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Canine Mast Cell
Tumors
Author: Kate Connick
©2002

Mast cell tumors (MCT) are cancerous proliferations of mast
cells. Although they can and will spread throughout the body, the danger
from mast cell tumors arises from the secondary damage caused by the
release of chemicals that they produce. These chemicals can cause systemic
problems that include gastric ulcers, internal bleeding, and a range of
allergic manifestations. Clearly, mast cell tumors affect both lifespan
and quality of life. Sometimes mast cell tumors are referred to as "the
great imposters," as there is no way to definitively identify them without
a biopsy and pathology report. Mast cell tumors vary widely in their size,
shape, appearance, texture, and location. It can be difficult not only to
recognize mast cell tumors but to predict their course. They may be
relatively innocent or aggressively malignant. As mast cell tumors are
very common in dogs, it is important for the regular pet owner to have at
least a basic understanding of what they are and how they work.

Mast cells are specialized cells that normally are found
distributed throughout the body and help an animal respond to inflammation
and allergies. Mast cells can release several biologically active
chemicals when stimulated, among them histamine, heparin, seratonin,
prostaglandins and proteolytic enzymes. Although these chemicals are vital
to normal bodily function, especially immune response, they can be very
damaging to the body when released in chronic excess.

Other names: Histiocytic mastocytoma, mast cell sarcoma,
mastocystosis (when there is systemic involvement).

Frequency/Location: Mast cell tumors are among the most common
tumors in dogs and are the most common type of skin cancer found in dogs.
Approximately 1/3 of all tumors in dogs are skin tumors, and up to 20% of
those are mast cell tumors. The most common location to find mast cell
tumors is, by far, the skin, followed by the spleen, liver, and bone
marrow. Approximately half of all cutaneous (skin) MCT�s are found on the
body proper, another 40% on the extremities (most frequently the hind
limbs), and the remainder on the head or neck. Approximately 11% occur in
more than one location.

Causes/Predispositions: No one fully understands what causes
cancer. Mast cell tumors are very common in dogs, yet they occur far less
frequently in cats and very rarely in human beings. They occur in dogs of
all breeds, ages, and genders and can occur anywhere on the body. There
appears to be a genetic component, as certain breeds are predisposed to
developing MCT. Among the most common victims are beagles, Boston
terriers, boxers, bulldogs, bullmastiffs, bull terriers, dachshunds,
English setters, fox terriers, golden retrievers, Labrador retrievers,
schnauzers, American staffordshire terriers, and weimaraners. Boxers are
at the highest risk, yet mast cell tumors are often not as aggressive in
this breed. There is some suggestion that mast cell tumor development may
be associated with golden/red coat color and with chronic immune
over-stimulation that occurs in dogs with allergies or other inflammatory
conditions. There may be environmental factors, viruses, or other
undetermined contributors. Mast cell tumors, as with all cancers, tend to
be associated with age. Older dogs are more likely to develop cancerous
growths, with the average age of a dog with MCT being 8-9 years.

Prevention: Because the cause is unknown, there is no known way
to prevent mast cell cancer. Presumably, the best one can do is to
maintain their pet�s overall health and be alert to signs of tumor growth
or ill health.

Diagnosis hopefully begins early when the alert pet-owner
notices a growth on his dog. The vet may take a fine-needle aspirate from
the growth to submit a sample for preliminary biopsy. The entire tumor
will then need to be fully removed, if possible, and submitted for biopsy.
Blood tests may include a complete blood count, serum chemistry profile,
and buffy coat. The CBC may reflect low or high white blood cell count,
low platelet count, elevated mast cell counts. The buffy coat is
diagnostic (although subject to false-positives) and reflects mast cells
circulating in the bloodstream where they are ordinarily not found in
large numbers. A positive buffy coat suggests bone marrow involvement.
Other tests may include urinalysis, lymph node aspirate, bone marrow
aspirate, x-rays, and ultrasound. The pathologist assigns a "grade" to the
tumor, a somewhat subjective assessment of how well differentiated the
cells are and therefore how aggressively malignant the cancer appears to
be. The practicing veterinarian and/or oncologist assigns a "stage" to the
cancer, as well. The supplemental lab work gives an estimate of how great
a foothold the cancer has, and this is reflected in the staging.

