Most types of cancer are classified by a process known as staging. Staging assigns numbered stages to cancers based on tumor size and how far the cancer spreads from the original site. Leukemia, however, is not staged because it involves all the bone marrow in the body and in many cases spreads to other organs. Instead, tests focus on establishing the type and subtype of leukemia. This information is then used to determine the prognosis (outlook) and predict which treatments will be most effective. Subtypes of ALL respond to treatment differently and their prognoses vary.

Acute lymphocytic leukemia (ALL) can be classified into subtypes based on cytogenic studies, flow cytometry and molecular genetic studies. These tests look for abnormalities in the genes and chromosomes of the cells. The cells are then further classified as either B-cells or T-cells.  Subtypes of ALL include:

• Early pre-B ALL
• Common ALL
• Pre-B-cell ALL
• Mature B-cell ALL (Burkitt leukemia)
• Pre-T-cell ALL
• Mature T-cell ALL

The classification of these subtypes is based on an older classification system known as the French-American-British (FAB) Classification of ALL. Based on the appearance of the cancer cells under a microscope (morphology), this system consists of three subtypes including:

• L1. This subtype accounts for approximately 30 percent of ALL cases in adults, and involves T-cells or pre-B cells. In this type, the lymphoblasts are small cells.
  
  
• L2. This subtype accounts for approximately 65 percent of ALL cases in adults, and involves T-cells or pre-B cells. In this type the lymphoblasts are larger.
  
  
• L3. This subtype accounts for approximately 5 percent of ALL cases in adults, and involves B-cells. Also known as Burkitt type leukemia, this subtype has poor prognosis with standard therapy. The disease is very similar to Burkitt lymphoma, and as a result is treated differently than most leukemias.

Information provided by cytogenic studies, flow cytometry and molecular genetic studies also provides information for predicting prognosis. Among the most important information provided by these studies is information about translocations (the transfer of DNA from one chromosome to another). Translocations are the greatest factor in determining a patient’s prognosis. Patients with the Philadelphia chromosome (a translocation between chromosomes 9 and 22) have a much worse outlook than those without it. It occurs in between 20 and 25 percent of ALL patients. Occurring in 5 percent of ALL patients, a translocation between chromosomes 4 and 11 is another translocation that carries a poor prognosis.  

In addition to translocation information, certain prognostic factors have been identified to help a physician determine if a patient requires more or less treatment. The patient’s age, white blood cell count, cytogenic test results, and initial response to chemotherapy are all considered when determining whether a patient will respond well to treatment.

Adult ALL prognostic factors include:

• Patients with T-cell ALL have the best prognosis, patients with mature B-cell ALL have the worst and those with pre-B-cell ALL have an intermediate prognosis.
  
  
• Younger patients have a better prognosis.
  
  
• Patients with an initial white blood cell count less than 50,000 have a better prognosis.
  
  
• FAB L3 morphology predicts a worse prognosis.
  
  
• Patients needing more than four to five weeks of therapy to reach complete remission have a worse prognosis.

Other factors that influence prognosis and treatment choices include:

• Whether the ALL has spread to the brain or spinal cord
• Whether the ALL has been treated before or has recurred

ALL is classified as a lymphocytic leukemia because it develops from bone marrow lymphocytes. Leukemias that develop from myeloid cells are known as myelogenous or myeloid leukemias. In some cases, however, the cancer cells have both lymphocytic and myeloid cell features on the same cell. In other patients, the leukemia may include some cells with myeloid characteristics and other cells with lymphocytic characteristics. These types of leukemias may be described as:

• ALL with myeloid markers (My+ALL)
• Acute myeloid leukemia (AML) with lymphoid markers
• Biphenotypic (2-type) leukemias