Chronic myeloid leukemia is the abnormal growth of relatively mature myeloid (white blood) cells. The disease is associated with a chromosomal abnormality, where genetic material from chromosome 9 is transferred to chromosome 22. The chromosome containing the genetic switch is called the Philadelphia chromosome and plays a role in the development of the disease. The median age at diagnosis is 67 years; however, clinical studies comparing different treatments for newly diagnosed patients have almost invariably involved patients younger than 65 years.

Initially in chronic myeloid leukemia, there is a gradual increase in mature, abnormal myeloid cells in the bone marrow. These cells eventually spill into the blood and other organs, causing symptoms such as fatigue from anemia or an enlarged spleen. The increase in leukemic cell numbers occurs slowly at first and is referred to as the chronic phase, but these cells invariably begin to increase more rapidly and/or include less mature cells, resulting in the accelerated or blastic phase. In order to understand the best treatment options available for chronic myeloid leukemia, it is important to know the phase of leukemia, since all new treatment information concerning chronic myeloid leukemia is categorized and discussed by the phase of disease.

Staging of Chronic Myeloid Leukemia

Chronic Phase : Patients in chronic phase have stable disease with only minor symptoms, no cancer outside the bone marrow or spleen and white blood cell and platelet blood counts easily controlled by low-dose chemotherapy or interferon.

Accelerated Phase : When chronic myeloid leukemia is difficult to control with low-dose chemotherapy or interferon, the white blood count begins to increase. New symptoms may appear and old symptoms may worsen. The spleen may enlarge and/or new abnormal chromosomes can be detected in the bone marrow cells. Eventually, the leukemia becomes completely resistant to treatment and the bone marrow becomes overburdened with large numbers of immature white blood cells known as "blasts". A diagnosis of accelerated phase requires at least one of the following:

• The persistent presence of 10-30% myeloblasts in the bone marrow or peripheral blood.
• A major increase of the white blood cell count to over 50,000, platelet counts that are increased or decreased and red blood cell levels that are low despite treatment.
• Progressive enlargement of the spleen.
• Growth of leukemia outside the bone marrow or spleen.
• The presence of any cytogenetic abnormality in addition to a Philadelphia chromosome.
• Persistent unexplained fever or bone pain.

Blastic Phase : Greater than 30% myeloblasts in marrow or blood