Subtypes

There is a variety of subtypes of AML. Although all subtypes cause decreases in normal blood counts, different types of AML are associated with specific symptoms and problems and can have widely different results after treatment.

Morphology

AML is initially defined according to its morphology (the appearance of the cancerous cells under the microscope). The classification of AML is named according to the type of normal, immature white blood cell it most closely resembles.

Most patients with AML are classified as having myeloblastic leukemia, which means the cancer is in the cells that normally produce neutrophils. Other patients have a type of AML called monocytic. In monocytic leukemia, the cells resemble other white blood cells called monocytes. Mixtures of myeloblastic and monocytic leukemia can occur.

Sometimes the AML seems to come from cells that produce red blood cells (erythroid) or platelets (megakaryocytic). Promyelocytic leukemia (APL) is a unique subtype of AML where the cancer cell stops maturing when the cell is at a stage called the promyelocyte or progranulocyte. Flow cytometry is a test that can detect the presence of particular proteins on the surface of abnormal cells and is sometimes useful in finding the difference among these subtypes.

The classification system from the World Health Organization (WHO) includes:

• AML with recurrent genetic abnormalities (that is, with specific chromosomal changes)
  
  
• AML with multilineage dysplasia (abnormalities in the appearance of other blood cells)
  
  
• AML with myelodysplastic syndrome, related to therapy
  
  
• AML that is not otherwise categorized

The French-American-British (FAB) classification is an older system for describing AML morphology, but it is still commonly used and is listed below for reference.

M0: Myeloblastic without differentiation

M1: Myeloblastic without maturation

M2: Myeloblastic with maturation

M3: Promyelocytic

M4: Myelomonocytic

M5a: Monocytic without differentiation (monoblastic)

M5b: Monocytic with differentiation

M6: Erythroleukemic

M7: Megakaryocytic

Cytogenetics

AML is also classified according to the cytogenetic (chromosome) changes found in the leukemia cells. Sometimes the doctor can find these changes visually by examining the chromosomes in dividing cells under the microscope, while other changes can be detected only with sophisticated molecular tests that can identify very small changes in the DNA.

Certain chromosomal changes are closely matched with particular morphologic subtypes of AML. More importantly, in many cases the chromosomal changes are highly predictive of the results of treatment and often influence the choice among different treatment approaches. These changes are commonly grouped according to the likelihood that the subtype will respond to treatment. (Note: all chromosomes are numbered from one to 22; sex chromosomes are called “X” or “Y.” The letters “p” and “q” refer to the “arms” or specific areas of the chromosome.)

Some of the most common chromosomal changes are grouped as follows:

Favorable. Chromosomal changes associated with a good result after treatment include abnormalities of chromosome 16 at bands p13 and q22, a translocation (exchange of genetic material) between chromosomes 8 and 21, and a translocation between chromosomes 15 and 17 (found in progranulocytic leukemia).

Intermediate. Findings suggestive of a less favorable prognosis include normal chromosomes (no abnormality detected) and a translocation between chromosomes 9 and 11.

Unfavorable. Examples of chromosomal changes predictive of unfavorable outcomes and low cure rates include extra copies of chromosomes 8 or 13, deletion of all or part of chromosomes 5 or 7, complex abnormalities involving many chromosomes, and abnormalities of chromosome 3 at band q26.

In general, the more favorable changes occur in younger patients, while the unfavorable changes are more common in patients older than 60. In each of these groups, there is still a wide range of how the cancer responds to treatment, with average expected rates of successful AML treatment ranging from 50% to 60% in the favorable group to less than 10% in the unfavorable group. Other factors, including the patient’s age and the level of the white blood cell count, also influence how well treatment works. It is not possible to predict exactly the likelihood of successful treatment in an individual person with AML.

Recurrent. Recurrent AML is cancer that comes back after treatment.