Most types of cancers are assigned numbered stages, based on the size of the tumor and how far from the original site in the body the cancer has spread.

There is no need to stage leukemia, as one would other cancers, because leukemia already involves all the bone marrow in the body, and, in many cases, has also spread to other organs such as the liver, spleen, lymph nodes, testes, and central nervous system. Laboratory tests focus on accurately determining the type and subtype of leukemia. This in turn determines the prognosis of the specific disease, and helps predict which treatments will work the best.

As leukemia treatment has improved over the past 20 years, research has focused on why some patients have a better chance of cure than others. Certain consistently observed differences among patients with good and poor responses to treatment are called prognostic features and help doctors decide if a certain type of leukemia should receive more or less treatment.

The French-American-British (FAB) Classification of Acute Leukemias

Several years ago, an international conference of prominent hematologists/oncologists specializing in leukemia treatment and pathologists specializing in laboratory tests for blood disease diagnosis was held to decide upon the best system of classification of acute leukemias. This group of French, American, and British doctors decided that acute leukemias should be divided into eight subtypes of AML and three subtypes of ALL.

Some subtypes of AML or ALL defined in the FAB classification are associated with certain symptoms. For example, bleeding or blood clotting problems are often a problem for patients with the M3 subtype of AML, also known as acute promyelocytic leukemia. Identifying M3 leukemia is very important for two reasons. The first is that these serious complications can often be prevented by appropriate treatment. The second reason is that M3 leukemias usually respond to retinoids (drugs chemically related to vitamin A). Addition of retinoids to the treatment program allows doctors to lower the doses of chemotherapy drugs and reduce the severity of certain side effects.

Some types of acute leukemia, such as the L3 subtype of ALL and M5 subtype of AML tend to have a worse prognosis and many doctors recommend more intensive chemotherapy for these patients.

The original FAB system was based only on appearance of leukemic cells under the microscope after routine processing or cytochemical staining. More recently, doctors have found that cytogenetic studies, flow cytometry, and molecular genetic studies provide additional information that is sometimes useful in classification of acute leukemias and predicting the patient's prognosis. In the coming years we will learn more about the underlying genetic defects that cause leukemia. These defects, rather than the appearance of the cells under the microscope, will be used to classify leukemias and understand their prognoses. These genetic defects might also form the basis for treating the leukemias.

The next few pages will discuss the details of acute leukemia classification. Some patients will find this interesting and helpful in understanding their leukemia. Others may be less interested in the details of these tests and may wish to skip ahead to the section on Treatment of Adult Acute Leukemias.

French-American-British (FAB) Classification of AML

FAB Subtype	Name	Approximate % of adult AML patients	Prognosis compared to average for AML
M0	Undifferentiated AML	5%	Worse
M1	Myeloblastic leukemia with minimal maturation	15%	—
M2	Myeloblastic leukemia with maturation	25%	Better
M3	Promyelocytic leukemia	10%	Best
M4	Myelomonocytic leukemia	25%	—
M4 eos	Myelomonocytic leukemia with eosinophilia	Rare	Better
M5	Monocytic leukemia	10%	Worse
M6	Erythroid leukemia	5%	Worse
M7	Megakaryoblastic leukemia	5%	Worse

French-American-British (FAB) Classification of ALL

FAB Subtype	Approximate % of adult ALL patients	Immunologic Type	Comments
L1	30%	T cell or pre-B cell
L2	65%	T cell or pre-B cell
L3	5%	B cell	Poor prognosis with standard therapy. Also called Burkitt's type leukemia.

Undifferentiated or Biphenotypic Acute Leukemias

More refined tests have shown that a number of acute leukemia cases have both lymphocytic and myeloid features. Sometimes leukemic cells have both myeloid and lymphocytic characteristics on the same cell. In other cases, a patient's leukemia may include some cells with myeloid features and other cells with lymphocytic features. Categorizing these acute leukemias is difficult and controversial. Sometimes these types of acute leukemias are called ALL with myeloid markers, AML with lymphoid markers, or biphenotypic (2 type) leukemias.

Status of Acute Leukemia After Treatment

Adult acute leukemia is either classified as being in remission (with no evidence of disease), or with active disease (with the patient either just newly diagnosed or in relapse). Minimal residual disease is a term which is used when there is chemical evidence (either molecular or cytogenetic ) that leukemic cells remain in the bone marrow, but there are not enough of these cells around to be found by routine examination under the microscope. For a patient to be in fulminant relapse they must have greater than 30% blast cells present in the bone marrow.