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Acute lymphoblastic leukemia (ALL) is the most common childhood malignancy, accounting for almost 25% of cancer diagnoses seen in children younger than age 15 years. Patients with more than 25% lymphoid blasts in the bone marrow are considered to have leukemia regardless of the presence of extramedullary disease. Patients who have an extramedullary lymphoid mass and 5% to 25% lymphoid blasts in the marrow are considered to have lymphoblastic lymphoma with marrow involvement. There are approximately 2400 new cases of ALL diagnosed in children and adolescents less than 20 years of age each year in the United States.1The peak incidence of ALL is in the 2- to 6-year-old age group. The incidence of ALL appears to be highest in Hispanic children and lowest in African American children.2

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Progress made in treating ALL has been remarkable. As recently as 50 years ago, cure rates for patients with ALL were less than 10%. In 2008, we expected to cure approximately 80% to 85% of patients with ALL.3-6 Patients are generally considered cured if they remain in remission more than 5 years from discontinuation of therapy. Chemotherapeutic principles and supportive care guidelines derived from the experience in treating ALL have had a major impact on the treatment of other types of cancer.

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Biology

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In the developmental pathway of lymphocytes (see Fig. 449-1), maturation to the mature B-cell and T-cell state occurs without the necessity of antigenic stimulation—so-called antigen independent differentiation. When a B or T cell combines with its specific antigen, proliferation and differentiation occur, resulting in long-lived memory cells for antigen recognition and plasma cells which secrete antibody or T cells with various effector functions, such as cytotoxicity, B-cell help, cytokine secretion, and so on. In B-lineage acute lymphoblastic leukemia (ALL), approximately 98% of patients show a B-precursor immunophenotype. The rare patient with mature B-cell leukemia has a form of Burkitt lymphoma and requires different treatment. The various stages of B-cell development cannot be judged by cell morphology. Surface antigen expression is used to determine the stage of B-cell development. The earliest stage in B-cell development is the pro-B cell, which is characterized by surface expression of histocompatibility complex (HLA-DR) and the lack of expression of CD10 (common ALL antigen). The common precursor B cell expresses HLA-DR and CD10 but does not express cytoplasmic immunoglobulin. The pre-B cell expresses cytoplasmic immunoglobulin, and may or may not express CD10. The incidence of the various subtypes of B-precursor ALL is approximately 75% common B, 20% pre-B, and 5% pro-B.4 The pro-B immunophenotype is most often seen in infants. In T-ALL, cases are evenly distributed between the early, mid-, and late thymocyte state determined by the presence or absence of various T-cell antigens such as CD 2, 4, 5, 7, 8, and cytoplasmic CD3. Rarely, ALL cases may show a biphenotypic, bilineage, or natural killer cell immunophenotype.

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