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Encyclopedia Entry |
Illness

Category:
Canine

Mast cell tumor
Mast cell tumor, histiocytic mastocytoma, mast
cell sarcoma

AffectedAnimals:
Mast cell tumors are very common in dogs. Dog breeds more
commonly affected include boxers, Boston terriers, bullmastiffs,
English setters, and golden retrievers. Older dogs are more commonly
affected, but MCT can occur in any age or sex.

Overview:
Mast cell tumors, or MCTs, are among the most common
tumors in dogs, with the skin being the most common primary site for
MCTs in this species. Mast cell tumors can occur anywhere in the
body. The systemic form of mast cell tumors, with visceral,
lymphatic or bone marrow involvement is referred to as mastocytosis.
Despite the sometimes "benign" appearance, mast cell tumors tend to
exhibit a very unpredictable biological behavior. Therefore many
veterinary oncologists consider all mast cell tumors malignant until
proven otherwise.

The mean age of dogs with mast cell tumors is nine years.
Predisposition to mast cell tumors appears to occur in Boxers,
Boston terriers, beagles, bulldogs, and Schnauzers. Although Boxers
appear to be at a higher risk for development of MCTs, most tumors
in this breed tend to be well differentiated.

When feasible, treatment involves addressing the tumor locally,
with surgery and/or radiation treatment. In some cases, systemic
treatment may be necessary, but is often not successful due to the
extent of the disease.

Clinical Signs:
The clinical signs of mast cell tumors in dogs are
variable, and depend upon anatomic location. Tumors may originate in
the dermis or in the subcutaneous tissues, and usually have
overlying, intact skin, although ulceration can also occur.
Palpation of these masses may produce Darier's sign, which is
erythema, or redness, of the skin secondary to the release of
histamine from the tumor. MCTs in dogs present as solitary or
multiple masses. They are found primarily in the trunk, perineum,
and extremities; and they are less common in the head and neck
region. Variations in tumor size, even on a daily basis, are a
relatively common historical sign in dogs with MCTs, as local
swelling occurs and subsides. The affected area may be painful or
sensitive to touch, especially if there is ulceration or marked
swelling. Limbs may become swollen, with pitting edema, secondary to
regional lymph node involvement and obstruction to venous and
lymphatic fluid return from the tissues below.
Vomiting,
diarrhea, poor appetite and weight loss may be noted in patients
with systemic involvement. Stools may be melenic or darkened by the
presence of digested blood, produced from gastrointestinal erosion
or ulceration. Anaphylactic shock, with rapid heart rate, pale
mucous membranes, weak pulses and collapse occurs in some patients
that experience massive release of tumor substances into the blood
stream.

Symptoms:
See Clinical Signs.

Description:
Mast cell tumors are among the most common tumors in
dogs. They may occur anywhere in the body, and may be benign or
malignant. Mast cell tumors, or MCTs, arise from mast cells, which
are normal components of the body. Mast cells originate in the bone
marrow and migrate to various locations throughout the body,
especially in connective and vascular tissues. Mast cells in dogs
normally contain histamine and heparin; these substances play a
significant role in the inflammatory response to various disease
processes, and in wound healing. Histamine released from mast cell
tumors causes some of the signs and symptoms of the disease, and may
produce secondary disease as well.
How a mast cell or clone of
mast cells becomes an MCT is not known. A genetic link is strongly
suggested by the relatively frequent occurrence of mast cell tumors
in certain breeds of dog, especially Boxers. Chronic inflammation
has been suspected as a facilitator of tumor formation; mast cells
tend to concentrate in chronically inflamed tissues. Viruses have
been proposed as a cause based on an experimental model. However, no
supporting evidence for a viral cause has been identified thus far.

In dogs, the skin and subcutaneous tissues are the most
common locations for mast cell tumors. Up to one out of five MCTs
occur in the skin. These tumors are classified as the cutaneous
form. These masses arise in or beneath the skin, and vary in size.
Palpation of the tumor may result in the release of histamine, which
causes local redness, hives and itchiness of the skin. Although MCTs
located elsewhere are more likely to be malignant, cancerous,
cutaneous-form mast cell tumors are not uncommon in dogs.

The systemic form of MCTs -- called mastocytosis -- is a
second class of these neoplasms, or growths. These mast cell tumors
form in organs and other deep tissues of the dog, including the
intestines, spleen, lymphatics, and other tissues of the
recticuloendothelial system. The systemic forms of MCTs are more
likely to produce signs and symptoms of systemic disease;
gastrointestinal tumors may produce ulceration of the stomach and
duodenum, and associated symptoms of diarrhea, vomiting, anorexia,
and melena.

Mast cell tumors in the preputial, perineal, and inguinal
regions tend to demonstrate more malignant behavior. MCTs can
metastasize, or spread, to any part of the dog's body. However,
metastatic mast cells will most likely spread to the regional lymph
nodes, spleen, and liver. Spread to the lungs is not common.

