Morphologic Assessment

A detailed discussion of the pathology of ALL is presented in Chapter 11. The initial morphologic classification for ALL was formulated by the FAB Cooperative Group,40 which proposed three morphologic subtypes (L1, L2, and L3). The L1 subtype is composed of small cells with a homogeneous nuclear chromatin pattern, regular nuclear shape, and scant cytoplasm that is only slightly to moderately basophilic. The L2 subtype contains cells of more heterogeneous size and are larger than those of the L1 subtype. These blasts also have a more heterogeneous nuclear chromatin pattern, a cytoplasm of variable amount and degree of basophilia, an irregular nuclear shape, and they may have nucleoli that are detected more readily compared to L1 blasts. The L3 subtype consists of blasts of large, uniform size with finely stippled chromatin and regular nuclear shape. Prominent nucleoli are commonly observed, and there are moderate amounts of cytoplasm with deep basophilia and lipid vacuoles. A "starry sky" appearance is often observed, which results from the ingestion of apoptotic neoplastic cells by histiocytes.41 Although many mitotic figures may be observed, the amount may be greater for T- compared to B-cell phenotypes.

Cytochemistry assessment demonstrates that the lymphoblasts in ALL are negative for myeloperoxidase and Sudan black B stains. Periodic acid-Schiff reaction and nonspecific esterase may be positive.42 Terminal deoxynucleotidyl transferase (TdT) is also positive in most cases of ALL.

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