MAST CELL
TUMORS
The
following information is simply informational. It's intent is not to
replace the advice of a veterinarian nor to assist you
in making a diagnosis of your pet. Please consult with
your own veterinary physician for confirmation of any diagnosis.
Your pets life may depend on it.
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OVERVIEW*
These tumors (also called
mastocytomas, mast cell sarcomas) are the most frequently recognized
malignant or potentially malignant neoplasms of dogs. In addition,
leukemic and visceral forms can occur. A viral etiology has been
speculated but remains controversial. These tumors may occur in dogs
of any age (average 8-10 yr). They may occur anywhere on the body
surface as well as in internal organs, but the limbs (especially the
posterior upper thigh), ventral abdomen, and thorax are the most
common sites; ~10% are multicentric. Many breeds appear to be
predisposed, especially Boxers and Pugs (in which tumors are often
multiple), Rhodesian Ridgebacks, and Boston Terriers. The tumors
vary markedly in size, and clinical appearance alone cannot
establish a diagnosis.
CLINICAL
SIGNS:*
Most commonly, they appear
as raised, nodular masses that on palpation may be soft to solid.
Although they often seem encapsulated, mast cell tumors in dogs are
seldom discrete. Rather, they consist of a highly cellular center
surrounded peripherally by a "halo" of smaller numbers of mast cells
that palpate as normal skin. Dogs can also develop clinical signs
associated with the release of vasoactive products from the
malignant mast cells. Most common is gastroduodenal ulceration that
may be present in up to 25% of cases.
This is Peanut, a Boston
Terrier presenting with what his vets have called, "the worse
case" of mast cell tumors they have ever
seen.
This is Cucumber, a beagle mix, with
Mast Cell in it's advanced stages. Hers began as a small lump on the
left side of her muzzle. As you can see in the photos above, the
cancer has caused deformity of her face, listing her nose to the
right and pushing her left eye from proper seating in it's socket.
The open area by her mouth was caused by scratching most likely due
to the discomfort it caused her.
DIAGNOSIS:**
The behavior of mast cell tumors is
variable in that some are rapidly fatal and others are benign. One
in eleven cases will appear as multiple nodules involving all the
skin. I like to refer to mast cell tumors as cancer and “tricksters”
because they can’t be trusted to behave according to their
classification. Most pathologists will report them as Grade II,
which means they don’t know how they’ll behave. The Grade III cases
are almost always fatal. Some will appear rapidly on the face feet
or axilla and resemble insect bites.
One can distinguish mast cell tumors from
benign fatty tumors with cytology, the examination of cells from a
fine-needle aspirate. It is excellent practice to perform
cytology before surgery. I like to
use New Methylene Blue stain on all my cytology specimens. The dark
blue storage granules of mast cells are easy to see under
microscopic examination of the stained aspirate. Early diagnosis and
aggressive treatment are most effective against this common cancer.
*Cytologic evaluation of
Wright's-stained, fine-needle aspirates or impression smears can be
used to establish the diagnosis of mast cell tumors in dogs.
However, cytology is not a substitute for histopathology—only the
latter has been correlated with prognosis. Two systems of
histopathologic grading have been defined, and to avoid confusion,
it is essential to know which of the two systems is being used.
*Although there
is believed to be a benign variant of canine mast cell tumor, there
is no clinical or microscopical means of identifying it. In
addition, small mast cell tumors may remain quiescent for long
periods before becoming aggressive. Thus, all should be treated as
at least potential malignancies.
TREATMENT:*
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Treatment depends on the
clinical stage of the disease. For Stage I tumors (a solitary
tumor confined to the dermis without nodal involvement), the
preferred treatment is complete excision with a wide margin;
at least 3 cm of healthy tissue surrounding all palpable
borders should be removed in an attempt to excise both the
nodule and its surrounding "halo" of neoplastic cells. If
histologic evaluation suggests that the tumor extends beyond
the surgical margins, reexcision should be attempted.
Alternatively, because mast cells are sensitive to radiation,
radiation therapy may be curative if the remaining tumor is
small or can only be seen microscopically. Combined radiation
and hyperthermia may be more effective than radiation alone.
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At present, there is no agreed upon mode
of therapy for Stage II-IV mast cell tumors. For Stage II
tumors (a solitary tumor with regional lymph node
involvement), options include excision of the mass and the
affected regional node (if feasible), prednisolone, and
radiotherapy, used either singly or in combination. Treatment
of Stage III (multiple dermal tumors with or without lymph
node involvement) or Stage IV (any tumor with distant
metastasis or recurrence with metastasis) tumors is generally
palliative. One recommended therapy is prednisolone (2 mg/kg
body wt, PO, for the first 5 days, followed by a maintenance
dose of 0.5 mg/kg, daily) or intralesional injections of
triamcinolone (1 mg/cm diameter of tumor, every 2 wk).
Treatment with H1- and H2-receptor
antagonists for the peripheral and gastric effects of
histamine, respectively may be indicated for animals with
systemic disease or clinical signs referable to histamine
release. Chemotherapy with vinca alkaloids (vincristine,
vinblastine), L-asparaginase, and cyclophosphamide has also
been used with some effectiveness.
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Since intraoperative
radiation therapy is the most aggressive approach being used on
sarcomas in the U.S.A. today, we treat dirty tumor beds and
recurrent mast cell cancer in this fashion. Our data suggests that
tumor bed implants with steroids and delivering 1,000 centiGray of
radiation during
surgery into the tumor bed increases long term remissions
and survival in animals with sarcomas of any histologic type and
especially mast
cell.
*We also deliver
intratumor injections on a weekly basis and evaluate the reduction
in size of non operated mast cell tumors. We also use cryotherapy to
freeze small mast cell tumors in patients who have multiple small
nodule disease.
These techniques
are attractive to clients who have old pets for which they decline
anesthesia and surgery.
CLINICAL TRIALS FOR MAST CELL
TUMORS
The Animal Cancer Institute
sites
A clinical trial is now open for selected mast cell
tumor patients. The objective of this study is to determine the
efficacy and tolerability of a novel, oral, investigational protein
kinase inhibitor for the treatment of dogs with recurrent mast cell
tumors.
Clinical Trial for Canine Mast Cell Tumor
Patients
Trial eligibility criteria include:
- measurable recurrent cutaneous mast cell tumor
- no more than one regional lymph node involved
- no visceral (liver, spleen, intestinal) metastases
- limited past use of chemotherapy and/or radiation therapy IS
acceptable
Trial Support/Funding Includes:
- diagnostic tests (to define eligibility and for follow up)
- oral treatment agent
- follow up examinations
Eligible patients will have the opportunity to
receive the investigational compound under closely monitored
conditions while participating in the study (limitations apply).
Dogs will receive the oral medication over a 6-week initial phase.
Follow-up will include weekly examinations during the initial phase
and then re-check examinations every 6 weeks pending response.
Participating Animal Cancer Institute Network
trial sites:
Animal Cancer Institute at Friendship Hospital
202-363-7300
Beltway Oncology and Internal Medicine
301-805-5680
Atlantic Veterinary Internal Medicine
410-224-0121
Regional Veterinary Referral Associates
703-451-8900
Southpaws Veterinary Referral Center
703-451-3635
VCA - Veterinary Referral Associates
301-340-3224
TOP
CLINICAL TRIAL
RESULTS:***
1.
Prostaglandins Leukot Essent Fatty Acids. 2003 May;68(5):317-22.
Faculty of
Veterinary Medicine, Institute of Physiological Chemistry, University of
Leipzig,