The chronic phase of CML usually gives way to the accelerated phase within 3-5 years, although it can occur at any point in time. Certain changes can occur, leading to the consideration of the accelerated form of the disease. These changes include:
- Very high (>1000 x 109/L) or low platelet counts (<100 x 109/L), that do not respond to treatment
- Steadily increasing WBC counts
- Development of additional genetic abnormalities
- Dysplastic development of megakaryocytes or granulocytes
- Greater than or equal to 20% peripheral blood basophils
Patients tend to experience an increase in the severity of the symptoms, and some do not survive this phase of CML.
The blast phase is the final phase of CML, which appears very similar to acute leukemia. Patients are identified in the blast phase if they were already diagnosed with CML and now have more than 20% blasts in their peripheral blood or bone marrow. Fibrosis in the bone marrow is also characteristic of the blast phase. Blasts can develop from any cell line; however, approximately 65% are myeloid in origin, while approximately 35% are lymphoid, with rare cases of erythroid or megakaryocytic. Various forms of laboratory analysis can be used to establish the blast lineage, including immunophenotyping and cytochemical staining, among others. While resembling acute leukemia, the CML blast phase is typically refractive to standard acute leukemia protocols.
