Healthy normal mast cells form part of the body’s immune system. They can be found in most tissues, but in particular in the skin, and the linings of digestive tract, the lungs, the mouth and the nose.
Mast cells perform functions in the body to help fight outside invaders such as microbes or foreign materials.
They are also involved in allergies and allergic reactions. The immune system, sensing the presence of invaders, triggers the mast cells to release large amounts of herapin, histamine, and enzymes.
Mast cell tumors are formed by DNA mutations of mast cells.
Mast cell tumors are the most common malignant canine tumors. They are very invasive and difficult to treat.
Mast cell tumors can occur in different areas of the body, but most commonly on the skin or under the skin surface. They account for 16% to 21% of all skin tumors in dogs.
Malignant aggressive mast cell tumors will locally invade cells in the neighborhood. They can also metastasize (spread) throughout the body, to the lymph nodes, spleen, liver, and bone marrow.
Very rarely, mast cell tumors can start internally and spread out to the skin.
When the entire body is affected, the disease is referred to as systemic mastocytosis.
Mast cell tumors can occur in all dogs but older dogs (9 years or older) are more prone to the disease.
Causes of Canine Mast Cell Tumors
Many vets believe that genetics plays a role in this form of cancer because there are definite breed predispositions.
Some breeds which are more susceptible to this cancer include:
- Boston Terriers
- Labrador Retrievers
- Rhodesian Ridgebacks
These breeds are anywhere between 4 and 8 times more prone to the development of mast cell tumors than other breeds.
There may also be an association between mast cell tumors and certain processes in the body, like allergies.
Symptoms of Canine Mast Cell Tumors
Mast cell tumors are commonly known as the “Great Imitators” because they can look like just about any kind of lumps or bumps.
About half of canine mast cell tumors occur on the skin at the rear end, under the tail. Forty percent of them occur on the skin of the legs, especially of the thighs. The remaining are found on the head, neck, and elsewhere on the trunk.
The most common appearance is a raised, button-shaped, red, hairless bump that is crusted or raw.
Some of the tumors are lumps under the skin which might just feel like a fatty tumor (lipoma).
Others are fleshy, thick masses that stick out. Sometimes the borders are discreet and easy to see. Other times they are not.
Also, mast cell tumors can come and go, only to re-appear later.
If you find a lump on your dog’s skin, you should monitor it carefully. Watch it for a day or two and seek veterinary attention immediately if you suspect mast cell tumor.
As mentioned above, mast cells contain chemicals such as histamine and heparin, and when a mast cell tumor is physically handled, the cells sometimes release these chemicals into the body.
Histamine can cause the stomach lining to produce a lot of extra acid, resulting in the formation of stomach ulcers. When this happens, the dogs may experience vomiting, loss of appetite, sometimes diarrhea (with blood in stools), and abdominal pain.
These chemicals also cause inflammation and swelling at the tumor site.
Diagnosis of Canine Mast Cell Tumors
A vet usually uses fine-needle aspiration to get an initial diagnosis. But this procedure cannot show how aggressive the tumor is (i.e. the tumor’s grade). To check that, the vet will recommend a biopsy.
Once mast cell tumor is confirmed by biopsy, the vet will grade the tumor on a scale from 1 to 3, with 1 being benign, 2 being intermediate, and 3 being malignant.
While this grading system can be unreliable at times, it is the only straighforward way of grading these tumors to predict their future behavior.
The tumor will also be “staged” to determine if it has metastasized (spread) and, if so, to what extent.
Armed with the information obtained from grading and staging, a veterinarian can design a treatment plan that is most suitable and effective for the dog patient.
Treatment of Canine Mast Cell Tumors
Depending on the grade and the stage of the tumor, conventional treatment include surgery, chemotherapy, and radiation.
Surgery is the treatment of choice for mast cell tumors in dogs.
Usually, the vet will remove the tumor with a wide excision (removing an additional 2 to 3 cm of the surrounding skin).
He will examine the edges of the removed cancer (surgical margin evaluation) to determine if there are still cancer cells in the dog. If there are, he will have to perform a second surgery or radiation.
Grade 1 tumors can usually be removed and normally they do not return.
Only about 5 to 10% of Grade 2 tumors will return if surgery is done with a wide excision. About 50% of Grade 3 tumors will return after surgical removal.
Radiation can be used on its own or in combination with other treatments.
If surgery is not an option (e.g. due to the location of the tumor), or if there is strong suspicion that there are cancer cells left in the dog’s body after surgery, the vet will suggest radiation once a week for 4 weeks. This may reduce the incidence of recurrence and increase survival rates.
Radiation of mast cell tumors in the skin seems to be more effective than that of lymph nodes or bone.
Chemotherapy is an option if surgery is not (e.g. due to the location of the tumor), or if there is strong suspicion that there are mast cell tumor cells inside the body after surgery.
Popular chemotherapy drugs include prednisone (or prednisolone), vincristine (or vinblastine), L-asparaginase, and cyclophosphamide.
Depending on the grade of the tumor and extent of metastasis, the vet may use single-drug therapy or a combination of drugs (e.g. prednisolone with vinblastine).
Prognosis of this form of cancer depends on the grade of the tumor.
According to statistics, with treatments of surgery, chemotherapy, and/or radiation, the approximate percentages of dogs that survive to 1500 days (4 to 5 years) are as follows:
- Grade 1 – 83%
- Grade 2 – 44%
- Grade 3 – 36%