A blast crisis is the final phase of chronic myelogenous leukemia (CML) — cancer of the white blood cells with uncontrolled proliferation and abnormal accumulation of the cells in the bone marrow and blood. Blast crisis is diagnosed when greater than 20 percent of the white blood cells and lymphocytes in the blood or bone marrow are immature, poorly differentiated cells, or blasts. Other key indicators include the finding of large clumps of blasts in bone marrow obtained by biopsy, and the formation of a solid tumor outside of the bone marrow — called a myeloid sarcoma. Chronic myelogenous leukemia usually evolves into the rapidly progressive blast crisis within approximately three to five years of the diagnosis, although patients ages 20 to 29, because of the more aggressive nature of their leukemia, may present in blast crisis. Treatment approaches are generally ineffective in this phase, with only about 20 percent of patients surviving the crisis.
The initial event in the sequence culminating in blast crisis is the acquisition within the bone marrow stem cells of the Philadelphia chromosome, named for the city in which it was isolated. Easily recognizable under a microscope, the Philadelphia chromosome is a translocation of genes between chromosomes 22 and nine. This genetic marker is present in 95 percent of CML patients. The abnormal chromosome causes uncontrolled proliferation and enhanced survival of abnormal blast cells. Despite many advances in leukemia treatments, the changes wrought by the Philadelphia chromosome make the blast crisis highly resistant to therapy, with favorable responses occurring in only 20 percent of cases.
Research shows that certain factors increase the risk for a patient to develop leukemia. Radiation, cigarette smoking, exposure to benzene, and chemotherapy have all been implicated in cases of leukemia. Down syndrome and other inherited conditions may also increase the risk of leukemia. There is also a rare leukemia linked to human T-cell leukemia virus type I.
Symptoms of blast crisis may include fatigue, malaise, low-grade fever, bleeding, bruising, and abdominal enlargement. Patients may have swollen lymph nodes and pain in their bones or joints. They may be susceptible to frequent infections, and they may have weight loss for no apparent reason. These symptoms occur due to the crowding of normal bone marrow components by abnormal stem cells, thereby decreasing the production of functional red blood cells, white blood cells, and platelets. The spleen, acting as a filter, enlarges as abnormal cells become trapped within its tissues.
Myeloid sarcomas, typically found in blast crisis, may develop in any tissue or organ, but the most commonly involved areas are the gums and the skin. Gum involvement produces swollen, tender areas that bleed easily with brushing and flossing. Skin sarcomas present as purplish-red, elevated nodules, which are infiltrated with white cell blasts. Other potential sites for myeloid sarcoma include the chest cavity, lymph nodes, lining of the brain, small intestine, ovaries, and uterus. Unlike the bone marrow sites, myeloid sarcomas usually respond positively to standard anti-leukemia chemotherapy.