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MAST CELL TUMORS IN
DOGS

Mast cell tumors (MCTs) are fairly common tumors in dogs. They
are most frequently found in the superficial layers of the skin, on
any part of the body. Frequently, there will be ulceration over the
area of the tumor, and the dog may scratch or bite at the affected
area. The appearance of the tumor does not reveal its potential for
spread or recurrence with any certainty. The tumors are usually
singular, but dogs may present with multiple nodules, or recurrent
ones. Some nodules occasionally enlarge and then regress in size on
their own, due to swelling within the tumor itself. This should
always raise the suspicion of the presence of an MCT.

DIAGNOSIS OF MAST CELL TUMORS

Mast cell tumors do not have a specific appearance. However, they
are fairly easily detected by a "needle aspirate and cytology."
Insertion of a small needle into the tumor (virtually painless) area
is followed by examination of the cells under a microscope. Mast
cells are large, round cells that usually have dark granules in
them. The granules contain substances which, when released, cause
swelling, itching, and redness. Infrequently, when a large number of
granules discharge their chemical contents into the bloodstream,
vomiting, stomach ulcers, shock and even death may result.

MCT TUMOR STAGING

Mast cell tumors can be somewhat unpredictable in their behavior,
relative to other types of tumors in dogs. Because of this, care is
taken to "grade" the tumors that are discovered. The grade reflects
the degree to which the malignant mast cells differ from normal,
non-malignant mast cells. The stage can generally be correlated with
tumor behavior, tumor recurrence, and survival of the patient. Mast
cell tumors affecting the limbs, head, or neck tend to correlate
with a more favorable prognosis than those found on the trunk or
groin. Multiple mast cell tumors or those exhibiting rapid growth
tend to have a more guarded prognosis. A pathologist determines the
tissue grade of the tumor after the tumor is biopsied or
removed.

*Grade I: well-differentiated-25% recurrence rate
post-surgery

*Grade II: moderately differentiated-44% recurrence rate
post-surgery

*Grade III: poorly differentiated-76% recurrence rate
post-surgery

TREATMENT FOR MAST CELL TUMORS

Treatment for mast cell tumors may involve surgery (the
mainstay), chemotherapy, and/or radiation therapy. Recommendations
for treatment are based on the type and grade of the tumor, surgical
feasibility, and the presence or absence of spread (dissemination)
of malignant mast cells throughout the body. Your veterinarian will
usually submit blood tests and request abdominal ultrasound or
radiographs (x-rays) to determine the likelihood of malignant mast
cells elsewhere in the dogs body. Bone marrow biopsies are no
longer routinely done, as they have not shown to have high
predictive value for tumor staging.

Surgery

For single mast cell tumors, a surgical procedure known as a
"wide resection" is performed. This means aggressively excavating
the tumor and surrounding tissues so that at least 3 cm of normal
tissue in all directions is removed. The margins of the removed
tissue are examined by a pathologist to determine the presence of
any lingering malignant cells. If negative, we refer to it as "clean
margins". If the pathologist suspects the presence of mast cells in
the remaining tissues of the surgery site, we refer to it as "dirty
margins".

An aggressive surgery early in the course of mast cell tumor
disease is associated with the best overall prognosis. A grade I or
II tumor that has been completely removed usually requires no other
immediate therapy. A grade III tumor, multiple tumors, recurrent
tumors, or tumors with dirty margins (those which for anatomical
reasons could not be subjected to further surgery) often require
follow-up or "adjunct" therapy.

Radiation Therapy

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Radiation therapy is an option for dogs whose mast cells tumors
are localized, but too large for a clean resection or in an area
difficult to resect such as tissues of the facial region, or as
follow-up therapy for tumors with dirty margins. Dogs tolerate
radiation therapy well, and it can offer long-term control for these
tumors. Radiation therapy would not be appropriate for dogs with
multiple tumors or those with evidence of disease throughout the
body since the radiation beam treats only a single focus of
disease.

Chemotherapy

Chemotherapy denotes the administration of certain anti-cancer
drugs in order to delay/prevent tumor growth or spread. It may be
used before or after surgery, or alone.

Prednisone (a cortisone) is the most commonly used drug
for therapy of mast cell tumors. It is well tolerated by dogs and is
usually employed for a minimum of six months. If no new tumors
appear within that time, your doctor may wean your dog off the
prednisone completely. The side effects of prednisone include weight
gain, increased appetite and thirst, bladder or skin infections, and
panting. Occasionally, stomach irritation or ulcers can occur, or
inflammation of the pancreas. Most of the time, the drug dose can be
titrated to the patient to minimize any overt symptoms. If the tumor
type is determined to be aggressive, additional drugs such as
stomach protectants may be prescribed to guard against untoward
tumor effects.

Vinblastine Protocol *

For recurrent or multiple tumors, and for those tumors that
cannot be surgically removed, combination chemotherapy can be
effective in controlling tumor growth and spread for weeks to months
or more. A cure per se is generally not realistic, but many dogs
tolerate therapy extremely well. The 6-month protocol involves:

· Predinsone: high dose at first, then
taper over 4 months

· Vinblastine: an outpatient injection,
given once every 21 days

· Cytoxan ®: an oral chemotherapy drug,
given by the owners on days 8, 9, 10, and 11 of a 21-day cycle.

Side Effects

The side effects of prednisone are discussed above. Vinblastine
and Cytoxan have the ability to cause nausea and or vomiting, though
this is not usual. The most important possible side effects are
lowering the bodys defenses so that frequent infections occurs, or
(rarely) causing many mast cells to release their contents at once.
Both situations can be life threatening. However, these are NOT
common, and the risk of these is significantly lower than the risk
of untreated mast cell disease. You will be given instructions on
what to do if any side effects occur, so do not hesitate to contact
us.

Lomustine (CeeNu)

CeeNu is a potent oral chemotherapy drug that can be used once
every three weeks in patients who MCTs have become resistant to
other treatments. It is very well tolerated in general. However,
because it can increase the risk of infection (especially seven days
after it is given), any noted fever, depression, weakness, or
refusal to eat should be reported to us immediately. Infection is
almost always completely reversible.

Prognosis

The factors that are known to influence patient outcome are grade
of tumor (I is best, III is most dangerous), completeness of
surgical removal (clean margins), and tumor location. Dogs with
high-grade tumors, multiple or recurrent tumors, or evidence of
spread to the bloodstream or to other organs have a much more
guarded prognosis for a lengthy survival.

Patient Monitoring

Close patient monitoring is essential in dogs with a history of
mast cell tumors. As with many tumors, early detection and treatment
increases the chances of successful treatment. You may be asked to
have your dog examined every 6-8 weeks following surgery, or every
21 days during chemotherapy. Blood tests and/or needle aspirates are
often requested to assess tumor control. Any time you suspect a new
or recurrent tumor, or your dog is physically ill, contact us
immediately. We welcome the opportunity to help you and your dog in
any manner we can.

Reference: Mast cell tumors in dogs; In: Managing The Veterinary
Cancer Patient; Ogilvie, OK and Moore, As. © 1995 Veterinary
Learning Systems. Trenton, NJ.

*Based on protocol published by Elnsslie, Robin; published in the
Veterinary Cancer Newsletter.

The information on this page is provided by

Bath-Brunswick Veterinary Associates, Inc.
Dr. Gail D.
Mason, DVM, MA, DACVIM

All materials copyright (c) 2000 2005
Bath-Brunswick Veterinary Associates,
Inc.

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