Most physicians classify ALL by its subtype using a system called the French American British (FAB) system, which takes into account a number of features of the cancer cells. Physicians use this information to help select the most effective treatment and to predict the course of the disease.
There are three primary subtypes of ALL, which are defined by the size and shape of the leukemic cells. L1 occurs in 30 percent of adults with the disease and is the most common subtype in children. L2 occurs in 65 percent of adult cases, and L3, also called Burkitt’s type leukemia, is the least common subtype, and is found in only 5 percent of adult cases.
More importantly, ALL cells are also classified into one of three immunophenotypes depending on whether they are derived from precursor B cells (the most common subtype in both children and adults), T cells, or “mature” B cells (also known as L3, or Burkitt’s type).
Another important method used to classify ALL is a test that assesses the chromosomal abnormalities in the diseased cells. Each cell in the body contains tightly coiled strands of DNA known as chromosomes, which contain 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)
- additions (when extra pieces of the chromosome are present)
Between 20 and 30 percent of adult patients with ALL have a chromosomal translocation called the Philadelphia chromosome in which chromosomes 9 and 22 have exchanged genetic material. The scrambled genetic material on chromosome 22 is called the BCR/ABL fusion gene. This gene directs the body to produce a protein that triggers the growth of leukemia cells. The Philadelphia chromosome is more common among older patients and historically has indicated a poorer prognosis and the need for more aggressive treatment. More recently, the introduction of new medicines that block the action of the BCR/ABL protein has led to improved treatments for patients with this abnormality.
ALL patients are also categorized into one of the following groups:
- untreated — patients who are newly diagnosed and have had no prior treatment for leukemia
- undergoing treatment
- in remission — patients who have received the initial remission induction treatment, have less than 5 percent blasts in their bone marrow, and have no signs or symptoms of disease
- having recurrent or refractory disease — patients whose disease has recurred after remission or whose disease did not respond to treatment (refractory disease)