Acute Myelogenous Leukemia (AML)

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Classifying AML

There are two classification systems for acute myelogenous leukemia (AML). The French-American-British (FAB) system is based on the type of cell from which the leukemia developed and how the cell looks under a microscope. More recently, the World Health Organization (WHO) has proposed a new system that’s based on the cytogenetic structure and arrangement of chromosomes in leukemic cells.

FAB Classification

The FAB classification system for AML was designed in the 1970s by a group of French, American and British doctors. The eight subtypes (M0 through M7) are determined by how the leukemia cells look when stained and examined under a microscope.

  • M0–Undifferentiated AML (about five percent of AML patients)
  • M1–AML with minimal maturation (about 15 percent of AML patients)
  • M2–AML with maturation (about 25 percent of AML patients)
  • M3–Acute promyelocytic leukemia (about 10 percent of AML patients)
  • M4–Acute myelomonocytic leukemia (about 20 percent of AML patients)
  • M4 eos–Acute myelomonocytic leukemia with eosinophilia (about five percent of AML patients)
  • M5–Monocytic leukemia (about 10 percent of AML patients)
  • M6–Acute erythroid leukemia (about five percent of AML patients)
  • M7–Acute megakaryocytic leukemia (about five percent of AML patients)

World Health Organization Classification

WHO classifies AML into four principal types.

1) AML with characteristic genetic abnormalities

  • AML with inversions in chromosome 16
  • AML with translocations between chromosomes 15 and 17
  • AML with translocations between chromosomes 8 and 21

These types of AML are more common in younger patients. Patients with the 15:17 translocation are said to have acute promyelocytic leukemia (APL). Treatment for APL differs from that for other types of AML.

2) AML arising from a previous hematologic disorder

About one-third of AML patients—particularly those over the age of 65—have abnormalities in blood counts for months before the diagnosis of AML. These patients often do poorly with standard therapy and consequently may be candidates for clinical studies of new therapies.

3) Therapy-related AML

About a quarter of AML patients have received chemotherapy for another malignancy, such as breast cancer or lymphoma, usually 5 to 10 years before development of AML but occasionally less than 2 years before.

4) Patients with AML not falling into one of the above categories

These patients are classified according to how their leukemic cells appear under a microscope. However, the prognostic and biologic significance of such microscopic appearance is uncertain, and classification based the appearance of leukemic cells will almost certainly be replaced by classification based on their molecular features.