Classification

To help plan treatment and predict prognosis (chance of recovery), doctors classify ALL based on the type of lymphocytes that are affected (such as T cells or B cells). For example, flow cytometry distinguishes between ALL involving T cells or B cells. About 5% of the B-cell cases have a unique subtype called Burkitt leukemia or Burkitt lymphoma. ALL can also be characterized by the appearance of the cells under the microscope (called L1, L2, and L3), although this is less important than the results of the flow cytometry or cytogenetic studies. Some specific chromosomal or genetic changes in the cancer cells are used to help predict how well the disease will respond to treatment and may guide the treatment choices.

About 20% to 30% of adults with ALL have a genetic abnormality or mutation called the Philadelphia chromosome. As described in Diagnosis, the Philadelphia chromosome refers to an exchange of genetic material, called a translocation, between chromosomes 9 and 22 that causes two genes called BCR and ABL to fuse into one gene called BCR-ABL. This mutation is found only in the blood-forming cells, not in other organs of the body, and is not inherited. Therefore, there is no concern about an increased risk to other family members.

The BCR-ABL gene causes cells in the bone marrow to produce an abnormal enzyme that allows specific types of white blood cells called B lymphoblasts to grow out of control. It is important to test for the presence of the Philadelphia chromosome because this could influence both the prognosis and the type of treatment that is recommended.

In a cancer where a solid tumor forms, doctors agree on a set of stages that describe how big the tumor is and where it has spread. Because leukemia usually does not form a solid tumor and is found throughout the body, there is no formal staging system for ALL. Instead, there are general classifications used to describe ALL:

Untreated. A patient has abnormal white blood cell, red blood cell, and platelet counts. The bone marrow contains abnormal lymphoblasts, and the person usually has symptoms as described in the Symptoms section.

In remission. A patient has received treatment for ALL. The bone marrow contains less than 5% blasts, and the patient has no symptoms. White blood cell, red blood cell, and platelet counts are back in the normal range.

Recurrent/refractory. Recurrent leukemia has come back after being in remission. Refractory leukemia means that the disease has not responded to treatment.