead, they divide CML into three phases based on how many abnormal cells they see in your blood and bone marrow samples.

These phases are:

About 90–95% of people diagnosed with CML are diagnosed in the chronic phase. CML in the chronic phase usually causes mild or no symptoms.

Read on to learn more about the chronic phase of CML, including how long it lasts, whether it’s curable, and how it’s treated.

CML phases are primarily determined by looking at how many blasts are in samples of your blood and bone marrow. Blasts are immature and abnormal cells that crowd out your healthy blood cells.

You’ll be diagnosed with chronic phase CML if less than 10% of the cells in your blood and bone marrow samples are blasts.

People in the chronic phase of CML usually have no or mild symptoms and often respond to standard treatment.

Doctors sometimes shorten chronic phase CML to CP-CML.

The chronic phase of CML usually lasts 3–5 years without treatment. Progression to later phases is associated with a much poorer outlook and a higher chance of death. Many people who receive treatment for CML never advance past the chronic phase.

Since the development of a type of targeted therapy called tyrosine kinase inhibitors, the number of people who advance from the chronic phase to the accelerated phase or blast phase has dropped from 20% to about 1–1.5% per year.

In a 2020 study, researchers found that 9.2% of a group of 206 people with chronic phase CML treated with tyrosine kinase inhibitors advanced to the accelerated or blast phase over a 10-year period. The overall survival rate during this period was 87%.

CML can be potentially cured with high dose chemotherapy and a stem cell transplant.

Stem cell transplant for CML

Allogenic stem cell transplants are the only proven cure for CML. During this procedure:

  1. You receive a high dose of chemotherapy and sometimes radiation therapy to destroy cancer cells in your bone marrow. Chemotherapy also damages healthy cells in your bone marrow that create blood cells.
  2. Bone marrow stem cells collected from a donor are infused into your bloodstream. The donor needs to be somebody who’s a genetic match or your body may mistake the stem cells as foreign invaders and attack them. Often the donor is a close relative like a brother or sister.
  3. The bone marrow stem cells replace your damaged cells and start creating new blood cells.

It’s worth noting that tyrosine kinase inhibitor therapy is the standard initial treatment for CML, so the use of stem cell transplants has gone down considerably. Stem cell transplants are most often recommended to younger patients and people who haven’t responded to tyrosine kinase inhibitors.

Other treatments for CML

According to the National Cancer Institute, there are six standard treatments used to treat CML:

  1. tyrosine kinase inhibitors (targeted therapy)
  2. chemotherapy
  3. surgery (splenectomy)
  4. high dose chemotherapy with stem cell transplant
  5. immunotherapy
  6. donor lymphocyte infusion, where white blood cells from a stem cell transplant donor are infused into your blood

A type of targeted therapy called tyrosine kinase inhibitors is the main treatment for CML. They’re almost always the first-line treatment for CML, except for people who are pregnant. You may receive:

If one of these drugs isn’t effective, your doctor may recommend switching drugs or increasing your dose.

Pregnant people are often treated with observation alone until after the pregnancy, at which time they usually start or restart tyrosine kinase inhibitors.

Learn more about CML treatment here.

The next phase of CML is called the accelerated phase, sometimes shortened to AP-CML. It’s characterized by more than 10% but less than 20% blasts in your blood and bone marrow samples. During this phase, your number of cancerous cells increases faster, and you may develop symptoms such as:

The final phase is called the blast phase, blast crisis, or BP-CML. During this phase, more than 20% of the cells in your bone marrow and blood samples are blasts. Symptoms in this phase tend to be more severe than during the accelerated phase, and the outlook is generally poor.

About half of people in the blast phase live less than a year.

Learn more about CML treatment by phase.

Most people with CML are diagnosed in the chronic phase. The chronic phase is the easiest phase to treat.

Most people who are diagnosed in the chronic phase never advance to other phases of CML if they receive standard treatment.

Chronic phase CML can potentially be cured with a bone marrow transplant. Even if you’re not eligible for a bone marrow treatment, CML can often be well-managed with a type of medication called tyrosine kinase inhibitors.