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Mast Cell Tumors

From Morris Animal Foundation

Older pets often develop cancer, and statistics suggest that cancer is the cause of death in 50 percent
of dogs older than 10 years of age. Skin cancer is the most common type of cancer found in dogs,
and mast cell tumors comprise about 20 percent of all canine skin cancers. Although cats have a
lower incidence of skin cancer compared with dogs, mast cell tumors are still the second most
common type of skin cancer in cats. Mast cell tumors can also develop in other organs, causing a
particularly serious form of cancer.

What is cancer?
Cancer is defined as uncontrolled cell growth. It is important to remember that even though cancer
is a genetic disease, it isn’t always an inherited disease. This means that cancer arises from changes in a
cell’s genetic code that lead to uncontrolled cell growth. Individuals may have an inherited tendency to
developing these changes, but other factors (such as the environment) also play a role in cancer’s

What are the six characteristics of cancer?

Cancers share six characteristics that define what cancer is and describe how it behaves. These
characteristics are:
1. Cancer cells have sustained growth, meaning they grow for longer periods of time
than normal cells.
2. Cancer cells don’t respond to normal antigrowth control signals.
3. Cancer cells are resistant to death, so they live longer than normal cells.
4. Cancer cells continuously divide, whereas normal cells have a limited number of
times they can divide.
5. Cancer cells are effective at making blood vessels, and by doing so, they can obtain
nourishment and compete with normal cells for nutrients.
6. Cancer cells invade and spread.

What is a mast cell?

Mast cells are a special type of white blood cell. They are concentrated in areas near the external
environment, including the skin and airways. Mast cells contain granules that give them a typical
microscopic appearance and make them easy to identify. These granules contain substances that are
important components of normal mast cell function.

What do mast cells do in animals?

Mast cells help protect the body against infections, which is why they tend to be located near the
outside environment. They are also involved in allergic reactions (both good and bad). Mast cell
granules contain chemicals that are secreted by the cells when they are stimulated. These substances
include heparin (a blood thinner), histamine (important in allergic reactions), enzymes and chemicals
that stimulate inflammation. Although most of us view inflammation negatively, it is actually one of
the body’s defense strategies. By causing inflammation, mast cells are trying to get the body’s
attention to address foreign material, such as parasites or allergens.

dog-mastcell7-300x199What are mast cell tumors?
Mast cell tumors are collections of mast cells that form a solid tumor, either in the skin or internally
in other organs. Because they contain inflammatory substances, mast cell tumors can sometimes be
very irritating or can cause problems such as vomiting and stomach ulcers.

Who gets mast cell tumors?

Any dog or cat can develop a mast cell tumor, but they are most commonly seen in Bulldogs and
their descendants (Boston Terriers, Boxers and Pugs), Labrador and Golden Retrievers, Cocker
Spaniels, Schnauzers, Staffordshire Terriers, Beagles, Rhodesian Ridgebacks, Weimeraners, Shar-Peis
and Siamese cats.
How can I tell if my pet has a mast cell tumor?

Mast cell tumors in the skin are often red in color because mast cell granules are filled with
substances important in inflammation. However, mast cell tumors can look like just about any type
of tumor, including fatty tumors. Because of this ability to mimic the appearance of many other
types of skin problems, it is important to have a veterinarian check any new lumps or bumps. These
tumors can be irritating, so pets will sometimes scratch or pick at them.
When mast cell tumors are growing internally, they are much harder to detect. Internally growing
tumors often produce irritating substances that can cause vomiting and stomach ulceration.
Unfortunately, there are no typical signs for internal mast cell tumors.

Dogs usually develop mast cell tumors on their trunks or limbs. Cats tend to develop them on their
heads and necks. When mast cell tumors develop internally, they are often found in the spleen and

How are mast cell tumors diagnosed?

As mentioned earlier, mast cells contain granules that are easily identified under a microscope.
Occasionally, mast cell granules are difficult to find, but in general this is one of the few types of
cancer that can be readily diagnosed by most general practitioners with a minimum of equipment.
Internally growing mast cell tumors are trickier to diagnose, but even they can be discovered using
ultrasound to look for suspicious growths.

What is staging and why is it important?
Cancer staging is a way of evaluating the extent and severity of a patient’s cancer in order to estimate
the prognosis and to guide treatment. Staging involves evaluating the amount of cancer present,
looking for evidence of tumor spread and evaluating whether the patient is feeling sick from the
tumor itself. All these factors together are used to determine the stage of cancer.
Why are mast cell tumors given a “grade”?
Mast cell tumors growing in the skin can take a variety of forms. Because they are such a diverse
group, the grading system helps to predict how a mast cell tumor will behave. Although the system
isn’t perfect, it is widely used in veterinary medicine. If your dog is (or was) diagnosed with a mast
cell tumor, you would likely be told something about the tumor’s grade. This grading system doesn’t
seem to work in cats, which have slightly different forms of mast cell tumors.
Canine mast cell tumors are divided into three grades, with grade 1 tumors being the most easily
treated and least aggressive type and grade 3 tumors being the most aggressive type. Grade 2 tumors
fall in between these two extremes. Most dogs have grade 1 or 2 tumors, which are more easily
How are mast cell tumors treated?

