At a Glance
Plasmablastic lymphoma (PBL) is an uncommon, highly aggressive type of lymphoma, which classically has been described with the highest incidence in HIV-positive individuals (typically presenting as a mass within the oral cavity; “oral cavity type”). However, it can present in other mucosal sites (i.e., Sinonasal, orbit, long, gastrointestinal (GI) tract), skin, and very rarely within lymph nodes (“nodal”). The median patient age is about 50 years.
PBL is a difficult diagnosis that requires correlation with clinical and other laboratory findings, since plasmablastic morphology and immunophenotype (special immunohistochemical stains) are virtually identical to plasma cell myeloma. The findings are CD45 patchy to negative, positive CD138 (a plasma cell marker), weak negative for most B-cell markers (CD 20, CD 19, and PAX-5). However, they are positive for CD79a (another marker of B-cell lineage) and restricted to expression of one of the immunoglobulin light chains (kappa or lambda). The proliferation index (assessed by Ki-67 immunohistochemical stain that marks proliferating cells) is usually very high (greater than 90%). Differentiation from plasma cell myeloma with plasmablastic morphology is the goal, as treatment and prognosis can differ considerably.
Until recently, PBL was classified by the World Health Organization (WHO) under the broader category of diffuse large B-cell lymphoma. The newest classification (2008) gives it its own category. The classic and most useful features that differentiate this lymphoma from plasma cell myeloma is the presence of Epstein-Barr virus (EBV), typically detected in the tissue by in situ hybridization for EBV-encoded RNA (EBER) in the majority (60-75%) of cases. Expression of the CD56 marker skews the diagnosis toward a plasma cell myeloma. Disseminated bone lesions can be present in both. It is important to provide the pathologist with a history of immunodeficiency, not restricted to HIV positivity, such as iatrogenic immunosuppression. Age itself can be a factor.
Since the CD45 (leukocyte common antigen, LCA) is negative, it is also important for the pathologist to exclude another poorly differentiated neoplasm that can express CD138, such as melanoma (ruled out by negative S-100 staining) and carcinoma (virtually ruled out by -10-cytokeratin staining).
Flow cytometry can be helpful to confirm immunoglobulin light chain restriction. Fluorescence in situ hybridization can demonstrate an IgH/myc translocation as seen in Burkitt lymphoma. However, the clinical utility of this additional information is unclear.
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
If not already known, the HIV status of the patient is quite crucial not only diagnostically, but also from a therapeutic standpoint, since treatment is very likely to include antiretroviral therapy. A serum protein electrophoresis with reflex immunofixation (if a monoclonal protein is detected, plasma cell myeloma is favored), a skeletal bone survey, a PET scan, and a bone marrow biopsy for staging are also indicated.
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