Leukemia: Overview and Types

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Leukemia: Overview and Types
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Leukemias are malignancies of the hematopoietic stem cells in the bone marrow. Although many varieties of leukemia exist, and can be distinguished by the affected cell type, four major categories are recognized. These are acute myeloid leukemia (AML), acute lymphoid leukemia (ALL), chronic myeloid leukemia (CML), and chronic lymphoid leukemia (CLL).

About 30,000 new cases of leukemia are diagnosed in the United States each year. The acute leukemias have a rapid clinical onset and are nearly uniformly fatal within months without treatment. Chronic leukemias often have a subacute onset of symptoms with an average survival of 4 to 5 years after diagnosis for CML, and 8 to 12 years for CLL. Over time, chronic leukemias transform into a more aggressive disease with a terminal blast phase that resembles acute leukemia.

Acute Leukemias

ALL is a proliferation of lymphoid precursors, primarily of B-cell origin. AML is characterized by proliferations of myeloid precursors, called blasts. Untreated, acute leukemias are uniformly fatal.

ALL accounts for about 12% of leukemias in the United States, but represents 60% of leukemias in persons younger than 20 years. With intensive chemotherapy regimens, about 80% of children and 40% of adults with ALL will be cured.

In contrast, AML is particularly common in older adults, with a median age at diagnosis of 68. Intensive chemotherapy, including stem cell transplants in selected patients, results in about a 50% long-term, disease-free survival.

Chronic Leukemias

Chronic leukemias more commonly affect older patients. They are, in general, more indolent than acute leukemias. However, many cases ultimately lead to a terminal phase that resembles acute leukemia.

CLL is a malignancy of mature B cells. It is the most common adult leukemia, with more than 80% of cases occurring in patients over the age of 60. Worldwide incidence is highly variable; North America has the highest rate, while CLL is rare in Southeast Asia.

CML is characterized by uncontrolled production of neutrophils, eosinophils, and basophils. It accounts for only 5% of childhood leukemia and has a peak incidence around age 53. CML is identified by a typical translocation, called the Philadelphia chromosome, which results in a fusion protein (BCR-ABL) that releases controls on stem cell proliferation and blocks apoptosis.

Clinical symptoms of chronic leukemia are generally nonspecific and often occur due to proliferation of the affected cell line or to decreased production of other blood cells. Common presentations include anemia (pallor, fatigue, palpitations), thrombocytopenia (epistaxis, menorrhagia, bleeding, bruising), and leukopenia (fever, frequent infections). Weight loss, lymphadenopathy, splenomegaly, gingival hypertrophy, and bone pain may also occur. Many patients with chronic leukemia remain asymptomatic, with discovery of the disease occurring during routine blood tests.