Classification of acute lymphoblastic leukemia
Different types of acute lymphoblastic leukemia or acute lymphocytic leukemia (ALL) start from different types of lymphocytes. Almost 85% of cases of ALL begin from
World Health Organization classification system
The World Health Organization (WHO) classifies ALL based on the immunophenotype of the leukemia cells. The immunophenotype is determined by lab tests including flow cytometry and cytogenetic tests. Rarely, acute leukemia may have characteristics of both ALL and acute myeloid leukemia (AML). These rare leukemias are referred to as mixed phenotype acute leukemias (MPAL).
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Type of lymphoblast |
WHO subtype |
|---|---|
|
Precursor B cell |
B lymphoblastic leukemia/lymphoma, not otherwise specified, NOS |
|
Precursor B cell |
B lymphoblastic leukemia/lymphoma with recurrent cytogenetic abnormalities:
|
|
Precursor T cell |
T lymphoblastic leukemia/lymphoma |
The older French-American-British (FAB) system classifies ALL based on what the leukemia cells look like under a microscope. It is based on the morphology (size, shape and structure) of the leukemia cells. Most doctors use the WHO classification system because it uses newer lab tests that more accurately classify ALL.
Other classification factors
Doctors use the following factors to describe ALL. They consider these factors, along with the WHO classification, to help them determine a prognosis and plan treatment.
Chromosome abnormalities
In about 70% of adults with ALL, the leukemia cells have certain changes to chromosomes, or chromosome abnormalities.
The Philadelphia chromosome
The Philadelphia (Ph) chromosome, or t(9;22), is a common chromosome abnormality in the leukemia cells of adults with ALL. The long arm of chromosome 9 is switched with the long arm of chromosome 22. This type of change is called a translocation. This translocation creates the BCR-ABL fusion gene, which leads to the development of ALL.
About 1 in 4 adults with ALL have leukemia cells with the Ph chromosome. They are referred to as having Ph-positive, or Ph+, ALL. Those who do not have the Ph chromosome are referred to as having Ph negative, or Ph–, ALL. Ph+ ALL is more common in older people.
It is not known why the Ph chromosome appears in leukemia cells in the blood and bone marrow. Although this chromosome abnormality is genetic, it is not inherited. It is an acquired abnormality, which means that it happens some time after birth. It does not seem to be passed on to other family members. This means that a person with ALL will not pass the BCR-ABL fusion gene to their children.
Other chromosome abnormalities
Other less common abnormalities include translocations between chromosomes. A translocation is when part of a chromosome is transferred to another chromosome such as t(4;11) which is a translocation of chromosomes 4 and 11.
Mixed phenotype acute leukemia
Sometimes the leukemia cells are considered mixed lineage because they have both myeloid and lymphoid characteristics. The leukemia cells may have both myeloid and lymphoid features on the same cell, or some leukemia cells have myeloid features and other leukemia cells have lymphoid features. Mixed phenotype acute leukemias (MPAL) are a special category of acute leukemias using the WHO classification system.
Leukemia cells in the central nervous system
About 5%–10% of people with ALL have leukemia cells in the central nervous system (CNS) at the time of diagnosis. The CNS is made up of the brain and spinal cord. ALL is known to spread to the CNS in about 35% of people who do not have prophylactic treatment.