Mast Cell Tumors in Dogs
Client Information Series
Mona P. Rosenberg, DVM DACVIM

Mast cell tumors are common on or just under the skin of dogs. Any breed of dog can develop a mast cell tumor (MCT), but certain breeds are predisposed, including Boxers, bulldogs, pugs, Boston terriers, golden retrievers, and cocker spaniels. Mast cells are normal cells within the body that are responsible for responding to allergic reactions. For example, if you are slung by a bee and the area becomes red, hot, and itchy, it does so because mast cells infiltrate into the area, releasing a variety of substances including histamine, causing these symptoms. Other than hereditary factors, we do not know why dogs develop these tumors.

Your veterinarian may have diagnosed this tumor on the basis of a procedure called fine-needle aspiration. This is a minimally invasive technique that involves sticking a needle into the tumor, sucking a few cells out, and smearing the cells on a slide for a pathologist to evaluate under a micro-scope. This procedure is not painful to your dog and allows us to make a diagnosis in most cases. It does not, however, allow us to predict the biologic behavior of ("prognose") MCTs; surgical removal of the tumor followed by the use of a grading system is required. Location of the MCT is also of prognostic significance.

Knowing that we are dealing with an MCT before surgery can be helpful, because MCTs are notorious for sending out long, finger-like projections of cells into the surrounding tissue. This means we must surgically remove a wider margin of "normal" tissue surrounding any visible tumor in an attempt to remove all the microscopic "fingers."

Grade I or well-differentiated MCTs are the least aggressive of the three classes. If we are able to surgically excise the entire tumor (the pathologist will comment that the margins of tissue removed are "clean" or free of cancer cells), the incidence of recurrence is typically small, with 93 per cent of dogs being disease free at 1 year. "Metastasis" or spread of this form of MCT to distant, internal locations is unusual.

Grade II or intermediately differentiated MCTs are more aggressive than their grade I counterparts. An as yet unidentified percentage of dogs with this form of MCT develop metastasis of their cancer to internal organs, typically to the bone marrow, spleen, or local lymph node. Provided there has been no spread of the cancer, 50 per cent of dogs with completely excised grade II or intermediately differentiated MCTs develop recurrence within 10 months of diagnosis; if no recurrence is detected in this period of time, there is a very good chance that the dog will survive for 5 years free of tumor.

Grade III or poorly differentiated MCTs carry a very poor prognosis, with 97 per cent of dogs succumbing to their cancer by 1 year. This is due to the high rate of metastasis or spread of the cancer to internal organs.

Mast cell tumors in the groin behave similarly to grade III MCTs, regardless of their histology grade. It is not currently understood, but a high potential for metastasis has been consistently observed. Some oncologist's believe that  MCTs in the armpits and mucocutaneous junctions (e.g., lip margins, vulva, anus) can be quite malignant as well.

Once a dog is diagnosed with a MCT, several diagnostic tests are recommended. First, a complete blood count, bio-chemical profile, and urinalysis are performed to ensure that your dog exhibits no negative effects of the cancer in his or her system. Sometimes, a blood test called a buffy coat test is performed. This test looks for mast cells circulating through the bloodstream. This test is useful if it is positive, but it is often negative even if the cancer has spread; thus, it is not very sensitive.

The next step is to grade the cancer if this has not yet been done. Again, this can be done only by surgically removing all or part of the tumor. Once the tumor grade is known, a decision regarding further testing and treatment can be made. If the local lymph node is enlarged, it will be aspirated to look for cancer cells. If the MCT has been graded as intermediate or poorly differentiated (grade II or III), aspiration of the bone marrow and the spleen is advised. This is the most sensitive technique for determining whether the cancer has metastasized. Unfortunately, dogs with mast cell cancer in the bone marrow or the spleen have a very poor prognosis; many dogs live only 90 days from the time of diagnosis because of the effects of the cancer cells on the body. Sometimes, even in dogs with advanced disease, treatment can improve both the quality and quantity of life.

Your veterinarian may refer you to a cancer specialist for the testing or further discussion of your options for treatment.

Treatment for dogs with MCTs is dependent on the grade of tumor and results of testing. Dogs with grade I tumors that have been completely excised (removed) are not typically treated with any additional therapy. The "gold standard" of treatment for dogs with grade II MCTs, because of their moderate incidence of local recurrence even with complete surgical excision, is radiation therapy.

We also recommend using radiation therapy to treat grade I and II tumors that cannot be completely excised, provided there is no evidence of metastasis. Eighty-eight per cent of dogs with incompletely excised grade II tumors survive for 5 years without disease when treated with radiation therapy.

For dogs with grade III MCTs, dogs with MCTs in the groin, or dogs that have been diagnosed with systemic spread of their mast cell cancer, drug therapy is often recommended. These drugs include diphenhydramine (Benadryl) and cimetidine (Tagamet) to counteract the effects of histamine on the body and prednisone and other chemotherapy drugs to attempt to kill the cancer cells.

These drugs are usually well tolerated by dogs. Signs of terminal stages of the cancer include lethargy and gastrointestinal signs such as vomiting, diarrhea, and poor appetite.

Our goal for all cancer patients is that their quality of life be excellent; we never want the treatment to be worse than the disease. This goal is often achieved by working as a close team with your veterinarian and often a board-certified cancer specialist
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This page is an introduction to Mast Cell Tumors written by Dr. Rosenberg.

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Introduction to cancer in pets and "What To Expect When You Go To The Vet"

Lymphoma: one of the most common types of cancer we see in pets


Osteosarcoma (Bone Cancer)

Squamous Cell Carcinoma (Often on the ears of white cats)  (Solar Cancers)


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