Pathology And Pathogenesis

Precursor T- and B-cell LBLs are neoplasms of lym-phoblasts of T- and B-cell lineage, respectively. Morphologically, they are very similar, comprising small- to medium-sized blast cells with little cytoplasm, inconspicuous nucleoli, and moderately condensed to dispersed chromatin.4 Convoluted nuclei are sometimes seen, but this has no clinical significance. Extensive mitotic figures are usually seen and widespread apoptosis is a common feature. The presence of apoptotic bodies being phagocytosed by macrophages produces the "starry sky" pattern typical of high-grade NHLs. Although this appearance is more characteristic of BL, it occurs relatively frequently in lymphoblastic disease, and in most cases, there are clear morphologic distinctions between the two entities. Additionally, immunohistochemical and cytoge-netic features of the two diseases are distinct, so that diagnostic confusion is rare. However, the distinction of lymphoblastic disease from other NHLs subtypes can be obscured in poorly processed or fixed tissue specimens. Additionally, lymphocyte-rich thymomas have similar morphologic features, but can usually be distinguished on the basis of the epithelial distribution characteristic of thymoma.

The typical immunophenotype for T- and B-cell LBL is summarized in Table 56.1.9-13 Most cases of T-cell disease have clonal rearrangement of T-cell receptor genes, most commonly 8, although this finding is not specific to T-cell lineage, and may also be seen in B-cell disease. Chromosomal translocations involving T-cell receptor genes have little value diagnostically, although they provide important information regarding the patho-genesis of lymphoblastic disease (see below).

One-third of patients have chromosomal translocations involving the a and 8 T-cell receptors (14q11.2), the p receptor (7q35), or the y receptor (7p14-15).14 These translocations typically result in high levels of coexpression of T-cell receptor genes with transcription factor genes (TAL1/SCL, TAL2 and LYL1, and HOX11/TLX1).15-17 These patterns of gene expression

Table 56.1 Immunophenotype of precursor lymphoblastic leukemia/lymphoma. Summarized from references 9-13

T cell B cell

0 0

Post a comment