Diagnosis:
Microscopic study of aspirated or excised tissues
provides important diagnostic information. A provisional diagnosis
of mast cell tumor based on history, physical exam findings, and
clinical signs is often confirmed with evaluation of tumor samples
obtained by the fine-needle aspirate technique. Granulated mast
cells are easily identified in fine-needle aspirates of mast cell
tumors. Mast cells are round and typically contain large, purple
cytoplasmic granules. However, undifferentiated tumor cells may not
always be identified with this method.
Examination of excised
MCT-tissues allows histologic grading and determination of the
completeness of excision; it may be required for a definitive
diagnosis in undifferentiated MCTs. Special staining techniques aid
the pathologist in determining the diagnosis and extent of tissue
invasion. Histologic classification of mast cell tumors typically
follows the system based on the degree of differentiation and
infiltration; it classifies MCTs as well differentiated, or grade I,
moderately differentiated, or grade II, and poorly differentiated,
or grade III. High-grade tumors have indistinct granules, with
variable staining. The cells may be bizarrely shaped, rather than
round, and are variably sized.

Additional diagnostic studies may be conducted to help
identify the presence of metastatic, or spreading, disease.
Diagnostic evaluation should include abdominal x-rays or
ultrasonography to identify hepatomegaly, or liver enlargement,
splenomegaly, or splenic enlargement, or lymph node involvement.
Ultrasound may be more sensitive than abdominal x-rays in assessing
the spread of mast cell tumors. Thoracic x-rays can detect lymph
node disease in the chest.

A complete blood count, or CBC, can detect the presence of
cytopenias, including low platelet count, low red blood cell count,
and low white blood cell count. The CBC will also demonstrate
mastocythemia, or elevated mast cell count, which suggests spread of
the mast cell tumor into bone marrow. White blood cells may be
elevated due to circulating inflammatory compounds or to the
presence of gastrointestinal ulceration. The recognition of mast
cells in circulation can be improved by concentrating the white
blood cell fraction of blood in a buffy coat preparation.

Prognosis:
Multiple factors determine the prognosis for dogs with
mast cell tumors. Dogs with rapidly growing tumors have a poorer
prognosis than dogs with slow-growing masses. Animals with
MCT-associated systemic signs+ loss of
appetite, vomiting, dark, tarry stools, gastrointestinal ulcers+ have a poorer prognosis. Location of the tumor
also has an impact on prognosis. Mast cell tumors in the inguinal
and perineal areas appear to be more aggressive than mast cell
tumors at other locations. Well-differentiated tumors tend to have a
better prognosis than undifferentiated tumors. Prognosis is better
if the tumor is in an early, localized clinical stage, rather than
in an advanced stage where the tumor cells have spread. Dogs with
tumors recurring after local surgical excision generally have a
poorer prognosis. Breed also may determine prognosis; MCTs in boxers
tend to be less aggressive than in other breeds.
Early detection
and aggressive treatment of mast cell tumors may result in a
complete cure in dogs. Even dogs with multiple mast cell tumors, or
with recurrent mast cell tumors in different regions of the body,
may have a good, long-term prognosis if treated early and
aggressively.

Transmission or
Cause:
The etiology, or cause, of
mast cell tumors in dogs and cats is unknown. Some breed
predisposition suggests a heritable cause. Other proposed causes of
MCTs include chronic inflammation and viral infection, although no
proof exists to support a viral etiology.

Treatment:
All mast cell tumors should be treated regardless of
their size. The type of treatment selected depends on the clinical
stage, histologic grade, size and location of the tumor. Aggressive,
local surgical excision is the primary therapeutic approach; other
treatment methods are employed adjunctively, or in certain
situations. Establishing a definitive diagnosis and treatment plan
prior to instituting therapy is therefore extremely important in
dogs with mast cell tumors.
Ideally, the goal of surgery is to
remove the entire tumor mass and any tissue surrounding it,
including lymph nodes, that may harbor spreading tumor cells. Wide
excisional margins, at least three centimeters in all directions,
should be obtained where feasible. Excised tissue is submitted to a
pathologist for careful histopathologic evaluation. The tissue
margins are examined for evidence of incomplete tumor excision.

In cases where the pathologist reports incomplete excision,
either additional surgery or radiation therapy may be necessary. If
adequate surgical margins are impossible to obtain despite
aggressive surgery, radiation should be considered as an additional
treatment.

Chemotherapy may be employed if surgery and radiation
treatments fail to eliminate all of the tumor cells, or if these
local treatment methods are not feasible. Prednisone has been shown
to be efficacious in certain cases of mast cell tumors. This
response appears to be variable, but complete responses have been
demonstrated. Chemotherapy has been partially successful with
lomustine (CCNU) and possibly a combination protocol consisting of
vinblastine, cyclophosphamide, and prednisone.

For cases where surgery alone, or surgery with radiation
therapy, have a good chance of completely eliminating the tumor and
any metastatic, or spreading, disease, then these local treatment
approaches should be attempted first.

Prevention:
Since the etiology of mast cell tumors is unknown except
for heredity, preventive measures do not currently exist. Owners may
detect tumors at an early stage by petting and grooming their
dogs.

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