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Precursor B- and T-Cell NeoplasmsAcute Lymphoblastic Leukemia/Lymphoma Acute lymphoblastic leukemia/lymphoma (ALL) encompasses a group of neoplasms composed of immature, precursor B (pre-B) or T (pre-T) lymphocytes referred to as lymphoblasts. The majority (~85%) of ALLs are precursor B-cell tumors that typically manifest as childhood acute "leukemias" with extensive bone marrow and variable peripheral blood involvement. The less common precursor T-cell ALLs tend to present in adolescent males as "lymphomas," often with thymic involvement. It is worth noting, however, that there is considerable overlap in the clinical behavior of precursor B-cell and T-cell ALL; for example, pre-B cell tumors uncommonly present as "lymphomas," and many pre-T cell tumors evolve to a leukemic peripheral blood picture. Malignant pre-B and pre-T lymphoblasts are also morphologically indistinguishable, and subclassification of ALL is thus dependent on immunophenotyping. Because of their morphologic and clinical similarities, the various forms of ALL will be considered here together. Approximately 2500 new cases of ALL are diagnosed each year in the United States, most cases occurring in individuals younger than 15 years of age. ALL is almost twice as common in whites as in nonwhites and is slightly more frequent in boys than in girls. The incidence of pre-B ALL is highest at about the age of 4, perhaps because the number of normal bone marrow pre-B lymphoblasts (the cell of origin) peaks in early childhood. Similarly, the peak incidence of pre-T ALL is in adolescence, the age when the thymus reaches its maximal size. Both pre- B and pre-T ALL occur in adults of all ages, but much less frequently than in children. Morphology. Because of different responses to chemotherapeutic agents, it is of great practical importance to distinguish ALL from acute myelogenous leukemia (AML), a neoplasm of immature myeloid cells that may cause identical signs and symptoms. Compared to myeloblasts, lymphoblasts have condensed chromatin, inconspicuous nucleoli, and scant agranular cytoplasm ( Fig. 14-5A ). However, TABLE 14-4-- Summary of Major Types of Lymphoid Neoplasms Diagnosis Cell of Origin Genotype Salient Clinical Features Neoplasms of immature B and T cells Precursor B-cell acute lymphoblastic leukemia/ lymphoma Bone marrow precursor B-cell expressing TdT and lacking surface Ig Diverse chromosomal translocations; t(12;21) involving CBFa and ETV6 most common rearrangement Predominantly children with symptoms relating to pancytopenia secondary to marrow involvement; aggressive Precursor T-cell acute lymphoblastic leukemia/ lymphoma Precursor T-cell (often of thymic origin) expressing TdT Diverse chromosomal translocations, many involving Tcell receptor loci; rearrangements of TAL1 most common Predominantly adolescent males with thymic masses; variable splenic, hepatic, and bone marrow involvement; aggressive Date: 2016-04-22; view: 1277
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