Round Cell Tumors in Dogs

My young Labrador retriever developed a raised, red tumor on his rear leg. My veterinarian removed it, had it analyzed, and reported to me that it was a histiocytoma. She said it was a type of round cell tumor that dogs can develop. 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 the help of a surgical pathologist who analyzes tissue under a microscope.

What are the different types of round cell tumors? How are they treated?

• Mast cell tumor

Mast cell tumors are the most common round cell tumor in the dog. They account for 20-25% 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 breeds descended from the bulldog (e.g., Boxers). Mast cell tumors start in the skin, but if they grow aggressively, they can spread to the internal organs causing systemic disease. If caught early, surgery alone may be all that is needed. Analysis of the tumor by a pathologist will allow your veterinarian to know if additional treatment like chemotherapy or radiation therapy is needed. See handout "Mast Cell Tumors in Dogs" for further information.

• Histiocytoma

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

• Plasmacytoma

Plasmacytomas are potentially malignant skin tumors that tend to occur in older dogs. They are small, raised, and red in appearance. Males are slightly more likely than females to develop plasmacytomas. Cocker spaniels seem to develop plasmacytomas more than other breeds. Surgery is the treatment of choice. See handout "Plasmacytic Tumors" for further information.

• Transmissible venereal tumor (TVT)

Transmissible venereal tumors 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, or mouth, and they bleed if disturbed. Surgery may be all the treatment that is needed, but if the tumors recur, chemotherapy may be indicated.

• 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 provides a definitive diagnosis of cutaneous lymphoma. Treatment will be guided by the pathologist’s analysis. See handout "Cutaneous Lymphoma in Dogs" for further 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.