Benign Fibrous Skin Tumors

By Ryan Llera, BSc, DVM; Debbie Stoewen DVM, MSW, RSW, PhD; Christopher Pinard, DVM

Fibrous tissue consists of bundles of collagen (protein) fibers that lie between rows of connective tissue cells. This 'connective tissue' is present throughout the body connecting and supporting organs and systems. The basic cell responsible for the production of collagen and other fibers is called a fibroblast.

What are fibrous tumors?dog_nodular_dermatofibrosis_2018-01

Chronic irritation and trauma can result in the formation of fibrous tissue masses called fibromas. Fibromas are mostly slow-growing and benign (non-cancerous).

Other common names for non-cancerous fibrous growths include hamartoma, fibroepithelial polyp, or skin tag. A hamartoma is a nodular disorganized proliferation of various kinds of skin cells. Some involve hair follicles and glands, and these are called fibroadnexal hamartomas.

Nodular dermatofibrosis is a syndrome in which multiple nodules form on and in the skin, most commonly on the hind limbs. These nodules are not malignant but can develop in conjunction with other types of cancers found elsewhere in the body. Nodular dermatofibrosis is rare in dogs but is most commonly seen in German Shepherds with tumors of the kidney or uterus in intact (unspayed) females.

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 at pressure points (e.g., the elbows), can cause changes in the way the skin rebuilds itself leading to these benign tumors.

What are the signs of these types of tumors?

These tumors typically grow in areas of increased pressure or repeat trauma. They appear as plaques, nodules, or small masses on or in the skin. They are typically slow-growing and not bothersome to your pet.

How are these tumors diagnosed?

If the tumors are nodular or mass-like, a biopsy may be required to obtain an accurate diagnosis. A biopsy is a surgical excision of a piece of the tumor. Pieces of the tumor are then examined by a veterinary pathologist under the microscope. This is called histopathology. Histopathology is not only helpful to make a diagnosis but can indicate how the tumor is likely to behave. Cytology (where some cells are acquired through a fine needle aspirate) is often unrewarding as tumors like this do not shed their cells very readily.

How do these tumors typically progress?

As these tumors are benign in nature, they do not spread to the surrounding tissues or elsewhere in the body. They may, however, continue to grow to the point of becoming irritating or bothersome, but generally are slow growing. The nodules of nodular dermatofibrosis may continue to appear over time, making it seem as if it is spreading, but each nodule is independent of the other.

What are the treatments for this type of tumor?

If the tumors grow to become bothersome for your pet, surgical removal may be recommended. In most cases, no treatment is necessary other than finding ways to control the underlying skin disease or allergy. Because nodular dermatofibrosis is a symptom of a cancer elsewhere in the body, they cannot be cured by surgical removal. Treatment for these nodules involves treatment of the underlying kidney or uterine tumor.

Is there anything else I should know?

If your pet has developed these tumors over the elbows (at pressure points), your pet may benefit from a bed to alleviate elbow discomfort. If your pet has developed these tumors as a result of repeated self-trauma due to allergies or skin disease, they may not resolve without attempting to control the underlying problem.

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