Histiologic Grade: The pathologist will apply specialized stains
to the tumor sample and microscopically examine it in order to determine
its grade. AgNOR staining appears to be the most conclusive prognosticator
for mast cell tumors. Tumor grade is associated with the degree of
differentiation of the mast cells. The majority of MCT are Grade 1 tumors
and are well differentiated and appear to have a very good prognosis with
no treatment beyond complete surgical removal. Grade 2 tumors are
moderately differentiated, and the prognosis and treatment options are
perhaps most complicated and difficult to predict. Grade 3 tumors are
poorly differentiated, very aggressive, and most likely to rapidly
metastasize. They carry the poorest prognosis but are fortunately the
least common. Histiologic grade is most predictive of prognosis.

Stage: Staging refers to degree to which the cancer has already
spread at the time of diagnosis. Tumors caught early - before they have
invaded other tissues or caused signs of systemic disease - logically
carry the most optimistic prognosis. MCT most commonly metastasizes to
lymph nodes, bone marrow, liver and spleen, so much of the lab work
focuses on these areas to detect and assess abnormalities. It is unusual
for MCT to spread to the lungs, as is so common with many cancers. In
simplest terms, Stage 1 refers to a single tumor with clean margins and no
signs of spreading. Stage 2 and Stage 3 show progressively greater signs
of invasion, perhaps to local lymph nodes, demonstrating dirty margins, or
presenting as multiple tumors. Stage 4 involves systemic metastasis and
carries a grave prognosis.

Local symptoms: The most obvious sign of mast cell cancer is
likely to be a tumor of some sort. Mast cell tumors can appear singly, in
groups, lie on the surface of the skin or underneath it, crop up anywhere
on the body, and defy easy description. You just don�t know it�s a MCT by
looking at it. Most (at least half of) mast cell tumors are found in or
under the skin on the trunk of the body itself, and the vast majority of
the remainder are found on the extremities, especially the hind limbs.
They are less commonly found on the head and neck, and less commonly still
arise from tissues other than the skin. If they are very swollen or
ulcerated, there may be pain, but most MCT�s are unlikely to be painful.
It has been observed that higher-grade tumors may be more likely to be
ulcerated in appearance and cause local irritation.

One characteristic quirk of mast cell tumors is the tendency for them
to change in size, even on a daily basis. A tumor that gets bigger and
smaller, seemingly on a whim, may be a MCT. Another idiosyncrasy is the
potential of the tumor to produce "Darier�s sign" if poked and prodded.
Handling these tumors - even a routine veterinary palpation or needle
aspirate - can cause a heavy release of histamine that results in
swelling, redness, itchiness, hives (wheal formation).

Systemic symptoms: Symptoms are variable, depending on the
location of the tumor and the degree to which is has developed and/or
spread. Signs of systemic involvement may include: loss of appetite,
vomiting, bloody vomit, diarrhea, abdominal pain, dark or black feces,
itchiness, lethargy, anorexia, irregular heart rhythm and blood pressure,
coughing, labored breathing, various bleeding disorders, delayed wound
healing, enlarged lymph nodes.

Treatment for mast cell tumors almost always first involves
surgically removing the entire tumor, if that is possible. It is
particularly important to remove a wide margin or large area of healthy
tissue (2-3 cm in each direction) around the perimeter of the tumor. The
idea is to capture any stray cancerous cells that aren�t immediately
obvious. In some areas, such as a leg or paw, it is virtually impossible
to remove sufficient tissue to satisfy this 2-3 cm oncological rule. The
tumor is then submitted to a laboratory for biopsy, and a pathology report
is generated. Very important is the determination of whether or not the
margins of the submitted tissue removal are clean (showing no signs of
cancerous cells) or dirty (showing invasion of cancerous cells). If the
margins are dirty, further surgery or radiation may be indicated to
attempt to remove or kill any remaining cancerous cells. Nonetheless, a
significant proportion of tumors that are incompletely excised (i.e.,
dirty margins) do not return.

Beyond complete surgical excision, treatment options depend on factors
that suggest the aggressiveness and status of the cancer. Low-grade tumors
are generally treated locally with surgery, with or without radiation.
High-grade tumors may be treated systemically with prednisone and/or other
chemotherapy. Sometimes the only "treatment" is supportive care intended
not to extend the dog�s life but to make what remains of it as comfortable
as possible.

