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DFS4 Friday August 24,
2007

Mast Cell
Tumors

Holly
Nash, DVM, MS
Veterinary Services Department, Drs.
Foster & Smith, Inc.

Canine mast cell tumors account for up to 20% of all skin
tumors
in dogs. While they often appear small and somewhat
insignificant, they can be a very serious form of cancer in
the dog. Some mast cell tumors are easily removed without the
development of any further problems and others can lead to a
life threatening disease. Proper identification and treatment
are very important in controlling these tumors.

What are mast cells?

Mast cells are cells that normally occur in the skin and
other tissues, such as the intestines and respiratory tract.
They are part of the immune system (defense mechanism) of the
body. They contain large amounts of histamine, heparin, and
proteolytic enzymes (enzymes which break down protein). These
can be toxic to foreign invaders, such as parasites, and are
released when the mast cell is triggered by the immune
system.

A mast cell tumor is formed from many of these mast cells.
Because of the histamine, heparin, and enzymes present in mast
cell tumors, they can create problems when damaged or removed.
Large amounts of these substances can be released into the
body and have significant effects on heart rate, blood
pressure, and other body functions. Sites where the tumors are
removed can sometimes refuse to heal and can become difficult
to manage.

Which dogs are at risk for developing
mast cell tumors?

Mast cell tumors can develop in all ages and breeds of
dogs. They are rare in cats and humans. There appears to be a
hereditary factor to these tumors as shown by some strong
breed predilections. They are most common in Boxers, Boston
Terriers, Pugs, English Bulldogs, and other brachiocephalic
breeds (those having a short, wide head). Golden Retrievers
may also be at increased risk. Most mast cell tumors develop
in older dogs, usually those 8.5 - 9.5 years of age.

The exact cause of mast cell tumors is still speculative. A
viral source has been mentioned, as well as hereditary and
environmental factors. It is quite possible that there are a
variety of different causes for the development of this tumor.
Because this tumor is not found in humans, there has not been
as much research and information available for the
veterinarian as there are for tumors that are found in both
humans and animals.

What are the symptoms of mast cell
tumors?

The appearance of mast cell
tumors can be widely variable. They can be either benign
or malignant
and can be found on any part of the body. They are found most
commonly on the trunk, limbs, and perineal (genital) area.
Tumors can be found on the skin or in the underlying or subcutaneous
tissue. They can be single or multiple and can be smooth,
bumpy, or even ulcerated.

Systemic
signs, such as vomiting, duodenal ulcers, blood in the stool,
and abnormalities in blood clotting occur in some dogs with
mast cell tumors. These signs result from the release of
histamine, etc. from the active mast cell tumors.

How are mast cell tumors
diagnosed?

Since they occur in a variety
of shapes and locations, a biopsy or needle aspirate
(collecting some tumor cells through a needle and examining
them under the microscope) is necessary to properly identify a
growth as a mast cell tumor.

Mast cell tumors are commonly graded and staged, meaning
classified as to how they are expected to behave. This is
performed by examining the tumor after it has been removed.
The grading and staging help determine what type of further
treatment may be necessary and the prognosis.

Grading Mast Cell Tumors

Mast cell tumors are "graded" as to how likely they are to
be malignant. The higher the grade, the more serious the
tumor.

Grade I: Occur in the skin and are considered benign.
Although they may be large and difficult to remove, they
tend to not spread to other areas of the body. Most mast
cell tumors are Grade I.

Grade II: Extend below the skin into the subcutaneous
tissues. Their cells show some characteristics of malignancy
and their response to treatment can be unpredictable.

Grade III: Invade areas deep below the skin, are very
aggressive, and require more involved treatment.

Staging Mast Cell Tumors

In addition to grading mast cell tumors, they are also
staged, which is a measurement of how they have spread in the
body. A tumor is staged after it is surgically removed and
examined, along with the neighboring lymph nodes. Staging is
based on how many tumors were present, the lymph node
involvement, and if all of the tumor was removed.

Stage 0: One tumor in the skin incompletely removed,
with no lymph node involvement.

Stage I: One tumor in the skin, with no lymph node
involvement.

Stage II: One tumor in the skin with lymph node
involvement

Stage III: Multiple large, deep skin tumors, with or
without lymph node involvement

Stage IV: One or more tumors with metastasis in the skin
with lymph node involvement. This stage is subdivided into
those that have no other signs (substage a) and those that
do have some other clinical signs (substage b).

How are mast cell tumors treated?

In determining the appropriate therapy for mast cell tumors
and their wide variety of forms, it is important to remember
that each animal needs to be evaluated and treated
individually.

Surgical Removal

Mast cell tumors are usually treated by surgical removal.
This is the treatment of choice, and if performed correctly,
will usually cure Grade I and Grade II tumors. It is important
that the tumor is carefully removed and a large area of
'healthy' tissue around the tumor is also removed. It is
sometimes difficult to determine exactly where the tumor
begins and healthy tissue starts, so a wide margin (large
portion of healthy tissue around the tumor, at least one inch)
should be removed along with the tumor.

Radiation

In some cases, surgical removal may not be an option, or it
may not be possible to remove enough margin around a tumor. In
these instances, radiation is recommended. Radiation therapy
after surgical removal appears to be beneficial and may reduce
the incidence of reoccurrence and increase survival rates.
Radiation is most useful when the tumors have not spread to
multiple areas of the body.

Chemotherapy

If the mast cell tumors have spread to multiple areas, a
combination of anti-cancer drugs are commonly used along with
surgery and radiation. These include vinblastine, lomustine,
and corticosteroids, such as prednisolone. Unfortunately, mast
cell tumors do not respond well to chemotherapy.

What is the prognosis for dogs with mast
cell tumors?

The prognosis depends primarily upon the grade and stage of
the tumor. The lower the grade, the better prognosis. In
addition, dogs with Stage I tumors have the best prognosis,
compared to those staged higher. The location of the tumor
also plays a role. Dogs with tumors on the limbs appear to
have the best prognosis. Those with tumors in the nail bed,
genital areas, muzzle, and mouth have a poorer prognosis. Dogs
with mast cell tumors in the internal organs, such as the
spleen or bone marrow, have the least favorable prognosis.

Conclusion

Mast cell tumors can present in a wide variety of ways and
can have an equally varied set of symptoms and outcomes.
Because it is difficult to diagnose a mast cell tumor by
visual inspection, it is important that all suspicious-looking
skin tumors be examined by a veterinarian and followed up with
diagnostic testing and identification. Treatment consists of
surgical removal and radiation, and possibly chemotherapy. As
with all tumors, prompt recognition and treatment is very
important in obtaining the best possible outcome.

References and Further
Reading

Bonagura, J. Current Veterinary Therapy 12. W.B. Saunders
Co. Philadelphia, PA; 1995.

Chun, R. Canine cutaneous mast cell tumors: One of these is
not like the other. Presented at the Western Veterinary
Conference, 2004. Las Vegas, Nevada.

Rogers, KS. Mast Cell Disease. In Ettinger, SJ; Feldman EC
(eds.) Textbook of Veterinary Medicine. W.B. Saunders Co.
Philadelphia, PA; 2005: 773-778.

Piscopo, S. Canine Mast Cell Tumors. Veterinary Forum.
June, 1999.

Vail, D. Dealing with Canine Mast Cell Tumors. Veterinary
Product News. December, 1999.

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