Leukemia

I

Introduction

Leukemia, any of several types of cancers that affect blood cells, including oxygen-carrying red cells; certain infection-fighting white cells, such as granulocytes, macrophages and lymphocytes; and platelets, which aid in blood clotting. According to the American Cancer Society, leukemia is the sixth leading cause of cancer deaths among men and the seventh leading cause of cancer deaths among women. Each year in the United States about 31,000 new cases of leukemia are diagnosed and the disease causes an estimated 22,000 deaths. It accounts for about one-third of all cancers in children under age 15.

Blood cells are made in the bone marrow, the spongy tissue in the center of bones. A leukemia begins when an immature blood cell in the marrow, known as a progenitor cell, becomes cancerous, dividing uncontrollably and overriding the body’s normal restrictions on cell division. Over time, the marrow becomes crowded with cancerous cells, all of them descendants of the first abnormal cell. The malignant cells may also accumulate in a patient’s lymph nodes, spleen, and elsewhere. At the time of diagnosis, up to a trillion leukemic cells may be present in the body.

The mass of leukemic cells in the marrow suppresses the production of healthy blood cells, giving rise to the symptoms typical of leukemia. Pale skin, fatigue, and shortness of breath are signs of anemia, a decrease in the concentration of red cells in the blood. Nose bleeds, gum bleeding, a tendency to bruise easily, and pinhead-sized red spots on the skin reflect the decrease in the concentration of platelets in the blood. A lack of functional white cells makes patients with leukemia prone to infection.

Leukemia was first described by European physicians during the mid-19th century. During autopsies, physicians noted cases of profoundly elevated white cell counts—today we know that many of these white cells were nonfunctional leukemic cells—and very low red cell counts. For this reason, the condition was referred to as weisses blut (German for “white blood”). Later, the term leukemia (Greek leukos, “white”; haima, “blood”) was applied to the disease.

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II

Types of Leukemia

The leukemias are classified by two principal characteristics: the lineage of blood cell that becomes cancerous, and how rapidly the disease progresses. A leukemia is classified as myelocytic or myelogenous if the malignant cells have descended from the progenitors of red cells, granulocytes, macrophages, or platelets. If the leukemic cells have descended from a lymphocyte precursor cell, the leukemia is referred to as lymphocytic.

Myelocytic or lymphocytic leukemia can be acute or chronic, terms that refer to the patient’s life expectancy if the disease remains untreated. Acute leukemias develop rapidly, and without prompt treatment, the suppression of normal blood cell production is so severe that death occurs in a matter of weeks. In the chronic leukemias, patients may survive for several years or more without treatment because the effects of leukemic cells on the structure and function of the marrow develop more slowly and are less severe. In chronic myelocytic leukemia, for example, the leukemic cells can often complete their development and become functional blood cells. In chronic lymphocytic leukemia, the leukemic lymphocytes do not function normally, but in many cases the abnormal cells do not severely inhibit normal blood cell development.

The four major forms of leukemia—acute myelocytic, chronic myelocytic, acute lymphocytic, and chronic lymphocytic—can be further subcategorized based on the appearance of the malignant cells, the presence of characteristic molecules on their surface, or their stage of development. For example, hairy-cell leukemia is an uncommon type of chronic lymphocytic leukemia in which the malignant cells have fine, hairlike projections on their surface.

Myelocytic leukemia—both acute and chronic forms—can occur at any age, but more than 90 percent of cases occur in adults, and the risk of developing the disease increases dramatically after the age of 50. Acute myelocytic leukemia is the most common form of leukemia in the United States, with 12,000 new cases diagnosed each year. About 4,600 new cases of chronic myelocytic leukemia are diagnosed each year.

Chronic lymphocytic leukemia occurs rarely before the age of 45 and increases in incidence with each succeeding decade. Acute lymphocytic leukemia, by contrast, can occur at any age, but about half the cases occur in children under the age of 19, with the peak incidence occurring at about 4 years of age. In the United States, chronic lymphocytic leukemia accounts for about 9,700 new cases of leukemia each year, and acute lymphocytic leukemia, for about 4,000 new cases.

III

Causes

In most cases of leukemia, the cause is unknown, but physicians have identified four known causes of certain types of leukemia. Intensive radiation exposure or moderately intense exposure for long periods (see Radiation Effects, Biological) increases the risk of acute and chronic myelocytic leukemia and acute lymphocytic leukemia, but not chronic lymphocytic leukemia. The high rate of leukemia among Japanese survivors of the atomic bomb detonations at Hiroshima and Nagasaki at the end of World War II dramatically demonstrated the role of high-dose radiation in causing leukemia.

Exposure to certain chemicals can also cause leukemia. Workers exposed to benzene over long periods have an increased risk of developing acute myelocytic leukemia. Tobacco smoking appears to increase the incidence of this form of leukemia. Chemotherapy drugs used to treat breast cancer, ovarian cancer, lymphomas, and certain other cancers also increase a patient’s risk of later developing acute myelocytic leukemia.

Two viruses, human T-cell leukemia viruses (HTLV) I and II, are known to cause T-cell leukemia, a very rare form of lymphocytic leukemia, in humans. However, only a small percentage of people who are infected with these viruses develop cancer. Although virus-related leukemia is rare in humans, it is quite common in other animal species, such as cats, chickens, and mice.

Genetic factors may also contribute to the development of leukemia. Some inherited conditions, such as Down syndrome, increase a person’s risk of developing leukemia. In addition, scientists have identified rare clusters of leukemia in several members of the same family, presumably due to an inherited genetic mutation.

IV

Diagnosis

Bone marrow biopsy and blood cell tests are the primary techniques used to diagnose leukemia. In a bone marrow biopsy, cells are collected through a hollow needle inserted into the outer edge of a hipbone, or a small cylinder of bone containing marrow is removed with a special needle. The marrow sample is examined with a microscope for the presence of leukemic cells.

Blood tests that monitor blood cell counts—the number of cells of different types in the blood—can also reveal abnormalities characteristic of various forms of leukemia. Patients with acute leukemias nearly always have decreased red cell and platelet counts. In some patients, white cell counts are also very low. In others, a large number of leukemic cells enter the blood from the bone marrow, making white cell counts very high. However, physicians can examine these cells under the microscope to determine that they are abnormal, leukemic cells, not healthy white cells, and that the elevated white cell count is not due to another cause, such as infection.

Patients with chronic leukemias usually have slightly decreased red cell counts. Platelet counts are usually normal or mildly increased in patients with chronic myelocytic leukemia, and normal or mildly decreased in chronic lymphocytic leukemia. Nearly all chronic leukemia patients have increased white cell counts. In chronic myelocytic leukemia, some of these leukemic white cells are capable of functioning as normal cells do by fighting infectious microbes. Hence, infection is no more common than in a healthy individual. In chronic lymphocytic leukemia the blood contains large numbers of malignant lymphocytes that do not function normally. Normal lymphocytes coexist but are suppressed, and the inability to respond to microbes with specific antibody production may increase the frequency and severity of infection in some patients.

Additional tests, such as staining of cells with various chemical dyes to help doctors examine their appearance, analysis of molecules on the surface of the cells, and analysis of the cells’ genetic material, are performed on leukemic cells collected from the marrow or blood. These tests help doctors determine the subcategory of leukemia, which, in turn, may affect a patient’s prognosis and the approach to treatment. Tests such as chest X rays and examination of the spinal fluid for leukemic cells can help doctors determine how far the disease has spread.