by Rosemarie Tremblay-LeMay MD MSc FRCPC and Vathany Kulasingam, PhD, FCACB
March 2, 2022
A plasmacytoma is a tumour made up of specialized immune cells called plasma cells. Normal plasma cells make different types of proteins called immunoglobulins (antibodies) that help protect the body from micro-organisms such as viruses. In contrast, all of the plasma cells in a plasma cell neoplasm make the same type of immunoglobulin. The abnormal plasma cells also make a lot more immunoglobulin than normal plasma cells.
When a plasmacytoma forms outside of a bone it is called an extraosseous plasmacytoma. If only a single tumour is found in a bone without evidence of abnormal plasma cells in other parts of the body, it is called a solitary plasmacytoma of the bone. Patients with a condition called plasma cell myeloma (multiple myeloma) can also develop masses outside their bone marrow that are made of plasma cells and identical to plasmacytoma.
Plasma cells start off life as a specialized type of immune cell called B-lymphocytes. Once a B-lymphocyte turns into a plasma cell, it has the ability to produce special proteins called immunoglobulins (Ig), which are also called antibodies. Immunoglobulins protect our body by sticking to bacteria and viruses, which makes them easier to remove from the body. Immunoglobulins can also stick to abnormal cells or cells that have stopped functionally normally.
Immunoglobulins are made up of four parts and each part is called a chain. One immunoglobulin is made up of two heavy chains and two light chains. There are five different kinds of heavy chains, called A, G, D, E, M, and two different kinds of light chains called kappa and lambda. Any combination of heavy and light chains can be used to make an immunoglobulin. These options allow your body to produce many different kinds of immunoglobulins (for example IgA kappa, IgG lambda, etc.).
While the immune system has the ability to make many different kinds of immunoglobulins, each plasma cell makes just one kind of immunoglobulin. Because our immune system makes millions of different plasma cells, it is normal to find many different kinds of immunoglobulins in the body at any time.
The diagnosis of plasmacytoma is usually made after a doctor removes a small tissue sample in a procedure called a biopsy. The diagnosis can also be made after the entire tumour has been removed in a procedure called an excision or a resection. The tissue is then sent to your pathologist who examines it under the microscope.
When examined under the microscope a plasmacytoma is made up of plasma cells arranged in large groups called sheets. These plasma cells can look abnormal; pathologists often use the word atypical to describe abnormal-looking cells.
Your pathologist will perform a test called immunohistochemistry to learn more about the plasma cells in your tissue sample and confirm that they are abnormal. They will also confirm that the tumour is made up of plasma cells and rule out other lymphomas that can look like plasmacytoma.
Immunohistochemistry is a test that uses antibodies to highlight different types of proteins produced by cells. When the cells produce a protein, pathologists describe the result as positive or reactive. When the cells do not produce the protein, the result is described as negative or non-reactive.
The tumour cells in plasmacytoma come from plasma cells and as a result, they produce proteins normally made by plasma cells, such as CD138, MUM1, or CD79a. They can also produce proteins that are not produced by normal plasma cells, such as CD20, CD117, CD56, or CyclinD1.
Your pathologist may also perform a test called in situ hybridization (ISH) to determine which immunoglobulins are being produced by the abnormal plasma cells. As described above, these abnormal plasma cells in a plasmacytoma will produce only one type of immunoglobulin, for example, IgG kappa or IgG lambda.
Sometimes, the abnormal immunoglobulins produced by the plasma cells in the plasmacytoma will build up in tissues. When this happens, it can create a substance called amyloid. In order to see amyloid, your pathologist may perform a special stain called Congo Red. Using this stain the amyloid looks red under normal light and apple green under a special light.