Benign Fibrous Skin Tumors

What are benign fibrous skin tumors?

Benign fibrous skin tumors are uncommon tumors composed of a type of skin cell called a fibroblast that normally produces collagen and other fibers that keep the skin strong yet flexible.

Benign fibrous skin tumors develop when fibroblasts overgrow (multiply uncontrollably), producing additional collagen and fibers in one area, creating a mass. These tumors, commonly called fibromas, are mostly slow-growing and benign (non-cancerous). Other common names for benign fibrous skin tumors include fibroepithelial polyp and skin tag.

A less common condition called nodular dermatofibrosis causes multiple nodules to form on and in the skin, usually on the hind limbs. These nodules resemble fibromas and are benign, but they are often associated with other cancers elsewhere in the body. Nodular dermatofibrosis is rare in dogs but is seen most often in German shepherds with kidney tumors or uterus tumors (in unspayed female dogs).

What causes these tumors?

The reason why a particular pet may develop this, or any tumor or cancer, is not straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary. Chronic trauma, especially over pressure points such as the elbows, can cause changes in the way the skin rebuilds itself and may lead to the development of these benign tumors.

What are the signs of benign fibrous skin tumors?

These tumors typically develop in areas of increased pressure or repeated trauma. They appear as plaques, nodules, or small masses on or beneath the skin. They are usually slow-growing and do not bother your pet.

How are benign fibrous skin tumors diagnosed?

If the growth is nodular or forms a distinct mass, a biopsy may be needed to make an accurate diagnosis. A biopsy involves surgically removing a piece of the tumor so it can be examined under a microscope by a veterinary pathologist. This examination is called histopathology and helps identify the tumor and predict how it is likely to behave.

Your veterinarian may first recommend cytology, in which a small sample of cells is collected using fine-needle aspiration (FNA). However, these tumors do not shed cells readily, so cytology is often not helpful, and a biopsy is usually needed to make a diagnosis.

How do benign fibrous skin tumors typically progress?

Because these tumors are benign, they do not spread to the surrounding tissues or elsewhere in the body. However, they may continue to grow slowly and eventually become irritating. With nodular dermatofibrosis, additional nodules may develop over time. Although this can make the condition appear to be spreading, each nodule develops independently.

How are benign fibrous skin tumors treated?

If the tumors become bothersome because of their size or location, surgical removal may be recommended. In most cases, no treatment is needed other than managing the underlying cause of repeated skin irritation, such as allergies or other skin diseases. Because nodular dermatofibrosis is a symptom of a cancer elsewhere in the body, surgically removing the skin nodules alone will not cure the condition. Treatment focuses on the underlying kidney or uterine tumor.

If these tumors develop over the elbows or other pressure points, your pet may benefit from a well-padded bed to alleviate elbow discomfort. If the tumors develop because of repeated self-trauma associated with allergies or skin disease, they may continue to recur unless the underlying problem is controlled.