In the United States, more than 40,000 adults and 3,500 children are diagnosed each year with leukemia, a cancer of the white blood cells (WBCs). While exposure to radiation, benzene, and some anticancer drugs have been shown to increase the risk of developing leukemia, and a few cases are associated with genetic disorders or rare viral infections, the cause of most leukemias is not known.
What is leukemia?
Leukemia is a bone marrow disorder that arises when one abnormal white blood cell begins to continuously replicate itself. These cells do not function normally, they do not fight infection as they should, and they do not die at the same rate as other WBCs. As they accumulate, they inhibit the production of the other normal blood cells in the marrow, leading to anemia, bleeding, and recurrent infections. Over time, the leukemic cells spread through the bone marrow and bloodstream, where they continue to divide, sometimes forming tumors and damaging organs such as the kidney and liver. Since the spleen is responsible for filtering the blood and destroying old cells, it may become enlarged and swollen with the abnormal cells, as can the liver and lymph nodes. If the cells reach the central nervous system and build up in the cerebrospinal fluid that supports the brain and spinal column, they can cause headaches and seizures.
The bone marrow, located in the soft center of the body's larger bones, produces precursors of red blood cells, platelets, and five different kinds of white blood cells. The most immature of these is called a blast. Most of these blood cells mature in the bone marrow before being released into the bloodstream. The WBCs created are grouped into two main categories: lymphocytes and myeloid cells (also called granulocytes for the granules found inside the cell). Myeloid cells (which include neutrophils, basophils, eosinophils, and monocytes) circulate in the blood, killing and digesting bacteria. Lymphocytes, which are found in both the blood stream and the lymphatic system, coordinate the immune response and produce antibodies. Leukemia arises from one of these white blood cell precursors. It is categorized both by the type of WBC involved and by how quickly it progresses. Although expanded classifications of the disease exist, the common types of leukemia are mentioned below.
- Acute lymphoblastic leukemia (ALL). This is a rapidly developing, abnormal growth (neoplasm) of the cells that are precursors of lymphocytes (lymphoblasts), characterized by 20% or more lymphoblasts in the bone marrow and/or the blood. It is the most common type of leukemia found in children (nearly 75% of cases occur in children under six years of age), although it affects both children and adults (usually adults age 65 and older).
- Chronic lymphocytic leukemia (CLL). This is a neoplasm composed of small, round to slightly irregular mature B lymphocytes that all look very much alike. It is the most common leukemia of adults in Western countries. It tends to be found in those over the age of 55 or 60, and the mean age at diagnosis is 65. This disease progresses more slowly compared with some other types of leukemia.
- Acute myeloid leukemia (AML). This is a disease resulting from one or more myeloid blasts that begin to continuously replicate (clone) themselves in the bone marrow, blood, or other tissue. The median age at diagnosis is 65 years. In children less than 15 years of age, AML comprises 15-20% of all cases of acute leukemia. Similar to ALL, the requisite blast percentage for an AML diagnosis is 20% or more myeloid blasts (myeloblasts, monoblasts/promonocytes, and megakaryoblasts) in the bone marrow and/or blood.
- Chronic myelogenous (myeloid) leukemia (CML). CML usually develops insidiously, with abnormal, harmful cells proliferating but producing no obvious symptoms. Between 20% and 40% of patients are asymptomatic and are diagnosed when a routine CBC is performed. The disease can occur at any age, but the median age at diagnosis is 50-60. It is an acquired condition that begins in an immature stem cell in the bone marrow when pieces from two chromosomes (9 and 22) break off and switch places (translocation). This results in an altered, fused gene (bcr/abl) on the derivative chromosome 22 (also known as the Philadelphia chromosome) that produces a protein called tyrosine kinase that deregulates cell growth. This leads to an overproduction of granulocytic white blood cells and the presence of both mature and immature cells in the bloodstream.