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 children, B-cell lineage ALL constitutes approximately 88 percent of cases. 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. Among children T-cell lineage represents approximately 12 percent of cases.
In addition to classifying ALL as either B-cell or T-cell lymphoblastic leukemia, the WHO further classifies ALL based on changes to certain chromosomes and genes.This identification of specific cytogenetic 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 (deoxyribonucleic acid) from one chromosome breaks off and becomes attached to a different chromosome. This results in a “fusion gene,” an abnormal gene that is formed when two different genes are fused together. For example, a translocation between chromosomes 9 and 22 is associated with a diagnosis of Philadelphia chromosome-positive (Ph+) ALL, a subtype of ALL that is treated differently than other subtypes.
Another type of genetic change in ALL is the result of numerical abnormalities. A numerical abnormality is either the gain or loss in the number of chromosomes from the normal 46. A change in the number of chromosomes can affect growth, development and the functioning of body systems. Approximately 25 percent of children with ALL have hyperdiploidy.
About 75 percent of adult and childhood cases of ALL can be classified into subgroups based on the chromosome number or DNA analysis, specific chromosomal rearrangements and molecular genetic changes. Not all patients who have ALL exhibit the same chromosome changes. Some changes are more common than others and some have a greater effect on the 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 and see pages 13-15.