by Rosemarie Tremblay-LeMay MD MSc FRCPC and Vathany Kulasingam, PhD, FCACB
November 10, 2023

A plasmacytoma is a tumour made up of specialized immune cells called 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.

How are the plasma cells in a plasmacytoma different from normal 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.

How is this diagnosis made?

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.

What does a plasmacytoma look like 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.

This picture shows the typical microscopic appearance of a plasmacytoma in the bone.

What other tests may be ordered to confirm the diagnosis?


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.

In situ hybridization

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.

What is amyloid and how is it related to a plasmacytoma?

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. 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.

About this article

This article was written by doctors to help you read and understand your pathology report. Contact us if you have any questions about this article or your pathology report. Read this article for a more general introduction to the parts of a typical pathology report.

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