MALIGNANT LYMPHOMA, PLASMABLASTIC TYPE.
Non-Hodgkin's lymphomas (NHL) are the second most common neoplasm in AIDS patients and are the most common neoplasm in certain AIDS risk groups (e.g. intravenous drug abusers, hemophiliacs) (1,2,3). AIDS-NHL are usually widespread at the time of presentation with frequent extranodal involvement and are associated with a poor prognosis. Most cases of AIDS-associated lymphoma can be classified as a small-noncleaved (Burkitts lymphoma) and large cell lymphomas including immunoblastic lymphoma with plasmacytoid differentiation (1). AIDS-NHL may occur anywhere in the head and neck area. They are morphologically similar to other AIDS-NHL that occur at other sites. A particular type of lymphoma has been recently described as a specific entity among AIDS-associated diffuse large cell lymphomas involving the oral cavity. These lymphomas have been designated as plasmablastic lymphomas in accordance with their plasmablastic morphology and immunophenotypic features.
The lymphoma presented here has the characteristic morphologic and immunophenotypic features of the recently described cases of plasmablastic lymphoma. Plasmablastic lymphoma can be considered as a subcategory of AIDS-associated diffuse large B cell lymphoma (4,5). They tend to grow diffusely with a high proliferation index and predilection for the oral cavity and jaws. It is known that most of the AIDS-related lymphomas are of B-cell origin expressing immunoreactivity for CD20 and CD79a antigens. However, plasmablastic lymphoma usually has a weak or absent expression of CD20 and CD45. Although CD79a was negative in our case, previously described cases showed variable expression. Expression of immunoglobulin light and/or heavy chains is also variable. The major feature shared by all described cases was very strong positivity for plasma cell marker CD138. This supports their plasmacellular differentiation.
Several entities should be considered in the differential diagnosis. The distinction from diffuse large B cell lymphoma of the immunoblastic type with plasmacytoid differentiation can be difficult. Plasmablastic lymphoma cannot be distinguished from that particular type of lymphoma on the basis of morphology only. High apoptotic rate can be helpful as it is rarely seen in diffuse large B cell lymphomas. Nevertheless, the absence of CD20 and CD45 allows the distinction most of the time between these two entities.
Plasmablastic lymphomas may also be confused with plasmacytomas, as they share a similar immunophenotype. A high proliferation index and apoptosis are unusual features of most plasmacytomas. Plasmablastic lymphoma is usually composed of blastoid appearing cells with prominent nucleoli, while plasmacytomas usually exhibit condensed chromatin and inconspicuous nucleoli. Major difficulties may occur when a solitary plasmacytoma displays anaplastic features, because they have mostly blastoid appearing cells with only a few clearly differentiated plasma cells. EBV staining can be very helpful, because previous studies showed that only 6.7% plasmacytomas are EBV positive in contrast to more than 50% of plasmablastic lymphoma associated with EBV (4,6).
Lack of CD20 and LCA expression in this undifferentiated large cell tumor of the oral cavity in HIV-infected patients can mistakenly lead to the exclusion of lymphoma from the differential diagnosis. In the cases where common lymphoid markers are negative, application of plasma cell markers and antibodies to immunoglobulin light chains together with cytokeratin can help in order to avoid confusion with undifferentiated carcinoma.
Even though distinct morphologic, immunophenotypic and clinical features suggest that plasmablastic lymphoma represents a new distinct subtype of AIDS- associated diffuse large B cell lymphomas, at present it is not formally recognized as a distinct type of AIDS-NHL. The diagnosis of plasmablastic lymphoma is not always straight forward. Recently, Rives and coauthors reported 12 patients fulfilling the criteria for plasmablastic lymphoma and suggested it may not be as distinctive as originally reported (7). Additional larger clinical and molecular studies are needed to further characterize this infrequent type of lymphoma.
Contributed by Sanja Dacic, MD, PhD.