Local treatments: Surgery is used to remove local cancerous
tissue when possible. Sometimes surgical sites for MCTs are resistant to
healing. Radiation may be used afterwards to locally kill off remaining
cancerous cells. Sometimes radiation is used in place of surgery to shrink
tumors that cannot be easily operated on. Radiation is highly effective in
controlling mast cell cancer, yet radiation is not for everyone. Dogs
treated for mast cell tumors may have 3-5 radiation sessions per week for
3-5 weeks and must be anesthetized for each treatment. This may cost
several thousand dollars, may require travel and/or boarding during the
treatment period, and may have short or long-term side effects that owners
find objectionable.

Systemic treatments: Prednisone (a corticosteroid) appears to be
the drug of choice in treating mast cell cancer, so much so that many
veterinarians will routinely prescribe several weeks or months of
prednisone subsequent to surgical removal of any mast cell tumor. In the
great scheme of things, prednisone is inexpensive and safe. Side effects
include increased drinking, urination, and appetite and potential
gastrointestinal upset. If a tumor has a reasonably high metastatic
potential, or if it has already metastasized, prednisone is likely to be
prescribed.

Sometimes other chemotheraputic drugs are used in combination with
prednisone if the mast cell cancer appears to have metastasized. These may
include: CCNU (lomustine), vinblastine, vincristine, doxorubicin,
mitoxantrone, cyclophosphamide (cytoxan), and L-asparginase. These are all
heavy-duty drugs with potential side-effects that include severe
immunosuppression, vomiting, diarrhea, liver damage. Several studies seem
to demonstrate their limited efficacy in conjunction with surgery, but in
and of itself, chemotherapy is no match for inoperable or metastatic mast
cell cancer. Responses to chemotherapy are minimal in cases of MCT.

Palliative treatment (supportive care): Sometimes one cannot do
anything more than offer supportive care to a dog with mast cell cancer.
The cancer may have spread too far. The dog may be old or have other
health impairments. The owners may object to more aggressive treatment. It
is the inability to manage such things as vomiting/diarrhea from gastric
ulceration that typically marks the end of quality of life for the dog
with mast cell cancer. Thus, controlling symptoms may be more important
than battling the cancer, per se.

Various drugs are used to prevent or manage potential problems caused
by the chemicals that mast cells release. In effect, one isn�t addressing
the cancer itself. One is simply trying to maintain quality of life by
fighting the deleterious effects of the chemicals secreted by the mast
cells. Corticosteroids like prednisone, antihistamines like benadryl, and
antacids like tagamet, zantac or pepcid are often used as a matter of
course after surgery and/or radiation and are routinely used to maintain
comfort in dogs where the cancer cannot otherwise be treated or has
spread. Sucralfate may be helpful with dogs that have bleeding ulcers, as
it coats the surface of the ulcer to protect it and allow for healing.
Further, there are now special foods available for dogs with cancer - like
Hills Prescription Diet n/d - which are intended to preferentially nourish
the patient but not the cancer itself, thereby increasing both quality and
length of life.

Recurrence: Dogs who have had mast cell tumors are more likely
to develop more mast cell tumors. 50% of surgically removed mast cell
tumors will re-grow in the same area.

Prognosis: Prognosis is highly variable and dependent on many
factors including tumor location, histiologic grade and clinical stage.
One statistic suggests that approximately half of all MCT�s are curable
with complete surgical removal and prednisone treatment. Dogs that are
tumor-free after 6 months are considered unlikely to have a recurrence.
Primary tumors that originate in areas other than the skin tend to be more
aggressive than cutaneous tumors. MCT in preputial (sheath), perineal
(groin), subungual (nail bed), and oral regions areas are generally the
most malignant. MCT of bone marrow or visceral tissue is particularly
grave. The higher the grade or stage, the worse the prognosis. Boxers are
especially at risk for developing mast cell tumors, yet they tend to have
less aggressive (lower grade) MCT than other breeds. Dogs showing systemic
signs and dogs whose tumors return after surgical removal have poorer
prognosis. Similarly, the faster the growth of the tumor, the worse the
prognosis. One study found that dogs with tumors that grow more than 1cm
per week have only a 25% chance of surviving 30 weeks. MCT's that exist
locally for several months without showing signs of rapid growth tend to
be benign.

