Phases and Risk Scores for Chronic Myeloid Leukemia (CML)

Chronic myeloid leukemia (CML) isn’t staged like most cancers. Instead, your cancer care team will assign a phase to your CML. They might also assign a risk score. This information is helpful for making treatment decisions and predicting your prognosis (outlook).

What are phases and risk scores for CML?

Most types of cancer are assigned a stage based on the size of the tumor and the extent of cancer spread. Stages can be helpful in making treatment decisions and predicting prognosis (outlook) for solid tumors. But CML is in the bone marrow and blood, so it isn't staged in this way.

Instead, CML is assigned a phase and often a risk score.

Phase: The outlook for someone with CML depends largely on the phase of the disease. The phase is based on the number of immature white blood cells (blasts) in the blood or bone marrow, as well as other factors (see below).

Risk score: Some other factors can also be important, including your age, your blood cell counts, and whether your spleen is enlarged. This information is used to assign a risk score. Like the phase of your CML, your risk score can also affect your outlook and help guide your treatment.

Phases of chronic myeloid leukemia

CML is divided into 3 phases, based on factors such as:

• The percentage of immature white blood cells (blasts) in the blood or bone marrow
• The percentage of other blood cells in the bone marrow
• If the CML cells have (or develop) certain gene or chromosome changes

Different groups of experts have suggested slightly different cutoffs to define the phases of CML, but a common system is described below. Not all doctors may agree with or follow these cutoff points.

If you have questions about the phase of your CML, ask your cancer care team to explain it in a way you understand.

Chronic phase CML

Most people with CML are diagnosed in the chronic phase. In this phase, people usually have mild symptoms (if any), and the CML usually responds well to standard treatments.

People whose CML is in the chronic phase typically have:

• Less than 10% myeloblasts (blasts) in their blood or bone marrow
• Less than 20% basophils in their blood
• None of the chromosome abnormalities in their CML cells that might be seen in more advanced phases (see below)

Accelerated phase CML

A person is considered to have accelerated phase CML if any of the following are true:

• The blood has at least 10% but no more than 20% blasts.
• Basophils make up 20% or more of the blood.
• Blasts and promyelocytes (another early form of myeloid cell) combined make up 30% or more of the blood.
• The platelet count is low (100 x 1,000/mm³ or less), and this isn’t caused by treatment.
• There are new chromosome changes in the CML cells, such as an additional copy of the Philadelphia chromosome, an extra copy of chromosome 8 (trisomy 8) or 19 (trisomy 19), loss of part of chromosome 17, abnormalities of chromosome 3, or a complex karyotype (many chromosome changes).

People with accelerated phase CML may have symptoms such as fever, poor appetite, and weight loss. Accelerated phase CML typically doesn't respond as well to treatment as chronic phase CML.

Again, not all experts agree on these definitions. For example, the most recent World Health Organization (WHO) classification no longer defines an accelerated phase CML because the outlook for this phase has improved so much with modern treatments.

Blast phase CML (also called acute phase or blast crisis)

The blast phase of CML is usually defined by at least one of the following:

• At least 20% of the cells in the blood or bone marrow are myeloblasts (blasts).
• Blast cells have spread to tissues and organs beyond the bone marrow and blood (known as extramedullary infiltrates).
• Leukemia cells have formed a tumor (myeloid sarcoma).

People with blast phase CML often have fever, poor appetite, and weight loss. In this phase, the CML tends to grow quickly, acting a lot like an acute leukemia.

Risk scores for chronic phase CML

Along with the phase of CML, other factors can help predict a person’s outlook. These factors are also sometimes helpful when choosing treatment. Some examples include:

• A person’s age
• The size of their spleen
• Their blood platelet count
• The percentage of blast cells in their blood
• The percentage of certain other types of cells in their blood

Doctors often combine these factors to assign people with chronic phase CML a risk score (low, intermediate, or high). This risk score can be used to help guide treatment.

There are different scoring systems. Each system looks at slightly different factors. Examples include:

• Sokal score
• Hasford (Euro) score
• EUTOS long-term survival (ELTS) score