Morphology

Until recently, ALL was classified based on morphology and cytochemistry according to the French-American-British (FAB) schema, which includes three major subtypes: L1, L2, and L3.1 However, recent studies have failed to show prognostic significance of the L1 and L2 categories, and their designation has not had a high reproducibility rate among morphologists. Based on these findings, and as a result of improved understanding of the biology of these disorders, the 2001 World Health Organization (WHO) classification considers ALL as a form of lymphoma with a distinctive presentation. Initial classification is based on immunophenotype, and according to FAB, cases considered L1 and L2 are classified under precursor B- and T-cell neoplasms as precursor B-lymphoblastic leukemia/lymphoma and precursor T-lymphoblastic leukemia/lymphoma.2 In addition to immunophenotype, the WHO classification incorporates molecular, cytogenetic, morphologic, and clinical features in defining disease entities, as described by the WHO Advisory Committee.2 Table 11.1 shows a comparison between the two classifications.

In the WHO classification, FAB L3 is classified as one of the mature B-cell neoplasms and is designated Burkitt lymphoma/leukemia. It remains a separate entity char acterized by its morphologic, immunophenotypic, cytogenetic, molecular, and clinical features. The blasts are characteristically medium in size, with dispersed chro-matin, multiple nucleoli, and a moderate amount of deep blue cytoplasm with vacuoles. The classic "starry sky" description is derived from the presence of light-colored macrophages (stars) interspersed among sheets of dark blue blasts (sky). Oil red staining can be used to highlight the vacuoles in Burkitt-cell leukemia blasts.

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