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What Is Leukemia?

Leukemia is cancer of the blood. Unlike other cancers, leukemia does not produce lump like tumors but results in rampant overproduction of cancerous white blood cells. Leukemia — the term derives from the Greek words for "white" and "blood" — is often considered a disease of children, yet it actually affects far more adults. It is more common in men than women, and in Caucasians than African-Americans; almost 30,000 cases are diagnosed in the United States each year.

Blood has three types of cells: white cells fight infection, red cells carry oxygen and platelets help to clot, all suspended in its liquid plasma. Every day, hundreds of billions of new blood cells are produced in the bone marrow — most of them red cells. In people with leukemia, however, the body starts producing more white cells than it needs. Many of the extra white cells do not mature normally, yet they tend to live well beyond their normal life span.

Despite their vast numbers, these leukemic cells are unable to fight infection the way normal white blood cells do. As they accumulate, they interfere with vital organ functions, including the production of healthy blood cells. Eventually the body does not have enough red cells to supply oxygen, enough platelets to ensure proper clotting, or enough normal white cells to fight infection, making people with leukemia anemic and susceptible to bruising, bleeding, and infection.

Cases of leukemia are classified as acute or chronic. Cancer cells in acute leukemias start multiplying before they develop beyond their immature stage. Chronic leukemias progress more slowly, with cancer cells developing to full maturity. Leukemias are further classified according to the type of white blood cell involved. Under a microscope, two main types of white blood cells are easily distinguishable: Myeloid cells contain tiny particles or granules; lymphoid cells usually do not.

Acute lymphocytic, or lymphoblastic, leukemia (ALL) is the most common type of cancer in children; acute myelogenic leukemia (AML) is the most common form of leukemia in adults. Without treatment, acute leukemias are usually fatal within months. Treatment effectiveness varies with the type and stage of the disease, but the younger the patient, the greater the chances of remission. In leukemia, remission means that no more cancerous cells can be detected and bone marrow appears normal. Adult patients treated for ALL have an 80% to 90% chance of attaining remission; about 40% of those who do so survive at least another five years, with a chance of a full cure. Patients treated for AML have a 60% to 70% chance of remission; about 20% of those survive at least three years, with a possibility of a full cure.

Chronic leukemias tend to affect middle-aged adults. Chronic lymphocytic leukemia (CLL) is the most benign, slowly progressing type. It can be controlled effectively with medication and may require no treatment in its early stages. Chronic myelogenic leukemia (CML) is more aggressive. Since it is difficult to prevent this disease from escalating to an acute phase even with treatment, average survival time is about four years. Each of these four main types of leukemia can be divided into many subtypes. Rarer forms of the disease include hairy cell, promyelocytic, megakaryocytic, basophilic, and eosinophilic leukemia.

What Causes It?

No one knows exactly what causes leukemia. But chromosome abnormalities are associated with leukemia and with a preleukemic disease called myelodysplasia. Nine out of 10 people with CML, for example, have an abnormal chromosome called the Philadelphia Chromosome in their blood cells. As with other leukemias, this chromosome is an acquired abnormality; it is neither inherited nor passed on to one's children. Chromosome abnormalities associated with congenital disorders such as Down syndrome strongly correlate with certain types of leukemia, and at least one virus in the same family as the human immunodeficiency virus (HIV) has been associated with a rare form of the disease.

Environmental factors seem to influence the risk of developing leukemia. Tobacco smokers are more prone to certain leukemias than nonsmokers. Research also suggests that prolonged exposure to radiation, various chemicals in home and work environments, and low-frequency electromagnetic fields may be associated with leukemia, but there is no definitive proof.
Leukemia is also a rare complication of chemotherapy or radiation therapy on other cancers.