Doctors classify acute lymphoblastic leukemia (ALL) into subtypes by using various tests. It's important to get an accurate diagnosis since your subtype plays a large part in deciding the type of treatment you'll receive. Depending on your ALL subtype, the doctor will determine
- The type of drug combination needed for your treatment
- The length of time you'll need to be in treatment
- Other types of treatment that may be needed to achieve the best outcomes.
Leukemia cells can be classified by the unique set of proteins found on their surface. These unique sets of proteins are known as “immunophenotypes.” Based on immunophenotyping of the leukemia cell, the World Health Organization (WHO) classifies ALL into two main subtypes.
- B-cell lymphoblastic leukemia/lymphoma: This subtype begins in immature cells that would normally develop into B-cell lymphocytes. This is the most common ALL subtype. Among adults, B-cell lineage represents 75 percent of cases.
- T-cell lymphoblastic leukemia: This subtype of ALL originates in immature cells that would normally develop into T-cell lymphocytes. This subtype is less common, and it occurs more often in adults than in children. Among adults, T-cell lineage represents about 25 percent of cases.
In addition to classifying ALL as either B-cell or T-cell, it is further classified based on certain changes to the chromosomes and genes found in the leukemia cells. This identification of specific genetic abnormalities is critical for disease evaluation, risk stratification and treatment planning.
Translocations are the most common type of genetic change associated with ALL. In a translocation, the DNA from one chromosome breaks off and becomes attached to a different chromosome. Sometimes pieces from two different chromosomes trade places. A translocation may result in a “fusion gene,” an abnormal gene that is formed when two different genes are fused together.
Another type of genetic change that occurs in ALL is the result of numerical abnormalities. A numerical abnormality is either a gain or loss in the number of chromosomes from the normal 46 chromosomes. A change in the number of chromosomes can affect the growth, development and functioning of body systems.
About 75 percent of adult ALL cases can be classified into subgroups based on chromosomal abnormalities and genetic mutations. Not all patients have the same genetic changes. Some changes are more common than others, and some have a greater effect on a patient’s prognosis.
- To see a list of all WHO classifications of ALL and a chart of the common chromosomal and molecular abnormalitites in ALL, order or download The Leukemia & Lymphoma Society's free booklet, Acute Lymphoblastic Leukemia (ALL) in Adults and see pages 13-15.