Prognosis/Grade-1 MCT: A grade 1, stage 1 cutaneous MCT is
likely to never return after surgical removal. The estimated long-term
survival rate for dogs with such tumors is over 90%.

Prognosis/Grade-2 MCT: It is reported that 50-75% of dogs with
Grade 2 MCT survive long-term (beyond 35 weeks). Another study concluded
that 44% of dogs with Grade 2 MCT survived long-term (over 4 years) after
nothing more than complete surgical removal of their tumors. Yet another
author reports a 45% mortality rate.

Prognosis with radiation: Radiation may increase survival. One
study concluded that 86% of its subjects, dogs with Grade 2 MCT, survived
long-term (over 5 years) with a combination of surgery and radiation
(compare that to the 44% mentioned in the previous paragraph). Another
author cited a 48-77% remission rate for treatment plans that included
radiation. Yet another study reported a 94% disease-free rate at one year
in dogs with Grade 2 MCT (dirty margins) after surgery and radiation, and
an 86% disease-free rate after 5 years. And another study still reflects a
75% survival rate two years after radiation and prednisone treatment. As
with anything, any research results need to be taken with a grain of salt
unless the entire research protocol is examined. These are merely
statistics that one can readily discover in various citations online and
in books. Nonetheless, they all appear to suggest that radiation after
complete surgical removal is statistically the most effective way to
combat (primarily Grade 2) MCT.

Prognosis with prednisone: Prednisone, with or without other
chemotherapy, is estimated to induce partial or complete remission in
approximately 20% of canine patients with MCT. The reason for prednisone�s
effectiveness is not fully understood, yet its use is pervasive in
treating this condition.

Prognosis/Grade-3 MCT or metastasis: Only 15% of dogs with Grade
3 MCT will be alive 7 months after surgery, and only 6% will be alive
after 2 years. Once mast cell cancer has metastasized, dogs tend to die
within 6 months as the symptoms of systemic illness (e.g., gastric ulcers)
can no longer be managed and the animal loses its quality of life.

Bottom line: Mast cell tumors occur frequently in dogs. They are
well documented and well understood in some respects, yet they are
unpredictable in appearance and behavior. No one knows what causes mast
cell cancer, nor how to cure it with certainty once it has metastasized.
The best one can do as a pet owner is be attentive to the general health
of one�s dog and get all unusual lumps and bumps examined carefully to
rule out MCT or to catch it in the earliest stages. MCT can be lethal, but
it is not necessarily so. If you�d like to send me the details of your
experience with mast cell cancer in your dog, I�d be fascinated to read
it. Perhaps I can compile a section with this information at some point in
the future.

Disclaimer: I am not a veterinarian, nor do I have any
particular veterinary/scientific/medical training or knowledge. I don�t
even play a vet on tv. This article represents my understanding, as a
layperson, and is intended merely as a starting point for the fellow
layperson/pet-owner. I assume no responsibility for inaccuracies. As
always, consult with a qualified veterinary professional to assess and
manage your pet�s health. Please notify me if you encounter factual errors
or would like to further my understanding of the above
condition.

Related References (opens new
window)

Treatment protcols for MCT. University of Florida
College of Veterinary Medicine.

An overview of mast cell disease in dogs and cats.
University of Georgia College of Veterinary Medicine.

Mast
cell tumors in dogs and cats. Columbia Animal Hospital, Maryland.

Mast cell
tumors. Vetinfo; Dr. Michael Richards, Virginia.

Mast cell tumors. Dr. Joe Bodewes; Drs. Foster and
Smith, Inc., Wisconsin.

Mast cell tumors in dogs. Oncolink; University of
Pennsylvania Cancer Center.

Canine mast cell tumor. Vetcentric; Maryland.

Mast cell tumors. Patricia Long; Bernese mountain dog
home page.

Mast cell tumors - Flat-coated retriever society of
America.

Mast cell tumors in dogs - Dr. Gail Mason, DVM,
Bath-Brunswick Veterinary Associates.

Mast cell tumors in dogs and cats - Michigan
Veterinary Specialists.

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