How is chronic myeloid leukemia classified?
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 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 other information, such as the phase of the disease, as well as 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 — myeloblasts (blasts) — that are seen 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.
Patients in this 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.
Patients are considered to be in accelerated phase if any of the following are true:
- The bone marrow or blood samples have more than 10% but fewer than 20% blasts
- High blood basophil count (basophils making up at least 20% of the white blood cells)
- High white blood cell counts that do not go down with treatment
- Very high or very low platelet counts that are not caused by treatment
- New chromosome changes in the leukemia cells
Patients whose CML is in accelerated phase may have symptoms such as fever, poor appetite, and weight loss. CML in the accelerated phase does not 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 more than 20% blasts. The blast cells often 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 much like an aggressive acute leukemia.
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.
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 number 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. Newer drugs like imatinib (Gleevec®), dasatinib (Sprycel®), and nilotinib (Tasigna®) have changed the treatment of CML dramatically in recent years. These models haven't been tested in people who are being treated with these drugs.
Last Medical Review: 09/23/2013
Last Revised: 02/10/2014