Phases of Chronic Myeloid Leukemia

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).

But because chronic myeloid leukemia (CML) is a disease of the bone marrow, it isn't staged like most cancers. The outlook for someone with CML depends on the phase of the disease and the amount of blasts in the bone marrow, as well as other factors like the age of the patient, blood counts, and if the spleen is enlarged.

Phases of chronic myeloid leukemia

CML is classified into 3 groups that help predict outlook. Doctors call these groups phases instead of stages. The phases are based mainly on the number of immature white blood cells (blasts) in the blood or bone marrow. Different groups of experts have suggested slightly different cutoffs to define the phases, but a common system (proposed by the World Health Organization) is described below. Not all doctors may agree with or follow these cutoff points for the different phases. If you have questions about what phase your CML is in, be sure to have your doctor explain it to you in a way that you understand.

Chronic phase

Patients in the chronic phase typically have less than 10% blasts in their blood or bone marrow samples. These patients usually have fairly mild symptoms (if any) and usually respond to standard treatments. Most patients are diagnosed in the chronic phase.

Accelerated phase

Patients are considered to be in accelerated phase if any of the following are true:

• The blood samples have 15% or more, but fewer than 30% blasts
• Basophils make up 20% or more of the blood
• Blasts and promyelocytes combined make up 30% or more of the blood
• Very low platelet counts (100 x 1,000/mm³ or less) that are not caused by treatment
• New chromosome changes in the leukemia cells with the Philadelphia chromosome

Patients whose CML is in an accelerated phase may have symptoms such as fever, poor appetite, and weight loss. CML in the accelerated phase doesn't respond as well to treatment as CML in the chronic phase.

Blast phase (also called acute phase or blast crisis)

Bone marrow and/or blood samples from a patient in this phase have 20% or more blasts. Large clusters of blasts are seen in the bone marrow. The blast cells have spread to tissues and organs beyond the bone marrow. These patients often have fever, poor appetite, and weight loss. In this phase, the CML acts a lot like an acute leukemia.

Prognostic factors for chronic myeloid leukemia

Along with the phase of CML, there are other factors that may help predict the outlook for survival. These factors are sometimes helpful when choosing treatment. Factors that tend to be linked with shorter survival time are called adverse prognostic factors.

Adverse prognostic factors:

• Accelerated phase or blast phase
• Enlarged spleen
• Areas of bone damage from growth of leukemia
• Increased number of basophils and eosinophils (certain types of granulocytes) in blood samples
• Very high or very low platelet counts
• Age 60 years or older
• Multiple chromosome changes in the CML cells

Many of these factors are taken into account in the Sokal system, which develops a score used to help predict prognosis. This system considers the person's age, the percentage of blasts in the blood, the size of the spleen, and the number of platelets. These factors are used to divide patients into low-, intermediate-, or high-risk groups. Another system, called the Euro score, includes the above factors, as well as the percentage of blood basophils and eosinophils. Having more of these cells indicates a poorer outlook.

The Sokal and Euro models were helpful in the past, before the newer, more effective drugs for CML were developed. It's not clear how helpful they are at this time in predicting a person's outlook. Targeted therapy drugs like imatinib (Gleevec^®) have changed the treatment of CML dramatically. These models haven't been tested in people who are being treated with these drugs.