Burkitt Lymphoma

General: Burkitt lymphoma (BL) is a highly aggressive B-cell lymphoma that can present as a lymphoma or acute leukemia. It often occurs in extranodal sites. Three clinical variants are recognized. The endemic form occurs in Africa, where it is the most common childhood malignancy. Epstein-Barr virus (EBV) is present in virtually 100% of cases. The sporadic form occurs in the developed world and children and younger adults. It is uncommon and represents about 1% of lymphomas with a median age of 31 years and male predominance.1 EBV is present in approximately 30% of cases. Immunodeficiency-associated BL is seen typically in HIV infected patients, where it often occurs early in the course of infection (sometimes as the AIDS defining illness). Its incidence appears to be decreasing in the era of more effective retroviral ther-apy.65 EBV is seen in approximately 40% of cases.66

Pathology: Burkitt lymphoma shows a starry sky pattern at low magnification because of the numerous tingible body macrophages that are present in the infiltrate containing apoptotic debris. The lymphoma cells are intermediate in size and uniform in appearance with vesicular chromatin and multiple inconspicuous nucleoli. There is a thin rim basophilic cytoplasm. The individual cells may show a retraction artifact around the cells. Mitoses are frequent and apoptosis of individual cells is present (Figure 52.14). Imprint morphol-

Figure 52.14 Burkitt lymphoma. Sheets of intermediately sized cells (compared to tingible body macrophage nuclei in left center) are seen with multiple inconspicuous nucleoli. The upper right inset shows lymphoma cells with basophilic cytoplasm and distinct vacuoles. The bottom inset is a Ki-67 antigen immunostain showing an extremely high proliferative fraction

Figure 52.14 Burkitt lymphoma. Sheets of intermediately sized cells (compared to tingible body macrophage nuclei in left center) are seen with multiple inconspicuous nucleoli. The upper right inset shows lymphoma cells with basophilic cytoplasm and distinct vacuoles. The bottom inset is a Ki-67 antigen immunostain showing an extremely high proliferative fraction ogy shows uniform cells with deeply basophilic cytoplasm and cytoplasmic vacuolization, which stains positively with oil-red O. In HIV-associated cases, plas-macytoid differentiation may be seen.

Immunophenotype: The immunophenotype is that of a mature B-cell expressing CD19, CD20, CD10, and surface immunoglobulin. The cells are negative for precursor-B-cell blast markers such as CD34 or TdT. Bcl-2 is also absent in most cases. Ki-67 is expressed in more than 90% of cells as a result of the underlying molecular genetic abnormality (described below).

Molecular genetics: Burkitt lymphoma is defined by the presence of a C-MYC translocation with immunoglobu-lin genes. Cytogenetically, this can be t(8;14)(q24;q32) or the variant translocations t(2;8)(p11;q24) or t(8;22)(q24;q11). These can be detected by FISH techniques and place C-MYC under the control of the immunoglobulin gene promoters, which leads to overexpression of C-MYC. This, in turn, causes expression of genes important in cell cycle such that virtually 100% of cells are cycling. Breakpoints in the immunoglobulin genes appear to vary depending on the type of BL. In endemic cases, the breakpoint is in the joining region suggesting transformation at the early B-cell stage, while in sporadic cases it involves the heavy chain switch region, suggesting a later B-cell stage of development at transformation.67 EBV viral genome can be detected in EBV positive cases and is clonally integrated, consistent with infection prior to malignant transformation.68

While C-MYC translocations are present in BL, it should be noted that it is not specific for this lymphoma since C-MYC translocation can occur as a secondary event in other types of lymphomas.

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