Your pathology report for dermatofibroma

by Glenda Wright MBBCh and Allison Osmond MD FRCPC
September 14, 2025

A dermatofibroma, also called a benign fibrous histiocytoma, is a very common non-cancerous skin tumour. It develops from the dermis, the thick layer of connective tissue just below the surface of the skin.

Dermatofibromas usually appear as small, firm bumps that may feel hard to the touch. They are most often found on the arms, legs, and trunk, though they can occur anywhere on the body.

These tumours are harmless, grow slowly, and do not turn into cancer. However, they may cause cosmetic concern or mild symptoms such as itching or tenderness.

What are the symptoms of a dermatofibroma?

Most dermatofibromas cause no symptoms and are noticed only because of their appearance or feel.

When symptoms occur, they may include:

• A firm, round bump less than 1 cm across.

• Colour ranging from pink to brown.

• A central dimple when the skin around the lesion is pinched (the “dimple sign”).

• Mild tenderness or itching in some cases.

What causes a dermatofibroma?

The exact cause is not always known. Some dermatofibromas appear to develop after minor skin injury, such as an insect bite, thorn prick, or other trauma. Others may arise without any clear reason.

Dermatofibromas are more common in adults, especially women, and they are thought to be a type of reactive growth in response to inflammation in the skin.

How is this diagnosis made?

Doctors may suspect a dermatofibroma based on its typical look and feel. However, because other skin tumours (some benign and some cancerous) can look similar, the diagnosis often requires examination of the tissue by a pathologist.

• Sometimes, a biopsy is done first, where a small piece of the lesion is removed.

• More often, the entire lesion is excised (removed) and sent for examination under the microscope.

Pathologists then look for microscopic features that confirm the diagnosis. In some cases, this can be difficult if only part of the lesion is sampled, so a complete excision helps ensure an accurate diagnosis.

What does a dermatofibroma look like under the microscope?

Under the microscope, a classic dermatofibroma has several distinctive features:

• The tumour grows in the dermis and sometimes extends slightly into the fatty tissue beneath the skin.

• It is made up of spindle-shaped cells (long, narrow cells) and other connective tissue cells mixed with immune cells, including lymphocytes and macrophages.

• A hallmark feature is collagen trapping, where bundles of collagen (the protein that gives skin its strength) are caught at the edges of the tumour. These trapped bundles can form round structures called collagen balls.

• The tumour cells often grow in a storiform pattern, which looks like swirls or the spokes of a wheel.

• The skin above the tumour may thicken, with elongated rete ridges (downward extensions of the epidermis).

• Sometimes, the tumour stimulates growth of nearby hair follicles, sebaceous glands, or pigment-producing cells.

Over time, older dermatofibromas may develop more fibrous (scar-like) tissue and fewer cells.

Can a dermatofibroma turn into cancer?

No. Dermatofibromas are benign tumours and do not become cancerous over time.

However, if a dermatofibroma is not completely removed, it may grow back. Rarely, certain subtypes can show more aggressive local growth, but they are still considered benign and do not spread to other parts of the body.

Subtypes of dermatofibroma

Pathologists recognize several subtypes of dermatofibroma based on microscopic features. These subtypes are all non-cancerous but may look unusual or mimic other tumours.

Cellular dermatofibroma

Cellular dermatofibroma is made up of tightly packed spindle-shaped cells that grow in bundles, a pattern known as fascicular growth. It often extends deeper into the fat beneath the skin and can sometimes contain small areas of cell death, although this is not a sign of cancer.

Aneurysmal dermatofibroma

Aneurysmal dermatofibroma contains blood-filled spaces and deposits of hemosiderin, which is a pigment left behind when blood breaks down. These features can make the lesion look like a tumour of blood vessels, but it is still benign.

Atypical dermatofibroma

Atypical dermatofibroma has some cells that look unusual in size or shape, known as pleomorphic cells. In some cases, abnormal patterns of cell division may be seen. Despite these atypical features, it remains a benign tumour.

Deep fibrous histiocytoma

Deep fibrous histiocytoma is a rare subtype that occurs entirely in deeper tissues, such as fat or soft tissue beneath the skin. Unlike superficial dermatofibromas, it usually has well-defined borders and a swirling storiform growth pattern under the microscope.

What happens after the diagnosis?

Most dermatofibromas require no further treatment once diagnosed. They are harmless and can be left alone unless they:

• Cause discomfort (pain, tenderness, or itching).

• Interfere with daily activities.

• Raise cosmetic concerns.

• Cannot be distinguished from other tumours without removal.

When removed, recurrence is rare unless part of the lesion remains.

Prognosis

The prognosis for dermatofibroma is excellent. All forms are benign, meaning they do not spread to other parts of the body or become cancerous. Classic dermatofibromas usually remain stable or slowly shrink over time. Subtypes such as cellular, aneurysmal, atypical, or deep fibrous histiocytoma may grow larger or recur locally if not completely removed, but even these do not metastasize (spread).

Questions to ask your doctor

• Do I need to have the entire tumor removed, or can it be safely monitored?

• If I have it removed, what are the chances it will grow back?

• Has the entire tumor been removed?