Physicians classify AML using a system devised by the World Health Organization (WHO). The WHO system bases classification on the type of cell from which the leukemia developed and how the leukemia cells look under the microscope, and also takes into account other prognostic features of the disease such as chromosomal abnormalities in the diseased cells. Each cell in the body contains chromosomes, tightly coiled strands of DNA that contain all the information cells need to function normally and reproduce.
In many forms of leukemia, changes can be detected in the chromosomes. These changes can include translocations (when pieces of DNA are exchanged between two chromosomes); inversions (when a piece of chromosome breaks off, turns upside down, and reattaches to the original chromosome); deletions (when a piece of a chromosome is missing); and additions (when extra pieces of the chromosome are present).
The WHO classification system divides AML into several broad groups:
- AML with certain genetic abnormalities, such as a translocation between chromosomes 8 and 21, a translocation or inversion in chromosome 16, changes in chromosome 11, or acute promyelocytic leukemia (M3), which usually has a translocation between chromosomes 15 and 17
- AML with multilineage dysplasia; this form of the disease is thought to evolve from a preleukemic condition known as myelodysplastic syndrome
- AML related to previous chemotherapy or radiation
- AML not otherwise specified. These subtypes include: undifferentiated AML (M0), AML with minimal maturation (M1), AML with maturation (M2), acute myelomonocytic leukemia (M4), acute monocytic leukemia (M5), acute erythroid leukemia (M6), and acute megakaryoblastic leukemia (M7). Subtypes M2 and M4 each account for 25 percent of AML cases; M1 accounts for 15 percent; M3 and M5 each account for 10 percent of cases; the other subtypes are rarely seen
Physicians also now test AML patients for a mutation in a gene called FLT3. In normal cells a cell-surface receptor called FLT3 helps signal the proper time for growth. In 20 to 40 percent of people with AML the FLT3 receptor is in the “on” position all the time, signaling uncontrolled proliferation. People with this mutation tend to have a more aggressive form of the disease that is more difficult to treat successfully.
Doctors also categorize AML patients in the following groups according to whether they have been treated before for the disease:
- untreated — these patients are newly diagnosed and have had no prior treatment for leukemia
- in remission — patients are considered to be in remission if they have received the initial remission induction treatment, the amount of blasts in their bone marrow is less than 5 percent, their blood counts have returned to normal, and they have no signs or symptoms of disease
- recurrent or refractory disease — these are patients whose disease has recurred after remission or whose disease did not respond to treatment (refractory disease)