Canine Mast Cell Tumors in Dogs

Mast cells are white blood cells involved in allergic response and inflammation. They house a substance containing histamine and heparin that can cause systemic complications when released into the bloodstream. These include allergic reactions, low blood pressure, gastrointestinal problems, and slow blood clotting in the area of the tumor. Mast cell tumors originate in the skin, then migrate to the local lymph node. From there, these tumors usually move to the liver and spleen. They can aggressively attack other body areas, such as the GI tract and bone marrow.

Mast cell tumors are the most common cutaneous tumors in dogs, accounting for 20% of all diagnosed skin tumors. These masses can occur on any part of the body, are very unpredictable, and can vary in appearance. They can often be successfully treated with removal and other therapies, but can reoccur in more progressed cases.

The tumors seen in this disease can vary in appearance considerably, and include:

• Solitary raised lumps or bumps under the skin
• Lumps are swollen, red, or ulcerated
• Lumps that are covered with hair or hairless
• Itchy lumps
• Multiple skin masses
• Tumor that increases, then decreases in size, sometimes by tumor agitation
• Masses with a stalk
• Painful tumors
• Presence of a lump for months or years without any change in size or appearance
• Rapidly growing lumps

Symptoms other than tumors include:

• Allergic reactions
• Gastrointestinal ulcers
• Vomiting
• Anorexia
• Bloody stools
• Slow blood clotting in the area of the tumor

Types

Mast cell tumors are categorized into three grades and react differently in the body depending on what grade they fall under. These are: 

• Grade I - Tumors are least aggressive in this grade and are often confined to the skin. These tumors can be cured with surgical removal
• Grade II – Tumors are in skin, and can be aggressive, spreading into lymph nodes, liver, spleen and bone marrow
• Grade III – These tumors are very aggressive, with a high level of metastasis

The actual underlying cause of mast cell tumors is unknown. There are a few factors being investigated, which include:

• Genetic predisposition
• Mast cell abnormalities
• Chronic inflammation
• Mutation of c-kit protein 

Any dog can develop mast cell tumors, but some breeds are predisposed to the condition. These include:

• Boxers
• Boston Terriers
• Bulldogs
• Pugs
• Retriever breeds
• Weimaraners
• Shar-Peis
• Rhodesian Ridgebacks
• Cocker Spaniels
• Schnauzers
• Staffordshire Terriers
• Beagles
• Older dogs

A visible skin tumor usually alerts a pet parent that there may be a problem. Your veterinarian will perform a thorough exam, focusing on any visible tumors. If your veterinarian suspects a mast cell tumor, then a fine needle aspirate biopsy is used to extract cells from the tumor. Mast cells can be identified through this biopsy.

Once it is diagnosed as a mast cell tumor, additional tests are performed to determine if it has spread to other areas of the body. These tests can include a biopsy of local lymph nodes, bloodwork, and a urinalysis. Abdominal ultrasounds and chest X-rays are used to assess the size of the lymph nodes, liver or spleen. An assessment of the bone marrow is made by collecting bone marrow cells for examination, or looking for circulating mast cells with white blood cells. These results will determine what kind of treatment is needed.

Treatment is based on the extent of the mast cell tumors, and their predicted behavior. 

Surgical removal is the most common treatment for mast cell tumors. It is important to determine if all of the tumor was removed, because it can reoccur at the same site if any was left behind. This is found in the biopsy evaluation following surgery. A second surgery may be required to remove the rest. Before surgery, antihistamines are often prescribed to counteract the effects of the histamine released from the mast cells.

After surgery, biopsied tumors are tested and evaluated to determine what grade they fall under. The grade is an important factor in determining the rate of aggressiveness of the tumor. A grade assessment can be aided by the history of the tumor, tumor location, and the health and breed of your dog. Factors such as tumor grade, tumor spread, and if there was enough of a tissue margin to adequately excise the tumor, are all taken into consideration in deciding if your dog needs additional treatments.

Radiation therapy can be prescribed and is often used for mast cells that could not be removed. Surgery and radiation therapy are generally recommended for Grades I and II. Chemotherapy can also be recommended and is often reserved for Grade III tumors.

Medications may be prescribed to fight the secondary issues caused by the tumors. These drugs can include antihistamines, antacids, and corticosteroids like prednisone.

Recovery is dependent on the grade of the tumor, whether the tumor was completely removed, and how much the tumor has spread.

For dogs with Grade I and II tumors, recovery is excellent after surgical removal and radiation therapy. No tumor reoccurrence within 3 years is seen for 90% to 95% of dogs. For Grade III tumors, recovery is fair, hindered by a likely tumor reoccurrence and spread. These tumors often require more aggressive treatments.

You may be prescribed medications to administer at home. Depending on the factors leading to recovery, you may need future veterinary visits for tests and treatments.

Dogs that have developed a mast cell tumor tend to develop more tumors, so watch your dog closely for the appearance of new growths. If caught when they are small, surgical removal is generally advised.