Pleomorphic Lipoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 24, 2026

A pleomorphic lipoma is a noncancerous tumor made of mature fat cells, long, thin spindle cells, and large cells with multiple nuclei. It is a type of lipoma, and it does not spread to other parts of the body.

Pleomorphic lipoma usually develops just under the skin of the back of the neck, the upper back, or the shoulder. It occurs mostly in men between about 45 and 70 years old. Removing the lump is normally curative.

This tumor is closely related to spindle cell lipoma. The two share the same genetic change and are considered part of one group, and many tumors show features of both. They are not the same diagnosis. A pleomorphic lipoma is distinguished by its large cells containing several nuclei.

This article explains how a pleomorphic lipoma is diagnosed and what the immunohistochemistry results and margins in your pathology report mean.

Does “pleomorphic” mean cancer?

No. Pleomorphic means that the cells vary in size and shape. In many tumors, this variation is a sign of cancer, which is why the word can be alarming when it appears on a report.

In a pleomorphic lipoma, the varied cells sit in an otherwise noncancerous tumor, and it lacks other features of cancer. There are almost no dividing cells, no dead tumor tissue, and no growth into surrounding structures.

A different tumor called pleomorphic liposarcoma is a cancer, and its name is similar. The two are distinguished under the microscope and with additional tests, as described below.

What are the symptoms of a pleomorphic lipoma?

A pleomorphic lipoma usually appears as a firm or rubbery lump under the skin that grows slowly over months or years. Most are painless.

A large lump on the back or neck can cause discomfort from pressure, particularly when lying down or leaning against a chair. These tumors rarely cause other symptoms.

What causes a pleomorphic lipoma?

Pleomorphic lipoma is caused by the loss of a small part of chromosome 13, in a region known as 13q14. This region contains a gene called RB1, which normally helps control cell growth.

This change happens in the tumor cells during a person’s lifetime. It is not inherited, and it does not carry the risks associated with the inherited condition that affects the same gene in the eye. The same change is found in several other noncancerous tumors, including spindle cell lipoma and cellular angiofibroma.

How is the diagnosis made?

A pathologist diagnoses pleomorphic lipoma after examining tumor tissue under a microscope. The tissue may come from a biopsy or from surgery that removes the whole lump.

Under the microscope, a pleomorphic lipoma contains mature fat cells, bland spindle cells, and thick rope-like bands of collagen. What sets it apart are large cells with several nuclei arranged in a circle around the cell rim. Pathologists describe these as floret-like giant cells because the nuclei resemble flower petals.

The stroma, the supporting tissue that holds the tumor together, often looks bright pink and may contain gel-like myxoid areas. The proportions of fat, spindle cells, and giant cells vary widely from one tumor to another.

Immunohistochemistry

Immunohistochemistry is a test that uses antibodies to show which proteins cells make. For a pleomorphic lipoma, it confirms the diagnosis and rules out tumors that can look similar. Your report may include some of the following:

CD34. The spindle cells and giant cells are strongly positive. This is the most consistent result in a pleomorphic lipoma.
RB1. This protein is lost in most pleomorphic lipomas, reflecting loss of the RB1 gene. Your report may describe this as loss of nuclear RB1 expression.
S100. The fat cells are positive, but the spindle cells are negative. A positive result in the spindle cells would suggest a nerve sheath tumor instead.
STAT6. This protein is negative. A positive result would suggest a solitary fibrous tumor, which is also positive for CD34.
Desmin and SMA. These muscle proteins are usually negative.

MDM2 testing

Some fatty tumors that can look similar have extra copies of a gene called MDM2, which is called amplification. These include atypical lipomatous tumor and, in some cases, cancers made of fat.

Normal cells and the cells in a pleomorphic lipoma have two copies of MDM2. Your pathologist may order fluorescence in situ hybridization (FISH) to count these copies. A normal result supports the diagnosis of a pleomorphic lipoma.

What other tumors can look like a pleomorphic lipoma?

Because the cells in this tumor vary in size and shape, it can resemble several other tumors. The tests above are used to tell them apart:

Spindle cell lipoma—a closely related noncancerous tumor without the large multinucleated cells.
Atypical spindle cell lipomatous tumor. A related tumor that shares many features but can come back after incomplete removal. It does not spread to other parts of the body.
Atypical lipomatous tumor. A fatty tumor that grows into surrounding tissue and has extra copies of MDM2.
Pleomorphic liposarcoma. A cancer made of fat. Unlike a pleomorphic lipoma, it contains many dividing cells and often dead tumor tissue, and it grows into surrounding tissue.

Tumor size and margins

Tumor size is the greatest dimension of the pleomorphic lipoma, measured in centimeters (cm). Most are small, and size does not change how this tumor behaves.

A margin is the edge of tissue cut by the surgeon. A negative margin means no tumor cells are seen at the cut edge, and a positive margin means tumor cells reach it. For a pleomorphic lipoma, a positive margin does not carry the meaning it does for cancer. It slightly raises the chance that the lump comes back, and further surgery is usually not needed.

What is the prognosis?

The outlook is excellent. A pleomorphic lipoma is noncancerous, does not spread, and does not turn into cancer. Simple removal is curative for almost everyone, and the tumor rarely comes back even when a margin is positive.

Routine follow-up imaging is not needed. Your doctor may ask you to return if a new lump appears or if the area changes.

What happens after the diagnosis?

Most pleomorphic lipomas are removed in a minor procedure, often under local freezing. Doctors usually recommend removal when the diagnosis is uncertain, the lump is large or growing, or it causes discomfort.

If the whole lump was already removed when the diagnosis was made, no further treatment is needed. If only a biopsy was done and the lump is small and comfortable, watching it is a reasonable option.

Questions to ask your doctor

Was the whole lump removed?
Does my report describe a pleomorphic lipoma or a spindle cell lipoma?
Did the report mention loss of RB1 or a positive CD34 result?
Was MDM2 testing done, and was it needed?
How was this separated from a cancerous fatty tumor?
Is there a chance this lump could come back?
Do I need any follow-up?