Plasmacytoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
October 6, 2026

A plasmacytoma is a tumor made of abnormal plasma cells. Plasma cells are immune cells that normally live in the bone marrow and make antibodies, also called immunoglobulins, to fight infection. In a plasmacytoma, a single abnormal plasma cell multiplies and forms a mass in one place.

A plasmacytoma is a cancer, but when it is truly solitary, it is confined to one area. It belongs to a family of conditions called plasma cell neoplasms, which also includes multiple myeloma.

This article explains how a plasmacytoma is diagnosed, what the findings in your pathology report mean, and why follow-up after treatment is so important.

What are the types of plasmacytoma?

Plasmacytomas are named by where they develop, and the two types behave differently:

Solitary plasmacytoma of bone. This type develops inside a bone, most often in the spine, pelvis, ribs, or thigh bone. It is the more common type.
Extramedullary plasmacytoma. This type develops outside the bone, in soft tissue. About 8 in 10 develop in the head and neck, especially the nose, sinuses, throat, and voice box. Less often, they develop in the digestive tract or lungs.

A plasmacytoma can also develop in a person who already has multiple myeloma. In that situation, the plasmacytoma is part of the myeloma rather than a separate diagnosis.

What does solitary mean?

A plasmacytoma is called solitary only when tests show that the disease is limited to one place. This means there is no evidence of multiple myeloma elsewhere in the body. Before a plasmacytoma is called solitary, your doctors will usually arrange the following:

A bone marrow biopsy. This looks for abnormal plasma cells in the bone marrow away from the tumor.
Imaging. A PET-CT or MRI looks for other areas of disease in the bones.
Blood and urine tests. These measure abnormal antibody proteins and check blood counts, calcium levels, and kidney function.

The bone marrow result places the plasmacytoma into one of three groups, and this matters for your outlook:

Solitary plasmacytoma. No abnormal plasma cells are found in the bone marrow.
Solitary plasmacytoma with minimal marrow involvement. A small number of abnormal plasma cells, fewer than 10%, are found in the bone marrow. This group has a higher chance of later developing multiple myeloma.
Multiple myeloma. If 10% or more of the bone marrow cells are abnormal plasma cells, or there is damage to the bones, kidneys, or blood counts, the diagnosis is multiple myeloma instead.

What causes a plasmacytoma?

The cause of a plasmacytoma is not known. It develops when a single plasma cell acquires genetic changes that let it multiply without control. All the cells in the tumor are copies of that one cell, so they all make the same single type of antibody.

Plasmacytomas are more common in older adults, usually after age 50, and are more common in men. They are not inherited.

What are the symptoms of a plasmacytoma?

The symptoms depend on where the tumor is:

Plasmacytoma of bone. Pain in the affected bone is the most common symptom. The tumor can weaken the bone enough to cause a fracture. A tumor in the spine can press on the spinal cord or nerves and cause back pain, numbness, or weakness.
Extramedullary plasmacytoma. A mass in the nose or throat can cause a blocked nose, nosebleeds, a sore throat, difficulty swallowing, or a change in the voice.

A truly solitary plasmacytoma does not cause anemia, high calcium levels, or kidney problems. These findings suggest multiple myeloma and lead to further testing.

How is the diagnosis made?

A pathologist diagnoses it after examining a tumor biopsy under a microscope. For a bone tumor, the biopsy is usually taken with a needle guided by imaging. For a soft-tissue tumor, a small piece may be removed, or the whole mass may be removed in an excision.

Under the microscope, a plasmacytoma is made of sheets of plasma cells. The cells may look larger and more irregular than normal plasma cells, with prominent nuclei and visible nucleoli. Your report may call them atypical plasma cells.

Immunohistochemistry

Immunohistochemistry uses antibodies to show which proteins tumor cells make. For a plasmacytoma, it confirms that the cells are plasma cells and helps rule out a lymphoma, a different blood cancer that can contain plasma cells. Your report may include some of the following:

CD138, MUM1, and CD79a. Normal plasma cells make these proteins, and they are positive in a plasmacytoma.
CD56, CD117, and cyclin D1. Normal plasma cells do not make these proteins. When they are positive, they support the diagnosis of an abnormal plasma cell tumor.
CD20. This protein is usually negative in a plasmacytoma. A positive result would raise the possibility of a lymphoma with plasma cells.

Kappa and lambda light chains

Every antibody contains one of two kinds of light chain: kappa or lambda. In normal tissue, plasma cells are a mixture, with roughly twice as many making kappa as making lambda.

Because all the cells in a plasmacytoma come from one cell, they all make the same light chain. Pathologists test for this using in situ hybridization (ISH) or immunohistochemistry. Your report may describe the cells as kappa-restricted or lambda-restricted, confirming they are a single abnormal group rather than a normal reaction to inflammation.

Amyloid

The abnormal antibody pieces made by a plasmacytoma can sometimes build up in the tissue as a protein called amyloid. Your report may mention amyloid if it was seen. Pathologists confirm it with a special stain called Congo red.

Amyloid limited to the tumor does not usually cause problems elsewhere. If your doctors suspect that amyloid is building up in other organs, such as the heart or kidneys, they will arrange further tests.

Molecular testing

Molecular tests are not usually needed to diagnose a plasmacytoma. Doctors may perform them if the diagnosis is uncertain or if the condition is later found to be multiple myeloma, where certain genetic changes help predict the outlook.

What is the prognosis?

Treatment controls the tumor itself very well. Radiation therapy keeps the plasmacytoma from coming back in the same place in more than 9 out of 10 people. The main concern after treatment is the risk of later developing multiple myeloma, and this risk differs between the two types:

Solitary plasmacytoma of bone. About half of people develop multiple myeloma within 3 to 5 years after treatment. Over 10 years, this rises to roughly two-thirds or more.
Extramedullary plasmacytoma. The risk is much lower. Between about 1 in 10 and 1 in 3 people develop multiple myeloma, and many are cured by radiation alone.

The risk is higher when abnormal plasma cells are found in the bone marrow at diagnosis. It is also higher when the abnormal antibody protein in the blood does not disappear after treatment, and when the tumor is large. Regular follow-up allows any change toward multiple myeloma to be found early, and multiple myeloma is a treatable condition with many effective therapies.

What happens after the diagnosis?

Radiation therapy. Radiation to the tumor is the main treatment for both types of solitary plasmacytoma.
Surgery. Surgery may be used for an extramedullary plasmacytoma that can be removed easily, or to stabilize a bone that has broken or is at risk of breaking.
Systemic treatment. The drugs used to treat multiple myeloma are not routinely given for a solitary plasmacytoma. Your hematologist may discuss them if you are at particularly high risk of progression, but their benefit in this setting is not yet established.
Follow-up. Regular blood and urine tests monitor the abnormal antibody protein, along with periodic imaging. Follow-up continues for many years, because multiple myeloma can develop long after treatment.

Questions to ask your doctor

Is my plasmacytoma in bone or in soft tissue?
Was a bone marrow biopsy done, and did it show any abnormal plasma cells?
Have imaging and blood tests ruled out multiple myeloma?
Do the tumor cells make kappa or lambda light chains, and is the abnormal protein detectable in my blood?
Was amyloid found in my tissue sample?
What is my risk of developing multiple myeloma?
How often will I need blood tests and imaging, and for how long?