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
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.
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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