Canine Mast Cell Tumors
Incidence/Cause
Mast cell tumors (MCT) in dogs are very common, accounting for approximately
20% of all skin tumors in dogs. For most dogs, the underlying cause
promoting the development of the tumor is not known. Mast cell tumors can
arise from any skin site on the body, and can have a variety of appearances.
Veterinary oncologists recommend that before any skin lump is removed, the
cells from the mass be collected for examination to rule out the lump as a
mast cell (or other malignant) tumor. And mast cells are something that are
easily identified on aspiration.
What you might see/ Clinical presentation
MCT most commonly are seen as solitary lumps or masses in or underneath the
skin; occasional dogs can have multiple masses from MCT. MCT can look like
just about anything, ranging from benign-appearing lumps (such as a lipoma),
to more angry or ulcerated lumps, masses with a stalk or focal thickenings
in the skin. MCT may change quickly in size (become larger then smaller )
because of reactions around the mass. Some dogs may have signs of systemic
disease, which can be caused by some of the biologically active compounds
found within mast cells. In most cases, evidence of a MCT is easily
generated by examination of a fine-needle aspirate of the suspect mass, and
aspiration is advised before removal of a mass to be sure it is not a MCT
(or other skin malignancy), a finding that would demand a more aggressive
surgical removal. Often, obtaining blood for a complete blood count and
biochemical profile, and a urinalysis will be advised as these can help
assess overall health and provide information that potentially influences
treatment recommendations.
Biological behavior of mast cell tumors
Most mast cell tumors
are considered locally invasive, and can be difficult to remove completely
because of the extent of local spread. The behavior of mast cell tumors
reflects their grade (a term used by pathologists and oncologists to
describe such things as how-well differentiated a tumor is, how frequently
it is dividing, how invasive to adjacent structures, and other criteria).
Mast cell tumors have 3 grades, with grade I being the least aggressive and
least likely to spread to other organs (metastasize), and grade III being
highly aggressive tumors with a high likelihood of metastasis; most grade II
tumors tend not to metastasize, although they can do so. Mast cell tumors
show a predilection to spread to regional lymph nodes, liver, spleen, and
bone marrow.
Clinical staging (determination of the extent of the tumor)
Because of the organs to which these tumors like to spread (metastasize) to,
staging a dog with a mast cell tumor (usually reserved for occasional grade
I tumors, most grade II tumors and all grade III tumors) entails collection
of cells from regional lymph nodes for microscopic examination, imaging the
thorax (radiographs) and abdomen (radiographs, abdominal ultrasound) for
enlargement of lymph nodes, liver or spleen, and some assessment of bone
marrow involvement, either a bone marrow collection for microscopic
examination, or examination of the white blood cells for circulating mast
cells (interpreted to mean that mast cells are in the bone marrow).
Treatment options
Surgical removal is the mainstay of treatment of canine mast cell tumors.
Because of their locally invasive behavior, wide margins of what appears to
be normal tissue around the tumor needs to be removed to increase the
likelihood that the tumor has been completely removed. For mast cell tumors
that were not, or because of location, could not be completely removed,
radiation therapy is often the best treatment for residual disease, although
a more aggressive second surgery is possible for some dogs. Chemotherapy is
sometimes used to treat mast cell tumors, but chemotherapy is usually
reserved for dogs with grade III tumors; mast cell tumors are notoriously
unpredictable tumors with regards to response to chemotherapy. In addition
to treatment of the tumors, some dogs will be treated with medications that
tend to help fight the secondary effects of the tumor. These usually include
drugs like prednisone, an anti-histamine like Benadryl, and an antacid type
medication like Pepcid.
Prognosis
The prognosis for completely removed grade I and grade II tumors is
excellent. The prognosis for incompletely removed grade I and II tumors
treated with radiation therapy after surgery is also excellent with
approximately 90-95% of dogs having no recurrence of tumor within 3 years of
receiving radiation therapy. The prognosis for dogs with grade III tumors is
considered guarded as local recurrence and/or spread is likely in most dogs.
If your dog is diagnosed with a grade III Mast cell tumor most likely
chemotherapy will be recommended as at least part of the protocol. Drugs
used at WSU include Lomustine and Vinblastine.
Future treatment options
Mast cell tumors have been examined for mutations in a certain gene known as
c-kit. This c-kit gene belongs to a family of genes codes for receptor
tyrosine kinases. Receptor tyrosine kinase inhibitors are an active part of
cancer treatment research in the human field and these drugs are being
looked at treatments for canine Mast cell tumors as well. No drug is
currently available commercially.
Key points
Dogs that develop Mast cell tumors seem to like to develop more of them and
this is not necessarily the same as metastasis. So any dog diagnosed with a
mast cell tumor must be watch closely in the future for the development of
new tumors. As long as tumors are caught when small, surgical removal is
usually adequate for treatment. Mast cell tumors can also be very
unpredictable tumors. Even grade I and II tumors can behave aggressively in
terms of metastasizing and or being difficult to control locally, in any
given individual dog. Statistics, while useful, can never predict how an
individual dog will fare with or without specific treatment.
Other helpful sites
http://www.marvistavet.com/html/body_mast_cell_tumors.html
http://www.vrcc.com/disease_mc_tumors.shtml
http://www.caninecancerawareness.org/CanineCancerMastCell.html
