Dermatofibroma

by Glenda Wright MBBCh and Allison Osmond MD FRCPC
November 20, 2024

A dermatofibroma is a common and benign (noncancerous) skin tumour that develops from a layer of connective tissue called the dermis in the skin. It often appears as a small, firm bump that may feel hard to the touch and is usually found on the arms, legs, or trunk. Dermatofibromas are harmless and typically do not cause any health problems, although they may occasionally be itchy or tender. These tumours are made up of a mix of connective tissue cells and immune cells. Another name for a dermatofibroma is benign fibrous histiocytoma.

What does a dermatofibroma look like?

Most dermatofibromas grow slowly, have a pink to brown colour, and are usually small and painless.

What causes a dermatofibroma?

Some dermatofibromas appear to develop after trauma, an insect bite, or prolonged inflammation of the skin. However, most tumours develop without preceding events, and the cause remains unknown.

Can a dermatofibroma turn into cancer over time?

No. A dermatofibroma is a non-cancerous tumour that will not become cancerous over time. However, tumours that are not entirely removed can re-grow.

How is this diagnosis made?

The diagnosis is usually made after the entire tumour has been surgically removed and sent to a pathologist for examination under a microscope.  Sometimes, your doctor will remove a small piece of the tumour in a procedure called a biopsy before removing the entire tumour.

However, it can be very challenging for a pathologist to make the diagnosis of dermatofibroma after only examining a small piece of the tumour because there are other types of tumours that can look very similar to dermatofibroma. In this case, your pathologist may offer a list of possible diagnoses which includes dermatofibroma. A final diagnosis will be given after the entire tumour is removed and examined.

Microscopic features of this tumour

Classic dermatofibroma is a benign tumour that typically grows in a deep layer of the skin, called the dermis. It often extends slightly into the fatty tissue beneath the skin and is surrounded by a mild inflammation caused by lymphocytes, a type of white blood cell.

Under the microscope, classic dermatofibroma contains a variety of cell types, including spindle-shaped cells, which are long and narrow, and rounder cells. These tumour cells are mixed with reactive cells, including dermal dendrocytes, which help regulate immune responses, and macrophages, which are cells that clean up debris and fight infections. A distinctive feature of this tumour is the trapping of collagen, the protein that gives the skin strength and structure, at the edges of the tumour. This trapped collagen forms small round structures called “collagen balls.” In some cases, the tumour cells grow in a swirling pattern known as a storiform pattern.

The skin above the tumour often thickens, with elongated and darkened projections called rete ridges. Sometimes, the tumour may stimulate the growth of hair follicles or sebaceous glands, which produce oil for the skin. It may also lead to an increase in melanocytes, the cells that produce pigment, in the area where the epidermis meets the tumour.

As the tumour ages, its microscopic appearance changes. Early lesions tend to have more cells, while older lesions become more fibrotic, meaning they develop more scar-like tissue, and contain fewer macrophages.

Dermatofibromas can show many additional patterns under the microscope, such as clear cells, cells with a granular appearance, or cells with vacuoles, which are small pockets inside the cells. These patterns do not affect the behaviour of the tumour and are simply variations in appearance. Multiple patterns can sometimes appear in the same tumour.

Less common histologic subtypes of dermatofibroma

Some dermatofibromas exhibit distinct microscopic characteristics known as histologic subtypes, which set them apart from the classic type. These subtypes include cellular dermatofibroma, aneurysmal dermatofibroma, atypical dermatofibroma, and deep fibrous histiocytoma. Below is a description of each subtype.

Cellular dermatofibroma

Cellular dermatofibroma is characterized by tightly packed spindle-shaped cells that grow in bundles, a pattern known as a fascicular growth pattern. Spindle-shaped cells are long and narrow, resembling a stretched-out oval. These cells have fine chromatin, which is the material inside the nucleus of the cell that carries genetic information, and pale pink cytoplasm, which is the substance surrounding the nucleus. This subtype often extends into the fatty tissue beneath the skin. In about 10% of cases, small areas of necrosis, which means cell death, may be seen. However, this is not a sign of cancer.

Aneurysmal dermatofibroma

Aneurysmal dermatofibroma contains blood-filled spaces and deposits of hemosiderin, a substance that forms when blood breaks down. These features can make the lesion look like a vascular tumour, which is a tumour made of blood vessels. Like the classic dermatofibroma, this subtype has a mix of cell types, including spindle cells, large cells called Touton-type giant cells, lymphocytes (a type of white blood cell), and macrophages (cells that remove debris and fight infection). The skin overlying the lesion is often thickened, a feature called epidermal hyperplasia, and collagen bundles, which are thick fibres that provide strength to the skin, may be trapped around the edges of the lesion.

Atypical dermatofibroma

Atypical dermatofibroma contains scattered cells with unusual shapes, called atypical or pleomorphic cells. Atypical cells look different from normal cells in size, shape, or structure. In some cases, atypical mitoses, which are abnormal patterns of cell division, and areas of necrosis may be seen. The remaining tumour cells may resemble those found in a cellular or classic dermatofibroma, depending on the case.

Deep fibrous histiocytoma

Deep fibrous histiocytoma is a subtype that occurs entirely in the deeper layers of tissue, such as the fat and soft tissue beneath the skin. Unlike superficial dermatofibromas, these tumours are well-circumscribed, meaning they have a clearly defined border. They also show a uniform appearance under the microscope, with cells arranged in a storiform growth pattern. A storiform pattern looks like the spokes of a wheel or a swirling pattern when viewed under the microscope.