Acute lymphoblastic leukaemia

Published on 03/04/2015 by admin

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Last modified 03/04/2015

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Acute lymphoblastic leukaemia

Acute lymphoblastic leukaemia (ALL) is a clonal malignancy of lymphoid precursor cells. In over 80% of cases the malignant cells are primitive precursors of B-lymphocytes and the remainder are T-cell leukaemias. The abnormal cell may arise at various stages of early lymphocyte differentiation (see p. 8).

ALL has a peak incidence in childhood with a gradual rise in incidence in later years (Fig 21.1). The disease has distinct characteristics in children and adults. Childhood ALL is often curable by chemotherapy whereas cure is elusive in adult ALL. Poorer outcome in adult ALL is due to a combination of a greater frequency of high-risk leukaemia with more drug resistance, and less effective treatment regimens.

Fig 21.1 Incidence of ALL at different ages.

Classification

The French-American-British (FAB) morphological classification is based on characteristics of the blast cells, including cell size, nuclear–cytoplasmic ratio, number and size of nucleoli and the degree of cytoplasmic basophilia (Fig 21.2). Morphological classification is now less important than that based on immunophenotyping, cytogenetics and molecular analysis (Table 21.1).

Table 21.1

Classification of ALL

¹ See also Figure 21.2.

Fig 21.2 Morphology of ALL blast cells.
(a) L1 type; (b) L2 type; (c) L3 type. Note that the L2 cells have more cytoplasm and more prominent nucleoli than L1 cells. L3 type cells have cytoplasmic vacuolation.

Definition of the immunological subtypes of ALL depends on the presence or absence of various cell surface and cytoplasmic antigens. A commonly used classification divides ALL into early pre-B, pre-B, B-cell and T-cell subtypes. Mature B-cell ALL typically has L3 morphology. The current WHO classification divides most ALL subtypes into B- or T-lymphoblastic leukaemia/lymphoma under the heading ‘precursor lymphoid neoplasms’. Mature B-ALL is included as Burkitt lymphoma.

In the selection of treatment it is important to differentiate between three broad groups; T-cell ALL, mature B-ALL and all other types of B-lineage ALL. Genetic abnormalities are becoming increasingly important in classification of ALL as they give vital prognostic information (Table 21.2).

Table 21.2

Chromosomal abnormalities in ALL

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Abnormality		Prognostic significance
Numerical change
High hyperdiploidy (over 50 chromosomes)		Favourable
Hyperdiploidy (47–50)		Intermediate
Pseudodiploidy (46 with structural/numerical change)		Intermediate
Hypodiploidy (less than 46)		Poor
Structural abnormality	Genes involved
Philadelphia chromosome, t(9;22)¹	BCR-ABL	Poor
t(12;21)²	TEL-AML1 (ETV6-RUNX1)	Good
t(1;19)	E₂A-PBX1	Good
t(v;11q23)