by Barry B. Burtis D.V.M.
Mast Cell Tumours
In my last column I wrote about Kirk, an eight year old male black Labrador. Last fall he was diagnosed with a mast cell tumour(MCT) in the skin over his left knee. Mast cell tumours are the most common skin cancer in dogs, comprising 20-25% of their skin tumours. They are the fourth most common skin cancer in cats. They tend to affect older animals but may occur at any age from 4 months to 18 years. They can affect any dog but several breeds including the boxer, Boston terrier, bull terrier, fox terrier and possibly the golden retriever and the Labrador appear to be predisposed to the development of this tumour. Siamese cats, of the breeds of cats, seem to be most susceptible to this tumour type.
When the story ended last time, the diagnosis of Kirk's problem had just been made. A fine needle aspirate from the affected area had provided data to confirm the presence of a MCT. However, all dogs with MCTs should be staged to determine the extent of their disease. This requires that a section of the tumour itself, called an excisional biopsy, will need to be collected. This will allow the biopsied tissue to be examined by a pathologist and the tumour graded. Grading will help in predicting the chance of recurrence and survival time following treatment for the MCT. In most situations the entire tumour is removed, if possible, at the outset of treatment and a portion of the tumour then is submitted for grading by a pathologist.
In preparation for Kirk's surgery, therefore, the first step was to collect some further information about his health status. A blood sample was collected for a complete blood count. We were checking to be sure there was no evidence that his tumour had already spread elsewhere in his body. We also needed to be sure he was not anemic nor had other changes in his blood cells occurred. A test called a buffy coat smear was also done. This is another way to be sure the tumour is not spreading via his blood circulation. Occasionally bone marrow aspirates are necessary to most accurately assess for MCT spread elsewhere. There was an additional fine needle aspirate of cells collected from the lymph node in Kirk's left rear leg. This also is needed in order to be sure the MCT is still confined to its primary site. Finally, Kirk had an abdominal ultrasound examination performed, checking for evidence of any change in his internal organs - particularly his liver and spleen. X-rays can be used for this purpose, as well. In Kirk's case, all these tests indicated that the tumour had not moved beyond its initial location.
Treatment for a MCT involves the use of surgery, chemotherapy or radiation, either individually or in combination. Surgical excision is the preferred treatment if the tumour is solitary and there is no evidence of spread to nearby lymph nodes or elsewhere in the body. Surgical excision of a MCT should be wide and deep to a minimum of 3 cm around the tumour. This was a cause for some concern with Kirk. The tumour was located in a place where healing might have complications because of such a large area of skin being removed over the knee joint, where there is constant movement. As a result, Kirk was referred to a board certified veterinary surgeon specialist for his operation.
Last October, Kirk had an operation that successfully removed the MCT, measuring approximately 5 cm long by 4 cm wide, located just below the skin, from his left rear leg. A pathologist was able to confirm that the margins of the removed tissue were free from any tumour cells, indicating a complete removal had been accomplished and the tumour was classed as a Grade 1 MCT. Grade 1 and Grade 2 tumours can usually be cured with appropriate local treatment, such as the surgery Kirk underwent. If there is evidence that a Grade 2 MCT has spread to nearby lymph nodes it may require additional surgery to remove those affected nodes and/or chemotherapy. If a MCT occurs in a part of the body that does not allow surgery to be performed, radiation therapy usually offers a good prognosis for recovery. Grade 3 tumours almost always spread elsewhere in the body or recur and the patient is unlikely survive more than 6 months.
Meanwhile, Kirk is doing just great. He has made a good recovery from surgery. He has had a 3 month follow-up visit and everything checked out just fine. His owner will continue to observe him carefully and perform regular home examinations to be sure no new lumps or bumps are detected. I know that if he could, he would encourage all pet owners to do for their pets, exactly what his owner did for him. That would be to become very familiar with your pet, be on the look-out for any change in your pet's body. If you notice any lump appear in or on your pet, do not delay having it examined by your veterinarian. Cancer can be beaten for pets, just as it can for people.