Precursor B- and T-Cell Neoplasms

Acute 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: 1150