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.
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.
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.
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 increases the risk of acute and chronic myelocytic leukemia and acute lymphocytic leukemia, but not chronic lymphocytic 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. 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.
Bone marrow biopsy and blood 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.
Treatment of leukemia depends on the type and extent of the disease and is tailored to each individual patient. In general, chemotherapy—the use of drugs that kill rapidly dividing cells—is the mainstay of treatment for both acute and chronic leukemias. In acute leukemias, chemotherapy is very intensive and uses several drugs, either simultaneously or sequentially, in order to kill as many leukemic cells as possible. Antibiotics and transfusions of red cells and platelets help sustain patients whose blood counts are dangerously low because they are receiving intensive chemotherapy.
Sometimes radiation is used to shrink collections of leukemic cells that accumulate in various parts of the body, such as on the lining of the brain and spinal cord in acute lymphocytic leukemia, or within lymph nodes in chronic lymphocytic leukemia. If left untreated, collections of cells on the lining of the brain and spinal cord can cause headache, blurred vision, and confusion, and elsewhere in the body can cause swelling and tenderness of the affected area.
Immunotherapy, though still in the experimental stages, is a promising new approach to treating leukemia. In this technique, highly specific molecules known as monoclonal antibodies are manufactured in the laboratory to target molecules on the surface of leukemic cells. The antibodies are attached to a radioactive substance, then injected intravenously into a patient. This method provides a convenient means of delivering the radioactive substance directly to leukemic cells, where it may kill these cells with minimal effect on healthy cells.
