Subtypes & Classification of Acute Lymphoblastic Leukemia
There are many subtypes of acute lymphoblastic leukemia (ALL), and the disease is further classified by treatment status and response to treatment. The subtype and classification are used to plan treatment and predict outcome.
Subtypes of ALL
ALL is divided into subtypes based on the type of lymphocyte—B-cell or T-cell—the leukemic cells come from, the maturity of the leukemic cells, and whether chromosomal abnormalities are present. The majority (approximately 80 percent) of people with ALL have a B-cell subtype. Some subtypes have an abnormal number of chromosomes; in other subtypes two chromosomes exchange some of their DNA in what is called a translocation.
Philadelphia Chromosome–Positive ALL
The most common chromosomal translocation in adult ALL is a rearrangement between parts of chromosomes 9 and 22—resulting in abnormal chromosomes. Discovered in Philadelphia in 1960, this was the first chromosome abnormality definitively linked to cancer. In this rearrangement, two genes called BCR and ABL fuse together in chromosome 22 to make one gene called BCR-ABL. If it has this feature, chromosome 22 is known as the Philadelphia chromosome. About 25 percent of adult ALL patients, and up to 50 percent of ALL patients over the age of 60, have this subtype, known as Philadelphia chromosome–positive ALL (PH+ ALL).
Most cancers are assigned a numbered stage based on the size of the tumor and how far it has spread. Because leukemia doesn’t typically form a solid tumor and is found throughout the body when it is diagnosed, there is no formal staging system for ALL. Instead it is classified as:
- Newly diagnosed/untreated
- In remission
Newly Diagnosed/Untreated ALL
Newly diagnosed ALL is described as untreated. Treatment may have begun for relief of symptoms, such as fever and bleeding, but treatment has not begun for the leukemia. Diagnosis has confirmed all of the following:
- The complete blood count (CBC) is abnormal.
- Greater than 20 percent of bone marrow or blood cells are leukemia cells.
- Signs and symptoms of leukemia are present.
ALL in Remission
After treatment, people with ALL may be considered in remission if all of the following are true:
- CBC is normal.
- No leukemia cells are present in the bone marrow.
- No signs or symptoms of leukemia are evident anywhere in the body.
ALL that returns after going into remission is called relapsed or recurrent. It may come back in the blood and bone marrow or in other parts of the body, for example, the lymph nodes, internal organs, or spinal fluid. Refractory means the leukemia has not responded to treatment.