Spindle Cell Lipoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 24, 2026

A spindle cell lipoma is a noncancerous tumor made of mature fat cells, long, thin cells called spindle cells, and thick bands of collagen. It is a type of lipoma, and it does not spread to other parts of the body.

Spindle cell 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 pleomorphic lipoma. The two share the same genetic change and are considered part of one group, so your report may describe features of both. They are not the same diagnosis, and pleomorphic lipoma also contains large cells with several nuclei.

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

What are the symptoms of a spindle cell lipoma?

A spindle cell lipoma usually appears as a firm or rubbery lump under the skin that grows slowly over months or years. Most are painless and are noticed because of their size or position.

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

What causes a spindle cell lipoma?

Spindle cell 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. When one copy is lost, the cells that make up this tumor begin to grow.

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 pleomorphic lipoma and cellular angiofibroma.

How is the diagnosis made?

A pathologist diagnoses spindle cell 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 spindle cell lipoma contains three parts in varying proportions. These are mature fat cells that look like normal fat, bland spindle cells arranged in short bundles, and thick, rope-like bands of collagen. The collagen is a very useful clue for this diagnosis, and your report may describe it as ropey collagen.

The stroma, the supporting tissue that holds the tumor together, often looks bright pink under the microscope and may contain gel-like myxoid areas. Some spindle cell lipomas contain very little fat, and these can be harder to recognize, which is one reason additional tests are performed.

Immunohistochemistry

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

CD34. The spindle cells are strongly positive. This is the most consistent result in a spindle cell lipoma.
RB1. This protein is lost in most spindle cell 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 in a spindle cell lipoma.

MDM2 testing

Some fatty tumors that can look similar have extra copies of a gene called MDM2, which is called amplification. The most important of these is atypical lipomatous tumor, which can come back after surgery.

Normal cells and the cells in a spindle cell 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 spindle cell lipoma.

What other tumors can look like a spindle cell lipoma?

Because a spindle cell lipoma contains fat, spindle cells, and collagen, it can resemble several other tumors. The tests above are used to tell them apart:

Pleomorphic lipoma. A closely related noncancerous tumor that also contains large cells with several nuclei arranged like the petals of a flower.
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.
Solitary fibrous tumor. A tumor that is also positive for CD34 but is positive for STAT6 as well.

Tumor size and margins

Tumor size is the greatest dimension of the spindle cell 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 spindle cell 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 spindle cell 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 spindle cell 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 tumor is small and comfortable, watching it is a reasonable option.

Questions to ask your doctor

Was the whole lump removed?
Did the report mention loss of RB1 or a positive CD34 result?
Was MDM2 testing done, and was it needed?
Does my report describe a spindle cell lipoma or a pleomorphic lipoma?
Is there a chance this lump could come back?
Do I need any follow-up?
Should any other lumps I have be checked?