by Jason Wasserman MD PhD FRCPC and Aleksandra Paliga MD FRCPC
September 18, 2023
The symptoms of PBL depend on where in the body the tumour develops. For example, a tumour involving the oral cavity often presents as a rapidly growing mass associated with bleeding and pain. A tumour in the nasal cavity may cause congestion, facial pain, and recurrent (multiple) nose bleeds. Tumours in other parts of the body may not cause any symptoms until late in the disease.
The exact cause of PBL is currently unknown. However, most patients have a history of immune deficiency secondary to long-standing HIV infection or immune suppression secondary to medications designed to decrease normal immune system function. For many patients, Epstein-Barr virus (EBV) appears to also play a role in the development of this type of cancer.
The diagnosis of PBL can only be made when a sample of the tumour is removed and the tissue is examined under the microscope by a pathologist.
When examined under the microscope, PBL is typically made up of large abnormal-looking plasma cells that may be described as plasmablastic or having an immunoblastic nuclear morphology because they resemble immature plasma cells and/or immune cells called immunoblasts respectively. However, unlike normal immunoblasts and plasma cells, the tumour cells in PBL are often arranged in large groups called sheets. Large numbers of mitotic figures (cells dividing to create new cells) are also seen along with necrosis (dead cells).
Pathologists often perform a test called immunohistochemistry (IHC) to confirm the diagnosis and to rule out other conditions that can look similar to PBL under the microscope. When IHC is performed, the tumour cells in PBL are often positive or reactive for some B cell markers including PAX-5 and CD79a but they are negative for CD20. The tumour cells are also often positive or reactive for some plasma cell markers including CD138, CD38, BLIMP1, and XBP1.
While normal plasma cells produce a combination of IgG lambda and IgG kappa immunoglobulins (antibodies), the tumour cells in PBL typically only produce one. This abnormal or restricted production can be demonstrated by IHC, flow cytometry, or in situ hybridization (ISH).
Tests may also be performed to confirm the presence of EBV in the tumour cells. The most common test used is called in situ hybridization (ISH) and it is performed to look for a marker called EBER which is produced by cells that have been infected with EBV.