What is a mast cell?
A normal mast cell is part of the immunologic defense system against invading organisms. These cells help fight against parasites and are found in tissues that interface with the external world such as the skin, respiratory system or intestinal tract. They do not circulate through the body.
The mast cell possesses granules that are released against the parasite and signal other immune cells that there is a problem. Sometimes mast cells are stimulated by antigens that are of similar shape or size as parasitic antigens. These "next best" antigens are usually pollen proteins and the result is an allergy. Instead of killing an invading parasite, the mast cell’s biochemicals produce local redness, itch, swelling and other symptoms we associate with allergic reactions.
And the mast cell tumor?
The mast cell can form a tumor made of many mast cells that release their toxic granules, creating allergic symptoms such as redness, swelling or itching. Mast cell tumors are especially common in dogs, accounting for approximately one skin tumor in every five dogs.
Boxers are at an especially high risk, as are related breeds, like the English bulldog and Boston terrier. Also at higher than average risk are the Shar Pei, Labrador retriever, golden retriever, schnauzer and cocker spaniel.
Most mast cell tumors arise in the skin, but technically they can arise anywhere that mast cells are found. It is not unusual for an owner to notice a sudden change in the size of the growth or that the growth has become itchy or bothersome to the dog.
Diagnosis can often be made with a needle aspirate, in which the doctor collects cells from the tumor with a needle and examines them under a microscope. The granules have distinct staining characteristics and can be recognized easily. An actual tissue biopsy, however, is needed to grade the tumor, as grading is crucial to determining prognosis.
How is the mast cell tumor graded? 
Once the mass has been submitted for biopsy, it can be graded. The grade is a reflection of the malignant characteristics of the cells under the microscope. These characteristics generally correlate to the behavior of the tumor. Grade I is benign, grade III is malignant, and grade II has the flexibility to go either way.
  • Grade I tumors: This is the best type of mast cell tumor to have. While it may tend to be larger and more locally invasive, it tends not to spread beyond its place in the skin. Surgery should be curative, if completely excised with wide margins. If the grade I mast cell tumor is not completely excised, it will grow back in time; it is best to get it all and be done with it as quickly as possible. About half of all mast cell tumors are grade I tumors and can be cured with surgery alone.
  • Grade II tumors: The behavior of this type of tumor is somewhat unpredictable. Recent studies have shown that radiation therapy administered to the site of the tumor can cure greater than 80 percent of patients as long as the tumor has not already shown distant spread.
  • Grade III tumors: This is the worst type of mast cell tumor to have. Grade III tumors account for approximately 25 percent of all mast cell tumors and they behave very invasively and aggressively. If only surgical excision is attempted without supplementary chemotherapy, a mean survival time of four to five months can be expected.
What kind of testing is necessary?
  • Basic bloodwork: A basic blood panel is part of this evaluation process and should be obtained at this point if it has not already. This will show any limitations to kidney or liver function, which is necessary to know prior to surgery. Bloodwork will also show if there are circulating mast cells in the blood or if there is anemia that might be related to the tumor.
  • Local lymph node aspiration: The lymph nodes near the site of the tumor are sometimes aspirated if they are found to be large or the doctor wants to see if tumor cells have potentially spread.
  • Aspiration of the spleen: The spleen is an organ of the lymphatic system. The presence of a tumor in the deeper lymphatic organs, such as the spleen and abdominal lymph nodes, can be assessed for the presence of mast cells. If the doctor feels an enlarged spleen or if the dog is having any systemic signs, X-rays and/or ultrasound imaging with splenic aspiration may be recommended.
  • Radiographs (X-rays): While the mast cell tumor does not spread to the lungs the way other tumors do, there are many lymph nodes in the chest and it is helpful to radiograph the chest to assess the size of these lymph nodes and help determine the extent of tumor spread.
How are mast cell tumors treated?
Therapy for mast cell tumors consists of surgery, and potentially, radiation therapy and/or chemotherapy. Which combination of the above is chosen depends on the extent of spread and malignant characteristics of the tumor.
  • Grade I: Complete surgical excision is curative, but deep and extensive margins are needed. The location of the tumor on the body can prohibit these wide surgical margins. If the margins are clean, the tumor should be completely removed, but it is still a good idea to keep an eye on the area in the future.
  • Grade II: Surgical excision is recommended, again with wide margins. If wide margins are not achieved (for either a grade I or II mast cell tumor), or if the pathologist believes the tumor is more aggressive, then radiation therapy should be considered.
  • Grade III: Surgical excision with chemotherapy is recommended. Depending on the location, radiation may also be considered.
  • Other medications to consider: Antihistamines such as Benadryl ® and Pepcid AC ® can help alleviate unpleasant secondary symptoms of mast cell tumors.
What are the potential complications?
Mast cell tumors can potentially be slower to heal post-operatively and suture sites have more potential to break down.
See Feline Mast Cell Tumor