Tumors of the Skin in Dogs

The short version

Tumors are abnormal growths of cells. Tumors affecting the skin or the tissue just under the skin are the most commonly seen tumors in dogs. Skin tumors are diagnosed more frequently than other tumors in animals in part because they are the most easily seen tumors and in part because the skin is constantly exposed to many tumor-causing factors in the environment. Chemicals, solar radiation, and viruses are just some of the things that can cause skin tumors. Hormonal abnormalities and genetic factors may also play a role in the development of skin tumors.

All of the various layers and components of skin have the potential for developing distinctive tumors. Distinguishing a tumor from an inflammatory disease can sometimes be difficult. Tumors are usually small lumps or bumps, but they also can occur as hairless, discolored patches, rashes, or nonhealing ulcers. Because skin tumors are so diverse, identifying them should be left to a veterinarian.

Basal Cell Tumors and Carcinomas

Basal cells lie at the base of the top layer of the skin (the epidermis). A benign growth of these cells is a basal cell tumor. A malignant growth is a basal cell carcinoma.

Basal cell carcinomas are less common in dogs than in cats. They are also often called basosquamous cell carcinomas in dogs. They can appear almost anywhere on the body. These carcinomas may be flattened or raised above the skin surface. However, they spread, forming new ulcers. Consequently, surgical removal is the treatment of choice. These tumors generally occur in older dogs. Saint Bernards, Scottish Terriers, and Norwegian Elkhounds are most at risk. Unlike basal cell tumors, basal cell carcinomas can be found almost anywhere on the body. These tumors spread to neighboring skin but seldom spread to other organs. Surgical removal is the treatment usually recommended. When performing this surgery, the veterinarian will remove a sufficient amount of normal skin around the tumor to make certain that the entire tumor has been removed.

Benign Fibroblastic Tumors

Collagenous nevi are benign collections of fibrous proteins known as collagen. They are common in dogs. Generally collagenous nevi are found in middle-aged or older animals, most frequently on the legs, head, neck, and areas prone to trauma. They are flat to raised lumps that develop in the skin or fat beneath the skin. Surgical removal of both forms is generally effective. Infrequently, some may grow too large to be surgically removed.

A disorder called generalized nodular dermatofibrosis (dermatofibromas) is rarely seen in German Shepherds. Affected dogs have multiple collagenous nevi that are associated with kidney and uterine tumors. The skin tumors are recognized first, and kidney disease develops 3 to 5 years later. There is no known treatment to prevent the formation of the kidney tumors.

Skin tags are distinctive, benign, skin lumps on older dogs. These are common, may be single or multiple, and can develop in any breed, although large breeds may be at increased risk. Most commonly, skin tags look like extended stalk-like growths, often covered by a wart-like surface. Surgical removal is optional, but a biopsy is recommended to confirm the diagnosis. Dogs that develop one are likely to develop others.

Benign, Nonviral, Wart-like Tumors

These tumors look a lot like warts, but they are not caused by a virus like true warts (papillomas). These tumors are often easy to remove and there is little threat to the overall health of the dog. For information on viral warts, see below.

Epidermal hamartomas (nevi) are dark, pointy bumps on the skin. Occasionally they are arranged in a line. They are rare and found most often in puppies. The disease may be inherited in Cocker Spaniels. Some form pimples or dark, thick skin folds. They are benign, but their appearance is unpleasant, and they are prone to secondary bacterial infection. For these reasons, epidermal hamartomas are usually removed or treated. Smaller hamartomas can be surgically removed. Dogs with large or multiple hamartomas may respond to drug treatment.

Canine warty dyskeratomas are rare, benign tumors that grow near hair follicles or sweat glands. They are bumps with a dark dot in the center. How they are formed is not known. They are usually treated by surgical removal.

Blood Vessel Tumors

Blood vessel (vascular) tumors of the skin and soft tissues are growths that closely resemble blood vessels. Some forms are benign while others are highly malignant.

Hemangiomas are benign tumors of adult dogs. Many breeds (including Gordon Setters; Boxers; and Airedale, Scottish, and Kerry Blue Terriers) are considered to be at risk. These tumors are most common on the legs and trunk. Hemangiomas are single to multiple, circular, often compressible, red to black lumps and can look like a “blood blister.” Although they are benign, they tend to develop ulcers and some grow quite large. Because of this, and because it is important to identify whether the tumor is cancerous, they should be removed.

Cutaneous (skin) angiosarcomas (also known as angioendotheliomas) start out looking like benign hemangiomas but then progress to become malignant blood vessel tumors. They occur most often in dogs with short, often white coats, with high amounts of sun exposure. The breeds prone to sun-caused angiosarcomas are Whippets, Italian Greyhounds, white Boxers, and Pit Bull Terriers. Irish Wolfhounds, Vizslas, Golden Retrievers, and German Shepherds are also prone to develop these tumors, but not in response to sun exposure. In dogs, they most frequently develop on the underside of the trunk, hip, thigh, and lower legs. Small surface tumors are easily controlled with freezing (cryosurgery) or laser surgery as needed. Avoidance of further sun exposure may reduce the development of new tumors; however, more tumors can appear over several years.

Hair Follicle Tumors

Trichilemmomas are rare, benign, hair follicle tumors of dogs, most commonly found on the head. Poodles may be predisposed. They appear as firm, oval masses, 0.4 to 2.75 inches (1 to 7 centimeters) in diameter that are compact but gradually grow.

Trichoepitheliomas are multiple small lumps in which an entire hair follicle is filled with condensed, yellow, granular, “cheesy” material. They occur mostly on the skin of the face. They can be either benign or malignant.

In dogs, they can occur at any age but are found most commonly during late middle age. Many breeds are predisposed, including Basset Hounds, Bull Mastiffs, Irish Setters, Standard Poodles, English Springer Spaniels, and Golden Retrievers. Tumors can develop anywhere on the body but most commonly on the trunk in dogs. Benign forms appear as cysts in or under the skin. Growth of the cysts or self-trauma may cause skin ulcers. Treatment is by surgical removal. However, dogs that develop one such tumor are prone to develop more at other sites. This is especially true for Basset Hounds and English Springer Spaniels.

Frequently asked questions

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