Unlike other types of cancer, doctors use phases rather than stages to describe the progression of chronic myeloid leukemia (CML). The phases of CML include the chronic phase, accelerated phase, and blast phase.

The chronic phase may cause few or no symptoms, while the accelerated and blast phases cause more significant symptoms. If doctors can diagnose and treat CML in the chronic phase, this increases the chance of a better outcome.

This article looks at the phases of CML, including their characteristics, symptoms, and factors that affect prognosis.

A vial of blood lead across a tray of other samples that a doctor is using to test for chronic myeloid leukemia.Share on Pinterest
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CML is a type of cancer that affects blood-forming cells, which are present in bone marrow. Blood-forming cells are cells that can grow and change into red or white blood cells, or into platelets. Each of these cells has a different function:

  • red blood cells carry oxygen around the body
  • white blood cells protect against pathogens, such as viruses or bacteria
  • platelets allow the blood to clot when a person is bleeding

In a person with CML, a type of white blood cell grows uncontrollably. These abnormal, immature white blood cells are known as “blasts.” As more blasts form in the bone marrow, it creates an overabundance of them and crowds out other types of blood cells.

CML progresses more slowly than other types of leukemia. This is what makes it a chronic leukemia, rather than an acute leukemia. However, sometimes CML can become more aggressive. It can also spread to other parts of the body via the bloodstream.

The three phases of CML are the chronic, accelerated, and blast phases. The sections below outline the general system doctors use to determine which phase a person is in, though not all doctors agree on the exact cut-off points.

Chronic phase

The chronic phase of CML is the earliest phase, and often causes few or no symptoms. It is the most common stage at which people receive their diagnosis. Most individuals in this stage respond well to standard treatments that reduce the number of leukemia cells.

Typically, doctors categorize a person as being in the chronic phase if their bone marrow or blood samples have a blast content lower than 10%.

Accelerated phase

Doctors usually say a person has moved from the chronic phase to the accelerated phase of CML if:

  • between 15–30% of their blood consists of blasts
  • at least 20% of their blood consists of basophils, which are a type of white blood cell
  • at least 30% of their blood consists of blasts and promyelocytes, which are a potential sign of blood cancer
  • their blood has a very low amount of platelets, and this is not due to treatment
  • their blood contains leukemia cells with new chromosomal changes

The symptoms a person may experience at this stage include:

  • fever
  • low appetite
  • weight loss
  • anemia, which may cause fatigue, headaches, or weakness

Compared to the chronic phase, CML in the accelerated phase does not respond as favorably to treatment. The goal of treatment is either to eliminate cancerous cells, or to return the CML to the chronic phase.

Blast phase

In the blast phase, the blood has a blast percentage of 20% or more in the bone marrow or blood. Doctors can typically see large groups of blasts in the bone marrow, and blast cells are visible on other organs and tissues.

In this phase, CML behaves in a similar way to acute or fast-growing leukemias, which is why some refer to it as the acute phase or blast crisis. The symptoms are often the same as in the accelerated phase.

People in the blast phase usually need treatment at a specialty center. There, a person may be able to enroll in a clinical trial, or get a combination of drug therapy and a stem cell transplant.

An individual can live with chronic leukemia for a long time before any symptoms arise, and due to advances in treatment, the life expectancy of those with CML is almost equivalent to that of the general population.

However, left untreated, CML is fatal, with a median survival rate of around 2–3 years. For this reason, early diagnosis and treatment is critical to a good prognosis.

The treatment for the chronic phase of CML is most often a tyrosine kinase inhibitor (TKI). Up to 70% of people have a complete cytogenetic response 1 year after starting a TKI. This means there are no remaining bone marrow cells that contain the chromosome responsible for causing CML.

It is hard for doctors to tell if someone taking a TKI is in complete remission from cancer. As a result, many people continue taking the medication indefinitely. Around 50% can successfully discontinue treatment without their CML returning.

There are many types of TKI, so if the first drug does not work, there are other options. Examples include:

If someone does not respond well initially to TKIs, doctors may consider:

Prognostic factors help predict the outcome of a person’s treatment. Doctors also find them useful in deciding on the optimal treatment plan. Adverse prognostic factors are associated with a shorter survival time.

Some of the adverse prognostic factors for CML include:

  • being in the accelerated or blast phase
  • bone damage caused by leukemia
  • having an enlarged spleen
  • blood samples that contain an increased number of eosinophils and basophils
  • very low or very high platelet counts
  • being 60 years of age or older
  • CML cells with multiple chromosome changes

Doctors can use scoring systems, such as the Sokal system or Euro score, to determine the best treatment for someone based on these factors.

The Sokal system predicts prognosis by developing a score that considers the individual’s age, number of platelets, blast percentage in the blood, and spleen size. Individuals are then divided into high-, intermediate-, or low-risk groups.

The Euro score also considers the percentage of blood eosinophils and basophils. A more significant portion of these cells correlates with a poorer outlook.

While these models of determining outlook were helpful in the past, newer forms of targeted therapy drugs have dramatically changed chronic myeloid treatment. Researchers have yet to adapt these models to the newer forms of treatment.

Chronic myeloid leukemia is a form of cancer that affects the myeloid cells. There are three phases of CML: chronic, accelerated, and blast. Classifying someone into these phases depends on the number of blast cells in the blood or bone marrow.

The phase helps determine the preferred treatment and overall outlook. However, relatively new therapies have made the outlook for this type of cancer very favorable, no matter the stage.

While the outlook for CML is favorable, many people must remain on medication for the rest of their lives.