How is Chronic Myeloid Leukemia Staged and Classified?

Chronic myeloid leukemia (CML) is not staged in the traditional sense like most cancers. Instead, it is classified into three phases based on the number of immature white blood cells (blasts) in the blood or bone marrow, and the severity of symptoms. These phases are the chronic phase, the accelerated phase, and the blast phase (also known as blast crisis).

Chronic Phase

In the chronic phase, less than 10% of the cells in the blood or bone marrow are blasts. Patients in this phase may have few or no symptoms. The chronic phase can last for several years, and the disease can be controlled well to a treatment called tyrosine kinase inhibitors. Since tyrosine kinase inhibitors have become the first-line therapy for CML, the 5-year survival rate has risen from 33% to over 90%. Individuals diagnosed with chronic phase CML are expected to reach normal or near-normal life expectancy.

Accelerated Phase

The accelerated phase is characterized by the presence of 10% to 19% blasts in the blood or bone marrow. Symptoms may become more severe, and the disease becomes harder to control. This phase indicates that the disease is progressing.

The World Health Organization (WHO) defines accelerated phase CML as having one or more of the following criteria:

Persistent or increasing high white blood cell count (greater than 10 x 10^9/L) unresponsive to therapy
Persistent or increasing spleen enlargement unresponsive to therapy
Persistently high platelet count (greater than 1000 x 10^9/L) unresponsive to therapy
Persistent low platelet count (less than 100 x 10^9/L)
Greater than or equal to 20% basophils in peripheral blood
10-19% blasts in peripheral blood or bone marrow
Additional chromosomal abnormalities in Philadelphia chromosome-positive cells at diagnosis
Any new clonal chromosomal abnormality in Philadelphia chromosome-positive cells during therapy

The WHO also considers resistance to tyrosine kinase inhibitors as provisional criteria for determining accelerated phase CML.

Blast Phase

The blast phase, or blast crisis, is the most severe phase of CML. It is characterized by the presence of 20% or more blasts in the blood or bone marrow. Symptoms are often severe, and the disease is difficult to control. Patients in blast phase often have a fever, an enlarged spleen, weight loss, and generally feel unwell. This phase is similar to acute leukemia.

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Chronic Phase

Accelerated Phase

The World Health Organization (WHO) defines accelerated phase CML as having one or more of the following criteria:

Persistent or increasing high white blood cell count (greater than 10 x 10^9/L) unresponsive to therapy
Persistent or increasing spleen enlargement unresponsive to therapy
Persistently high platelet count (greater than 1000 x 10^9/L) unresponsive to therapy
Persistent low platelet count (less than 100 x 10^9/L)
Greater than or equal to 20% basophils in peripheral blood
10-19% blasts in peripheral blood or bone marrow
Additional chromosomal abnormalities in Philadelphia chromosome-positive cells at diagnosis
Any new clonal chromosomal abnormality in Philadelphia chromosome-positive cells during therapy

The WHO also considers resistance to tyrosine kinase inhibitors as provisional criteria for determining accelerated phase CML.