The blastic phase of CML resembles acute leukemia.
Two criteria are used to define the blastic phase of CML:
Blast percentages of 20% or more in the peripheral blood or bone marrow
Extramedullary blast proliferation in the lymph nodes, skin, spleen, bone, central nervous system, lungs, or any other tissue
In approximately 70% of patients in the CML blastic phase, the blasts are myeloid. The blasts may be granulocytic, monocytic, erythroid, or megakaryocytic in nature.
In 20-30% of the CML-blastic phase, the blasts are lymphoid in nature (commonly precursor B cells).
A small percentage of cases show a mixed phenotype of blasts (myeloid & lymphoid blasts).
It is vitally important to use immunophenotyping to determine the blast's lineage for the purpose of treatment.
