Primary effusion lymphoma (PEL) is classified as a diffuse large B cell lymphoma. It is a rare malignancy of plasmablastic cells that occurs in individuals that are infected with the Kaposis sarcoma-associated herpesvirus (i.e. KSHV/HHV8). Plasmablasts are immature plasma cells, i.e. lymphocytes of the B-cell type that have differentiated into plasmablasts but because of their malignant nature do not differentiate  into mature plasma cells but rather proliferate excessively and thereby cause life-threatening disease. In PEL, the proliferating plasmablastoid cells commonly accumulate within body cavities to produce effusions (i.e. accumulations of fluid), primarily in the pleural, pericardial, or peritoneal cavities, without forming a contiguous tumor mass. In rare cases of these cavitary forms of PEL, the effusions develop in joints, the epidural space surrounding the brain and spinal cord, and underneath the capsule (i.e. tightly woven collagen fibers) which forms around breast implants. Less frequently, individuals present with extracavitary primary effusion lymphomas, i.e., solid tumor masses not accompanied by effusions. The extracavitary tumors may develop in lymph nodes, bone, bone marrow, the gastrointestinal tract, skin, spleen, liver, lungs, central nervous system, testes, paranasal sinuses, muscle, and, rarely, inside the vasculature and sinuses of lymph nodes. As their disease progresses, however, individuals with the classical effusion-form of PEL may develop extracavitary tumors and individuals with extracavitary PEL may develop cavitary effusions.PEL typically occurs in individuals who are immunocompromised, i.e., individuals whose immune system is weakened and therefore less able to fight infectious agents and cancers.
One factor that appears to improve the treatment of Type II PEL is the addition of rituximab (a monoclonal antibody directed against and killing CD20-bearing cell) to the intensive chemotherapy regimens used to treat Type I PEL: the malignant cells in Type II PEL commonly express CD20 whereas the malignant cells in Type I PEL rarely express this cell surface marker. However, there are several cases of KSHV/HHV8-negative EBL that presented with pericardial effusions without evidence of more extensive disease that have experienced complete responses and favorable prognoses without chemotherapy or other cancer treatment (including rituximab) after simple drainage of the effusion. These cases suggest that, in addition to the presence of rituximab-sensitive CD20-bearing malignant cells, Type II PEL may be a less severe disease than Type I PEL, at lease in certain cases. Treatment
PEL is generally resistant to cancer chemotherapy drugs that are active against other B-cell lymphomas and therefore carries a poor prognosis. Overall median and 1 year survival rates in a series of 28 patients treated with chemotherapy for PEL were 6.2 months and 39.3%, respectively. In this study, the complete response rate (presumed to be temporary) to a standard CHOP chemotherapeutic regimen (i.e. cyclophosphamide doxorubicin, vincristine, and prednisone) was only 10% whereas a more intensive CHO chemotherapy regimen which included high dose methotrexate and bleomycin achieved a compete response rate (presumed temporary) of 70%.