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Name
Mast Cell Tumors, Canine
Short Description
Mast cell tumor
Affected Animals: 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.
