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.