Your pathology report for plasmacytoma

by Rosemarie Tremblay-LeMay MD MSc FRCPC and Vathany Kulasingam, PhD, FCACB
October 18, 2025

A plasmacytoma is a tumor made up of plasma cells, a type of immune cell that lives in the bone marrow and makes antibodies (immunoglobulins) to help fight infections. In a plasmacytoma, these plasma cells grow abnormally and form a single tumor or mass.

When the tumor forms inside a bone, it is called a solitary plasmacytoma of bone. When it develops outside of the bone, in soft tissues such as the sinuses, lungs, or digestive tract, it is called an extramedullary (extraosseous) plasmacytoma.

A plasmacytoma is part of a spectrum of diseases known as plasma cell neoplasms. Some people with multiple myeloma (plasma cell myeloma), a cancer involving many bones, may also develop one or more plasmacytomas in addition to abnormal plasma cells in the bone marrow.

What causes a plasmacytoma?

The exact cause of plasmacytoma is not fully understood. It develops when mutations (genetic changes) occur in a plasma cell, allowing it to multiply uncontrollably and produce large amounts of a single type of antibody (immunoglobulin).

While most cases occur by chance, certain factors can increase risk:

• Older age (most cases occur after age 50).

• Male sex (men are affected more often).

• Chronic immune stimulation or previous radiation exposure.

Unlike multiple myeloma, a solitary plasmacytoma is localized—meaning it involves one area of bone or tissue and does not affect the rest of the body. However, a small number of patients may later develop multiple myeloma over time.

What are the symptoms of a plasmacytoma?

Symptoms depend on the location of the tumor.

• Plasmacytoma of bone: Bone pain, tenderness, or fractures.

• Extramedullary plasmacytoma: A painless mass, nasal blockage, cough, or difficulty swallowing if the tumor forms in the head, neck, or upper airway.

• General symptoms: Fatigue or weakness if anemia develops, and occasionally signs of high calcium levels or kidney problems if the tumor produces large amounts of antibody.

Many patients do not have symptoms related to other organs, since a plasmacytoma is usually confined to one area.

How is the diagnosis made?

The diagnosis of plasmacytoma is made after a biopsy, a procedure in which a small sample of tissue is removed from the tumor and examined under the microscope by a pathologist.

If the tumor is inside a bone, the biopsy may be guided by imaging. If the tumor is in soft tissue, it may be removed with a needle or minor surgery. In some cases, the entire mass is removed (an excision) and then sent for examination.

To confirm that the disease is limited to one site, your doctor may also order:

• Bone marrow biopsy to check for plasma cells elsewhere.

• Imaging tests (such as MRI, PET, or CT scans) to look for additional lesions.

• Blood and urine tests to measure abnormal immunoglobulins.

What does a plasmacytoma look like under the microscope?

When examined under the microscope, a plasmacytoma is made up of sheets or clusters of plasma cells. These cells may appear larger and more irregular than normal plasma cells, sometimes showing prominent nuclei or visible nucleoli. Pathologists often describe these as atypical plasma cells.

Normal plasma cells produce many different types of antibodies. In contrast, all the abnormal plasma cells in a plasmacytoma produce a single type of antibody, such as IgG kappa or IgG lambda, confirming that the cells originated from a single clone.

What additional tests may be performed?

Immunohistochemistry

Pathologists perform immunohistochemistry (IHC) to confirm that the tumor cells are plasma cells and to rule out other types of blood cell tumors such as lymphoma.

This test uses antibodies to detect specific proteins in the cells. The abnormal plasma cells in a plasmacytoma usually produce proteins normally made by plasma cells, such as CD138, MUM1, and CD79a. They may also express proteins not seen in normal plasma cells, such as CD56, CD117, or Cyclin D1, which help confirm the diagnosis.

In situ hybridization

An additional test called in situ hybridization (ISH) is often used to determine which light chain—kappa or lambda—the plasma cells produce. Normal plasma cells are roughly half kappa and half lambda, but in a plasmacytoma, all tumour cells produce only one type, confirming that the tumour is monoclonal.

Molecular testing

In some cases, molecular tests are performed to look for specific genetic changes (mutations or translocations) that help doctors assess prognosis or identify related diseases such as multiple myeloma.

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

In some plasmacytomas, the abnormal antibodies made by plasma cells can build up in tissues and form a material called amyloid. When this happens, the condition is called amyloidosis.

Pathologists use a special stain called Congo red to detect amyloid. Under regular light, amyloid appears red, and under polarized light it shows a distinctive apple-green color. The presence of amyloid can explain symptoms such as swelling, tongue enlargement, or heart or kidney dysfunction.

How is plasmacytoma different from multiple myeloma?

Plasmacytoma and multiple myeloma are closely related conditions that belong to the same family of plasma cell neoplasms.

• Plasmacytoma involves a single, localized tumor made up of abnormal plasma cells, either in bone or soft tissue.

• Multiple myeloma involves many areas of bone marrow throughout the body and causes widespread problems such as anemia, bone destruction, kidney injury, and high calcium levels.

For this reason, patients with plasmacytoma undergo blood, urine, and imaging studies to rule out multiple myeloma. Even when none is found, ongoing monitoring is important, as some patients later develop myeloma.

Is plasmacytoma a cancer?

Yes. A plasmacytoma is a form of cancer, but unlike multiple myeloma, it is localized—meaning the abnormal plasma cells are confined to one area of bone or soft tissue.

Because it is made up of malignant (cancerous) plasma cells, plasmacytoma has the potential to grow and, in some cases, to progress over time to multiple myeloma, which affects many areas of the bone marrow.

Most plasmacytomas behave as slow-growing cancers that respond well to treatment such as radiation therapy or surgery, especially when detected early. Long-term follow-up is important to make sure no new tumors develop.

What is the prognosis for a person with plasmacytoma?

The prognosis for a solitary plasmacytoma is generally excellent, especially when it is treated early with radiation therapy or surgery. Most patients remain disease-free for years.

However, some cases may eventually progress to multiple myeloma, particularly when small numbers of abnormal plasma cells are later found in the bone marrow or when abnormal proteins reappear in the blood. Long-term follow-up with regular blood tests and imaging is recommended.

Questions to ask your doctor

• Is my plasmacytoma located in bone or soft tissue?

• Have tests shown any evidence of multiple myeloma?

• What type of antibody (immunoglobulin) is being produced?

• Was amyloid found in my tissue sample?

• What treatment is recommended for my tumor?

• How will I be monitored after treatment?