|
Mast Cell
Tumors in Dogs
Dr.
Gail Mason, DVM, MA, DACVIM
Kathi
L. Smith, RVT
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 spontaneously discharge their
chemical contents into the bloodstream, vomiting,
stomach ulcers, shock and even death may result.
|
|
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 dog's 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" by an
experienced surgeon is performed. This means
aggressively excavating the tumor and surrounding
tissues so that at least 2-3 cm of normal tissue
in all directions is removed. This must include
a "deep margin" which involves removing
tissue below the tumor. The margins of the removed
tissue are marked and 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". If the remaining, malignant cells
are less than 2 cm from the edge of the biopsy
specimen, it is referred to as "close 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
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.
Radiation treatment can be accessed at New England
Veterinary Oncology Group (NEVOG) in Waltham
MA., Angell Memorial Animal Hospital in Boston,
MA. and Tufts University School of Veterinary
Medicine in Grafton, MA. This is a highly specialized
form of therapy and we are pleased to refer
your pet to these centers if need arises.
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. By itself, prednisone
is considered to have only mild anti-cancer
effects on MCTS.
Chlorambucil
Protocol
If
your pet is deemed to have an increased risk
of tumor recurrence, we may recommend combination
oral therapy with prednisone and chlorambucil
(Leukeranš). Chlorambucil is a chemotherapy
drug that is extremely well tolerated in most
patients and yet offers more tumor protection
than prednisone alone. This drug is usually
administered twice weekly and requires that
your pet be monitored at least every 6-8 weeks.
The minimum treatment period is 6 months.
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 six-month protocol involves:
- Prednisone:
high dose at first, then taper over 4 months
- Vinblastine:
an outpatient injection, given once every
21 days
- Cyclophosphamide
(Cytoxanš): an oral chemotherapy drug, given
by the owners on days 8,9,10, and 11 of a
21-day cycle.
This protocol has produced a 1 and 2 year survival
rate for grade 2 tumors of 91%. For patients
with grade 3 tumors, the 1-year survival rate
is 66%.
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 body's defenses so
that 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
(CeeNuš)
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 or your veterinarian,
or an emergency hospital immediately.
Infection is almost always completely reversible.
This drug is more potent than Cytoxanš and maybe
substituted for it if indicated. Occasionally,
this drug may decrease patient blood cell counts,
which requires a dose reduction or discontinuance.
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
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 and monitored
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 that your dog is physically
ill, contact us immediately. We welcome the
opportunity to help you and your dog in any
manner we can.
|
