Plasmacytoma

What is a plasmacytoma?

A plasmacytoma is a tumour made up of specialized immune cells called plasma cells. Unlike normal plasma cells, all of the plasma cells in a plasmacytoma 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.

The immune system

Your immune system is made up of many different kinds of cells.  Each type of immune cell plays an important role in protecting your body from infections and helping you heal after an injury. Unlike other types of organs, your immune system is spread throughout your body. Most immune cells are found in lymph nodes, although many are also found in the blood, skin, gastrointestinal tract, and bones.

Plasma cells

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.

Plasma Cell

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.

How do pathologists make the diagnosis of plasmacytoma?

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.

plasmacytoma

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 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.

Patients with plasmacytoma can either have normal bone marrow or only a small population of abnormal plasma cells that make up < 10% of all bone marrow cells. Some of these patients will go on to develop a condition called plasma cell myeloma over time.

Amyloid

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.

by Rosemarie Tremblay-LeMay MD MSc FRCPC and Vathany Kulasingam, PhD, FCACB (updated July 20, 2021)
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