Mast Cell Tumors in Dogs

These notes are provided to help you understand the diagnosis or possible diagnosis of cancer in your pet. For general information on cancer in pets ask for our handout "What is Cancer." Your veterinarian may suggest certain tests to help confirm or eliminate diagnosis, and to help assess treatment options and likely outcomes. Because individual situations and responses vary, and because cancers often behave unpredictably, science can only give us a guide. However, information and understanding about tumors and their treatment in animals is improving all the time.

What is a mast cell?

Mast cells originate in the bone marrow but mature in other tissues. They are found in all tissues of the body but are concentrated in the skin, respiratory tract, and digestive tract. Mast cells produce many chemicals with differing effects on the body (histamine, proteoglycans, neutral proteases, and chemotactic growth factors). These chemicals are present in granules within the cells. Mast cells release their granular contents in response to various stimuli, inducing an inflammatory reaction.
In addition, mast cells interact with cells of the immune system that produce antibodies as part of an allergic reaction. The mast cells present foreign molecules to immune system cells, and recruit specialized cells to engulf foreign or invading material.

On top of being a cellular barrier to external agents, mast cells have a regulatory function on nerves in the skin, blood circulation, fibrous tissue, and other immune cells. They are important in allergic responses, tissue remodeling, wound healing, and non-allergic skin diseases. Mast cells in hair follicles also help to regulate the growth cycles of those follicles.

Not surprisingly, with all these functions, mast cells are not a single cell type. In dog skin, most are a highly reactive type. 

What is a mast cell tumor?

A mast cell tumor (MCT) originates in the dog's mast cells and results from a mutation of those cells. MCTs can be relatively harmless with a low level of malignancy, or highly malignant with more life threatening consequences. Some are in multiple locations when diagnosed. Recurrence and spread to other parts of the body (metastasis) is also possible even if the MCT is treated.

What do we know about the cause of mast cell tumors?

MCTs occur due to a mutation in the mast cells which leads to unregulated reproduction and growth of the cells. Up to 30% of MCTs contain a specific mutation which has been identified.

Is this a common tumor?

MCT is the most common malignant skin tumor in the dog. Occasionally MCTs involve the internal organs, but this is relatively rare. Most dogs are middle-aged when MCT is diagnosed, but MCTs have been reported in dogs as young as 4 months of age.
Some breeds are more susceptible to mast cell tumors. MCTs are particularly common in Boxers, Bull Terriers and Labrador Retrievers. They are rare in German Shepherds.

How will this cancer affect my dog?

Most MCTs are diagnosed when there is a lump in the skin. The tumor cells produce many chemical mediators (histamine, proteoglycans, neutral proteases and chemotactic growth factors). These mediators have local effects on blood vessels and cells of the immune system that may make the tumor look like an inflammatory reaction or infection. MCTs may also itch. The tumors can sometimes bleed and may vary in size from day to day. The chemical mediators released by MCTs may cause inflammation elsewhere in the body.

How is this cancer diagnosed?

In dogs, MCTs are called the “great imitators,” as they can clinically resemble many other tumor types.
 
Before surgery, microscopic examination of cell samples obtained by aspiration or fine needle biopsy is helpful in identifying this tumor and planning surgery. Some MCTs do not have a distinct border differentiating them from the surrounding normal tissue, so an unusually wide surgical margin around the obvious lump is required in order to make sure that all of the mutated cells are removed.

Once removed, the MCT will be sent to a surgical pathologist for microscopic evaluation. The pathologist will report the grade of the MCT and will also indicate if the mass was completely removed.

What are the grades of mast cell tumor and their respective prognoses?

MCTs are classified as one of three grades microscopically. The grade helps to predict how the tumor will behave in the body.

Grade I MCTs contain cells that are 'mature' or well differentiated. These are the least well-defined tumors in terms of their borders. Their diffuse nature makes excision difficult. They are usually clinical stage I, or one tumor confined to the skin without spread to local lymph nodes ("glands").  Grade I MCTs are considered benign, and approximately 36% of MCTs fall into this category.

Grade II MCTs represent approximately 43% of those diagnosed, making them the most common.  The biological behavior of grade II mast cell tumors is not easy to predict. Sixty-five percent are cured surgically, but both recurrence and metastasis (spread of the tumor) are possible.  The prognosis is less favorable if:

•    The MCT is found in multiple locations
•    There is rapid growth of the mass
•    The dog is one of the susceptible breeds
•    The MCT is located at the junction of skin and mucous membranes
•    The MCT represents recurrence

Grade III MCTs are clinically aggressive and will spread to multiple locations. They represent approximately 20% of all MCTs. Grade III MCTs are treated with chemotherapy, but the prognosis is exceptionally poor. Most dogs with Grade III MCT, even when treated appropriately, survive less than one year.

What will help to predict what will happen to my dog?

To help predict what might happen in any individual case, a surgical pathologist performs histopathological grading. This involves close examination the sample or tumor using a microscope and the pathologist’s specialized skills and knowledge to figure out what grade the tumor falls under. Because pathology is somewhat subjective, researchers are looking for molecular markers that will provide improved prognostic information.

Tumor staging, or estimating the extent of spread of the tumor, may also assist in determining the prognosis. Staging may include lymph node biopsy, abdominal ultrasound, and bone marrow biopsy.

What types of treatments are available for MCT?

Surgery to remove the tumor with a wide margin of tissue around the lump is the first step in treatment whenever possible for all grades of mast cell tumors.

Grade I MCTs can generally be cured surgically with a complete resection and "clean" margins.

Grade II MCTs are often considered cured with surgery that includes removing a margin of surrounding normal tissue. Incomplete surgical removal of a Grade II MCT requires additional treatment which may include a second surgery or local radiation therapy.

Grade III MCTs are difficult to treat, and even with surgery and chemotherapy, the prognosis is very poor. Most dogs with Grade III MCT will succumb to the effects of their MCT in less than a year.

Can this cancer disappear without treatment?

Poorly differentiated mast cell tumors do not disappear spontaneously.

How can I nurse my pet?

After surgery, the operation site needs to be kept clean and your pet should not be allowed to interfere with the site. Report any loss of sutures or significant swelling or bleeding to your veterinarian. Hematoma (a localized collection of blood) in the surgical wound is not uncommon after surgery for this tumor. If you require additional advice on post-surgical care, please ask your veterinarian.

How will I know if my dog’s MCT is permanently cured?

With mast cell tumors, the absence of any recurrence beyond 6 months is encouraging, because recurrence usually occurs within a few months of surgery.

Histopathology and the associated report of the veterinary pathologist will give your veterinarian the information that helps to indicate how an individual tumor is likely to behave (the prognosis) including the probability of local recurrence or metastasis (spread to other areas of the body). Clinical staging tests may establish that such distant spread has occurred.

Are there any risks to my family or other pets?

No, this is not an infectious tumor and it is not transmitted from pet to pet or from pet to people.