Round Cell Tumors in Dogs

By Malcolm Weir, DVM, MSc, MPH; Robin Downing, DVM, DAAPM, DACVSMR, CVPP, CRPP

What are round cell tumors?

Round cell tumors are among the most common skin tumors in dogs, and they typically form just under the skin, although they may change the surface of the skin above them. Some round cell tumors are more worrisome than others. It is impossible to diagnose any of them without a veterinary pathologist analyzing a tissue sample of the tumor under a microscope.

What are the different types of round cell tumors, and how are they treated?

Mast cell tumor
Mast cell tumors (MCT) are the most common round cell tumor in dogs. They account for 7-21% of all skin tumors in dogs. They can occur in any breed, age, or gender, but mast cell tumors tend to occur in older dogs and are more common in the Boxer, Rhodesian Ridgeback, Boston Terrier, Pitbull Terrier, Shar-Pei, and Weimaraner. MCTs start in the skin, and if they grow aggressively, they can spread to the internal organs, causing systemic disease. If caught early, surgery may be the only treatment needed. Tumor analysis by a pathologist will allow your veterinarian to know if additional treatment, such as chemotherapy or radiation therapy, is required. See the handout "Mast Cell Tumors in Dogs" for more information.

Histiocytomas are benign tumors that tend to arise on the head, neck, legs, and feet. They look like mast cell tumors, although they tend to be smaller and are less common. While they can occur in dogs of any age, they are most common in younger dogs. Surgery is the treatment of choice. See the handout "Cutaneous Histiocytoma in Dogs" for more information.

Plasmacytomas are potentially malignant skin tumors that occur in older dogs. They are small, raised, and red in appearance. Cocker Spaniels seem to develop plasmacytomas more than other breeds, with the Yorkshire Terrier, Boxer, West Highland White Terrier, Kerry Blue Terrier, Scottish Terrier, German Shepherd, and Airedale Terrier also reported as being predisposed. Surgery is the treatment of choice. See the handout "Plasmacytic Tumors" for more information.

Transmissible venereal tumor (TVT)
Transmissible venereal tumors (TVT) are sexually transmitted tumors that pass easily between breeding animals. They look like small, red cauliflower heads. TVTs tend to occur around the genitals, nose, and mouth and bleed if disturbed. Surgery may be the only treatment needed, but if the tumors recur, chemotherapy may be indicated. See the handout “Transmissible Venereal Tumor” for more information.

Cutaneous lymphoma
Cutaneous lymphoma is a rare form of the relatively common canine cancer lymphoma. Cutaneous lymphoma tends to occur in older dogs and generally develops in multiples. Surgical removal and analysis by a pathologist provide a definitive diagnosis of cutaneous lymphoma. The pathologist’s examination will guide treatment. See the handout "Cutaneous Lymphoma in Dogs" for more information.

Should I be worried about long-term issues from my dog’s round cell tumor?

When caught early, most round cell tumors are removed easily, and surgery is generally curative. The most important take-home message is to be vigilant and to have any skin lumps or bumps assessed by your veterinarian promptly.

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