MAST CELL TUMORS IN DOGS
Mast cell tumors (MCTs) are the most common skin tumor in dogs. Mast cells are inflammatory cells which are normally present in body tissues such as the skin, spleen, liver, and intestines. They are important players in the body's inflammatory response, containing histamine, heparin and other inflammatory mediators. Because of the nature and function of mast cells, MCTs can often appear red, swollen, and bruised. They can also change in size and appearance. Boxers, Labrador retrievers, Golden retrievers, Shar-peis and Boston terriers are commonly affected breeds, however MCTs are seen in all ages and types of dogs. The cause of MCTs is unknown.

Clinical Presentation
A hallmark of MCTs is their extreme variability in clinical appearance. They may be single or multiple (about 10% of the time), hairless or haired. They are often raised masses, from 1/2 to 2 inches in diameter. They may be ulcerated, red and swollen. Some MCTs are slow growing and may have been present for several months, others can grow rapidly over days. Subcutaneous MCTs can be soft and feel like a benign fatty tumor, or lipoma. Mast cell tumors that occur in abdominal organs such as the gastrointestinal tract, spleen or liver may cause signs including decreased appetite, vomiting, diarrhea, dark tarry stool, weight loss, or depression.

Diagnosis
Most MCTs are diagnosed based on cytology samples which are obtained by a fine needle aspirate of the mass. Fine needle aspirates are a fast, relatively noninvasive diagnostic test involving insertion of a small needle into the mass to obtain cells from it. This test can almost always be done with the animal awake. Samples are evaluated in the hospital to make sure that they are adequate to send to a laboratory for diagnosis by a pathologist, which takes 1-2 days. Because many MCTs feel and look similar to a benign fatty mass, I recommend doing a fine needle aspirate on all lumps and bumps. This is especially true for a dog with a history of a MCT. In some cases, a cytologic sample does not contain enough information for a definite diagnosis. Then a biopsy is required. Depending on the size and location of the mass, either a small piece of the tumor (incisional biopsy) or the entire thing (excisional biopsy) is removed. General anesthesia may be required for a biopsy. These results usually take 3-5 days.

Histological grade
Pathologists determine the tumor grade based on the microscopic characteristics of the tumor cells on biopsy samples. This information is very important in determining the treatment recommendations and prognosis for dogs with MCTs. Grade 1 MCTs have a low chance of metastasis (spreading) to other areas of the body and are usually cured with surgery. Grade 3 MCTs have a high chance of metastasis and therefore carry a poor prognosis. These masses usually are growing quickly and are associated with a lot of inflammation. Grade 2 MCTs are somewhere in between. These tumors have a variable growth rate and chance of metastasis.

Clinical staging
Clinical staging is the process of doing diagnostic tests to determine where in the body there is disease. Metastasis occurs when tumor cells break away from the primary tumor site and travel through the lymphatic system or blood vessels to distant sites where they begin to multiply and grow. MCTs metastasize most commonly to the nearest draining lymph node. They can also go to the liver, spleen and bone marrow. Thorough diagnostic staging of dogs with MCTs includes abdominal ultrasound, +/- x-rays of the chest, blood work and either a bone marrow aspirate or buffy coat test (a blood test). The last 2 tests are both ways to look for evidence of mast cell disease in the bone marrow.

Treatment:
The treatment of choice for all mast cell tumors is wide surgical excision. Ideally, we want to remove 2-3 cm (about 1 inch) of normal appearing tissue on all sides including deep to the mass. In some locations such as the legs this is obviously difficult to impossible. For MCTs that are large or in difficult locations, the tumor grade and clinical stage are important pieces of information for the oncologist and surgeon to have when making a decision about how aggressively to approach surgery. After surgery, the tissue that is removed is sent to the pathologist for evaluation which includes looking at the edges of the tissue for evidence of tumor cells. We use the term "dirty" margins when tumor cells are seen at the borders of the tissue taken out and "clean" margins when no cancer cells are seen. Unfortunately, this determination is sometimes not as clear cut as we would like and we rely on the surgeon's impression of whether good margins were achieved. If the margins are "dirty," further options would include performing a second surgery if it is feasible, adjunctive radiation therapy, and/or chemotherapy.

Radiation therapy (RT) is used most commonly for grade 2 mast cell tumors when clean margins cannot be obtained surgically. The vet school at Louisiana State University in Baton Rouge is the nearest facility for veterinary RT. Treatments are usually given on Mondays, Wednesdays and Fridays for approximately 6 weeks, and they require general anesthesia. Adverse effects primarily occur on the skin and involve varying degrees of inflammation and hair loss. An estimate for full course treatment is approximately $4000. Most dogs with grade 2 MCTs enjoy long term tumor control after radiation therapy.

Chemotherapy is used for MCTs when there is systemic involvement, when there are multiple tumors, or when the tumor is grade 2 or 3. In general, chemotherapy is only moderately successful in shrinking MCTs. Lomustine ( trade name CCNU) and Vinblastine are the most affective drugs. In studies of dogs with tumors that were not surgically removable who were treated with 1 of these 2 drugs, about 50% of the masses decreased in size. I usually use Lomustine first until we determine it is not working, then switch to Vinblastine. Lomustine is an oral chemotherapy drug which is given in the form of a capsule once every 3 weeks. The most common adverse effect of this drug is suppression of the white blood cell count most profoundly 7 days after treatment. After long term use, damaging effects to the liver can be seen. We recommend regular monitoring of blood cell counts and liver enzymes. Vinblastine is given intravenously every 1-3 weeks and is sometimes used in combination with another chemotherapy drug called cyclophosphamide. Adverse effects are uncommon but can include suppression of the white blood cell count and gastrointestinal signs such as decreased appetite, vomiting and diarrhea. These drugs range from approximately $150-250 per treatment.

Prognosis
The prognosis for dogs with MCT is variable and depends on tumor grade, size, location, and rate of growth. Dogs can live with grade 1 MCTs for months to years and most experience long term tumor control with wide surgical excision. Fifty to 75% of dogs with grade 2 MCTs have long term tumor control with wide surgical excision. Most dogs with grade 3 MCTs are lost to their disease within 6 months. MCTs around the penis, prepuce, anus, vulva, mouth, and toes may have a higher risk of metastasis than those in other locations.