Fibrous Histiocytoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 25, 2026

A fibrous histiocytoma is a common noncancerous skin tumor. It is made of fibroblasts, which build connective tissue, histiocytes, which are a type of immune cell, and collagen. It develops in the dermis, the layer of skin beneath the surface.

Another name for this tumor is dermatofibroma, and the two terms mean the same thing. A fibrous histiocytoma does not spread to other parts of the body and does not turn into cancer.

This article explains how a fibrous histiocytoma is diagnosed, what the subtype in your report means, and what happens after the diagnosis.

What are the symptoms of a fibrous histiocytoma?

A fibrous histiocytoma usually appears as a small, firm bump in the skin, most often on a leg. The color ranges from pink to brown. Most are painless and grow slowly, and many stay the same size for years.

A characteristic finding is that pinching the skin on either side of the bump makes it dimple inward instead of rising. Some fibrous histiocytomas are tender, and some itch.

What causes a fibrous histiocytoma?

For most people, doctors find no cause. Some appear after minor trauma such as a scratch, an insect bite, or long-standing inflammation of the skin. Many develop with no preceding event.

Fibrous histiocytoma is not inherited, and having one does not raise your risk of skin cancer.

How is the diagnosis made?

A pathologist usually makes the diagnosis after the whole bump is removed and examined under a microscope. Sometimes a small sample is taken first in a biopsy.

Under the microscope, a fibrous histiocytoma is made of spindle cells arranged in a swirling pattern that pathologists call storiform, meaning it resembles a woven mat. At the edge of the tumor, cells surround and trap bundles of normal collagen, a key feature for this diagnosis.

The skin above the tumor is often thickened, and the pigment-producing layer may be more active, which explains the brown color some of these bumps have. Mitotic figures, which are cells in the process of dividing, may be present but are not numerous.

What are the subtypes of fibrous histiocytoma?

Your report may name a subtype. These describe what the tumor looks like under the microscope, and all of them are noncancerous:

Cellular. Contains more tightly packed cells and more dividing cells than usual. It is often found on the legs and may be larger. This subtype comes back more often than the usual type.
Deep. Extends below the skin into the fat or deeper tissue. These tumors are often larger and can be mistaken for a soft tissue cancer before examination.
Aneurysmal. Contains large blood-filled spaces, which can make the bump grow quickly and look red or blue. Rapid growth and color often raise concern before a diagnosis is made.
Lipidized. Contains fat-filled cells, giving the bump a yellowish color. It is most often found on the lower legs, usually around the ankle.
Clear cell. Contains cells with pale, clear centers. It can resemble other clear-cell tumors, so additional tests are sometimes used.
Palisaded. Cells line up in rows around areas of collagen, which can resemble a nerve sheath tumor. A negative S100 stain separates the two.
Myxoid. Contains a gel-like background called myxoid material, which can make the bump feel soft.
Epithelioid. Contains round cells that resemble epithelial cells and often form a raised, polyp-like bump. Most of these tumors have a change in the ALK gene, and many experts now consider this a separate tumor rather than a subtype.

Immunohistochemistry

Immunohistochemistry is a test that uses antibodies to show which proteins cells make. It is not needed for most fibrous histiocytomas, since the microscopic appearance is usually enough. When it is used, your report may include some of the following:

CD34. This protein is negative in a fibrous histiocytoma, or positive only at the edge. It is the main stain used to separate this tumor from dermatofibrosarcoma protuberans, described below.
Factor XIIIa and CD163. These support a fibrous histiocytoma, although the results can vary.
ALK. This protein is positive in the epithelioid subtype and negative in the others.
S100. This protein is negative. It is used to rule out a nerve sheath tumor or melanoma.

How is this different from dermatofibrosarcoma protuberans?

Dermatofibrosarcoma protuberans is a different skin tumor that grows into surrounding tissue and often recurs after surgery. Despite the similar name, it is not related to dermatofibroma.

The two can look alike under the microscope, particularly when a fibrous histiocytoma is cellular or extends deeply. The CD34 stain usually settles the question, since it is strongly positive throughout a dermatofibrosarcoma protuberans and negative or only focally positive in a fibrous histiocytoma. Trapped collagen at the edge also favors a fibrous histiocytoma.

Margins

A margin is the edge of tissue cut by the surgeon. Your report may say that the tumor extends to a margin.

For a fibrous histiocytoma, a positive margin does not carry the meaning it does for cancer. Many of these bumps are removed by shaving them off at skin level, so tissue at the base commonly remains. This is usually why a bump comes back, and further surgery is often unnecessary unless the bump is bothersome.

What is the prognosis?

The outlook is excellent. A fibrous histiocytoma is noncancerous, does not spread, and does not become cancer over time.

A tumor that is not completely removed can grow back in the same place. This happens more often with the cellular, deep, and aneurysmal subtypes. A bump that comes back is still noncancerous.

What happens after the diagnosis?

No treatment. A fibrous histiocytoma that causes no symptoms can be left alone. Many people never need anything done.
Removal. The bump may be removed if it is painful, growing, repeatedly irritated by shaving or clothing, or bothersome in appearance.
Complete removal. For the cellular and deep subtypes, removing the tumor with a rim of normal tissue lowers the chance it comes back.
Follow-up. Routine follow-up is not needed. Your doctor may ask you to return if the bump changes or a new one appears.

Questions to ask your doctor

Was the whole tumor removed, and did it extend to a margin?
Which subtype does my report describe, and does it change anything?
Is there a chance the bump will come back?
If it comes back, what’s the next step?
Were any additional stains needed to confirm the diagnosis?
Should any other bumps on my skin be checked?