Surgery is the most effective treatment. If a mast cell tumor can be completely removed, prognosis
can be excellent, especially for grade 1 tumors. Even if the tumor can only be reduced in size, the
patient can achieve extended remissions. Higher grade mast cell tumors require big incisions to
achieve disease-free, or “clean,” margins, so surgeons must be skilled in these larger surgeries.

Margins in cancer treatment describe the edges of tumors that have been removed surgically. When
surgeons cut out a tumor, they try to take some normal tissue as well. The idea is to completely
remove the cancer, not leaving any cells behind that could begin growing again. After removing the
tumor, the surgeon will identify the edges of the removed tissue for the pathologist, a doctor trained
to look at tissues, to determine a diagnosis.

The pathologist will not only decide what type of tumor
is present and how aggressive it is, but will also look closely at the margins to see whether any tumor
cells are along the edge. If the pathologist identifies cancer cells at the edge, this is usually a sign that
the surgeon didn’t remove all of the cancer. If the margin is “clean,” the pathologist didn’t find any
obvious cells. This suggests the surgeon removed all of the cancer from that area. This system isn’t
foolproof, but many studies provide surgeons with guidelines for the ideal margins for removing
certain types of tumors. Margin guidelines also factor into treatment decisions when growths are in
areas that are difficult to address surgically, such as in the head or lower limbs.

When a grade 1 or grade 2 cannot be completely removed by surgery, radiation therapy is often very
effective in combination with surgery to “debulk,” or reduce, the tumor’s size. Chemotherapy is also
sometimes used after a debulking surgery, but results aren’t always as good as using radiation

Unfortunately, there are few treatment options for grade 3 mast cell tumors, and most of them
aren’t very effective. The rapid spread of grade 3 tumors makes them less amenable to surgery or
radiation treatment. Grade 3 tumors are also not very sensitive to most chemotherapy drugs.

What is the prognosis for dogs with mast cell tumors?

A mast cell tumor’s grade is highly correlated with long-term prognosis. Grade 1 tumors have the
best prognosis and can often be cured, while grade 3 tumors have the worst. Most patients die of
grade 3 tumors within a year. Prognosis also depends on the location of the tumor, the stage of the
tumor and if the patient has any other signs, such as vomiting or poor appetite.

What is the prognosis for cats with mast cell tumors?

The prognosis for cats depends on the location of the mast cell tumor. Mast cell tumors located in
the skin tend to have a good prognosis, provided they are in a location that is surgically accessible.
Tumors located in the spleen also have a surprisingly good prognosis, and many cats retain an
excellent quality of life for one to two years after spleen removal. Intestinal mast cell tumors are
associated with a poor prognosis, and survival times are a few months at best.

What is Morris Animal Foundation doing to help?

Mast cell tumor research has been a funding focus at Morris Animal Foundation for the last seven
years, and many studies are currently underway. Research studies have been concentrated in three
areas: development of methods to better predict which mast cell tumors are more likely to spread;
identification of dogs at risk for the development of mast cell tumors; and identification of potential
therapeutic targets for dogs. In addition to these basic grants, Morris Animal Foundation has funded
four Fellowship Training Grants for young scientists studying mast cell tumors.
Golden Retrievers develop cancers at high rates and have a high incidence of mast cell tumors, so
the Foundation anticipates that results from the multiyear Golden Retriever Lifetime Health Study
will generate data on the genetics and behavior of mast cell tumors in a large population of dogs.
Morris Animal Foundation remains committed to finding the underlying mechanisms of mast cell
tumor growth and developing more effective treatments for this common cancer.


1 MARY COIRA { 05.16.17 at 5:47 pm }

My dog, a Golden Retriever, has cancer that shows in the skin in the form of tumors. My husband and I has taken him to vets around the area but their opinions vary. One says it is too risky to do surgery. The other vet said he will charge $1600.00 to remove two tumors. The third vet says that if they are removed they will comeback bigger. We are desperate and don’t know what to do. Any suggestion?

2 al { 05.17.17 at 9:00 am }

Our Millie had a mass cell tumor about the size of an egg just behind her right front leg when we adopted her in January 2009. It was remove by an aggressive surgeon (ensure good margins) and was a low grade 1. Her estimated age was 6 years old and she’s doing fine today at 14 with no new tumors.

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