Canine Mast Cell Tumors
Mast cell tumors comprise 20-25% of all cutaneous
and subcutaneous tumors in dogs, making them the most prevalent
canine skin tumor. Mast cell tumors may occur in the intestines,
liver, spleen, and bone marrow at a much lower prevalence.
Mast cell tumors most frequently develop in dogs at an average
age of eight years.
The clinical signs of mast cell tumors are variable
and dependent on the location and grade of the tumor. Mast
cell tumors can be found on all areas of the skin. Mast cell
tumors most commonly appear as small raised nodular masses
that vary from firm to solid on palpation, but no typical
morphology of mast cell tumors exists. Mast cell tumors can
imitate the appearance of most other skin tumors, making diagnosis
based on physical exam difficult. Most mast cells are solitary
tumors, but approximately 10% are multicentric or infiltrative.
Other clinical signs of mast cell tumors relate
to the release of mediators stored in their intracellular
granules or cytoplasm. These mediators include histamine,
heparin, proteolytic enzymes, and other cytokines. The most
common sign is gastric ulceration via histamine, which can
manifest as vomiting, diarrhea, or anemia. Similarly, local
ulceration near the cutaneous neoplasm may develop as a result
of inflammation and edema. Delayed wound healing may be caused
by the proteolytic enzymes and the activation of macrophages
via histamine to release fibroblastic suppressor factor.
In addition, coagulation abnormalities due to heparin release
or hypotensive shock due to a massive histamine release may
occur.
A diagnosis of a mast cell tumor can be accomplished
through cytological examination of a fine needle aspirate
of the tumor. Cytology of a typical mast cell tumor will
reveal a large number of discrete round cells with abundant,
small, uniform, basophilic cytoplasmic granules located intracellularly
and extracellulary. The granules may stain poorly with Diff
QuickÒ stains. The nucleus will appear round to oval, but
may be hidden in heavily granulated cells. Mast cells can
vary greatly in appearance, and some tumors may contain agranular
mast cells. Varying numbers of eosinophils and neutrophils
may also be scattered throughout the smear.
Preliminary staging of the tumor should include
palpation of the local lymph nodes, which are the most common
sites of metastasis. The liver and spleen should also be
palpated because hepatomegaly and splenomegaly are common
with disseminated mast cell neoplasia. Cytology of the regional
lymph nodes and bone marrow, as well as an enlarged liver
or spleen, can reveal increased numbers of mast cells signifying
metastasis. Greater than 10 mast cells per 1000 nucleated
cells in the bone marrow is indicative of neoplastic infiltration.
.
The preferred treatment for a mast cell tumor
confined to the dermis with no nodal involvement is complete
excision with a wide margin of at least 3 cm. Although most
tumors palpate as discrete masses, most mast cell tumors are
not discrete but surrounded by small numbers of neoplastic
cells. Histologic examination of the tissue removed is imperative
to confirm the diagnosis, grade the tumor, and evaluate completeness
of excision. If histology reveals a poor margin of excision,
a second aggressive surgery is the treatment of choice. If
a proper margin of excision cannot be obtained, treatment
usually involves surgery to remove as much of the tumor as
possible followed by radiation or chemotherapy. No protocol
has been agreed upon or proven to be markedly effective for
malignant tumors, multiple tumors, and tumors that cannot
be excised. A treatment plan for these tumors usually involves
corticosteroids as well as chemotherapeutic drugs and radiation
therapy. Palliative treatment with cimetidine and histamine
blockers can help prevent gastric ulceration and some of the
other secondary effects of the tumor.
Mast cell tumors are graded histologically based
upon the number of granules, mitotic index, and cellular characteristics
of malignancy. The grading scale divides mast cells into
three groups. The distribution within these groups is: 40%
differentiated, 40% intermediately differenti-ated, and 20%
undifferentiated. Prognosis is excellent for a solitary well
differentiated tumor that can be easily excised with good
surgical margins.
- by Matt Renninger, Class of 1999
- edited by Janice Lacey, DVM
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