What is a mast cell tumour?

Mast cells are a type of inflammatory cell, and are found mostly in the skin. They manufacture histamine, which they store in granules, to be released in response to some types of allergic or inflammatory injury or stimulus. A mast cell tumour (MCT) is a growth of mast cells, usually in the skin. It is the most common form of skin cancer in dogs, making up around 20% of all malignant skin tumours.

Most MCTs present as a small swelling in the skin, but they can have a variety of appearances. These can include both soft and hard swellings, which sometimes can swell alarmingly quickly due to the release of histamine. Some MCTs also itch for the same reason. Occasionally we just see a red, very angry patch of skin. Occasionally the patient presents with signs of excessive histamine release alone, such as vomiting due to stomach ulcers.

While the skin tumours can appear well circumscribed (i.e. with a well defined edge), they actually have numerous microscopic tentacles or satellite tumours spreading out into the surrounding tissue. This results in the need for more radical surgery than would at first appear.

A few patients will present with systemic mast cell disease, where the cells are distributed throughout the body. This is a much less common form of the disease.

Why has my dog developed mast cell tumours?

Most cases have no specific cause. We are aware of some breed predispositions, such as Boxers (which tend to have low grade tumours) and Shar Peis (which tend to have high grade tumours), but any breed of dog can develop MCTs, and at any age.

What is the outlook for my dog?

The prognosis for a dog with a skin MCT is very variable, and depends mainly on the grade of the tumour. Tumours are graded depending on their cellular characteristics, as described during a pathologist’s examination of the tumour tissue. They are divided into one of three grades:

Grade 1 tumours are the least aggressive, and are usually cured by good surgery, although this depends on their complete removal. This can be difficult, depending on where the tumour appears.

Grade 2 tumours (the majority) can be moderately aggressive, and care must be taken to be as thorough as possible in eliminating the chance of leaving tumour tissue behind.

Grade 3 tumours are the most aggressive, and the hardest to treat as a consequence. If we diagnose a grade 3 tumour, we assume that the tumour has already spread to elsewhere in the body. The majority of dogs with grade 3 tumours die within a couple of months of diagnosis if no further treatment is given.

Unfortunately, the appearance of an individual tumour bears no relation to its grade, so histology (examination by a pathologist) is essential for both grading the tumour, and checking the surgical margins. However, MCTs on digits or around the mouth, eyes or genitals do tend to have a higher grade, as well as often being harder to completely
remove.

What happens next?

Your dog has probably been diagnosed as having a MCT based on a fine needle aspirate of the skin swelling. This unfortunately cannot give us a grade of tumour, but does tell us that the tumour contains mast cells.

The first step is to check your dog over thoroughly to look for any more MCTs (more than one tumour will be present in around one in eight dogs). We can then plan the surgery needed to remove the tumour(s).

Depending on the case, we may wish to biopsy the mass to grade it, or check for further disease elsewhere before surgery, as this may change the approach to treatment. Most cases will go straight to surgery at this point to cut out the tumour(s).

Treatment

Most MCTs require surgery alone, as long as all of the tumour tissue can be removed. Mast cell tumours require removal of a 2-3cm margin of normal tissue as well as the tumour itself, to ensure complete removal of all tumour tissue (including satellite tumours). This margin also needs to go underneath the tumour, to include some of the deeper tissues.

This excised piece of tissue is then sent to the lab for examination under the microscope. The result will grade the tumour (see above) and examine the margins of the excised piece, to assess the completeness of removal.

If the result shows a grade 1 or 2 tumour that has been completely excised, then no further treatment is usually necessary. These dogs are effectively cured of their disease, although very occasionally we may see another MCT spontaneously appear elsewhere.

If the result shows an incompletely removed grade 1 or 2 tumour, then further surgery may be suggested to complete the excision. This would require removal of the previous surgical site, with a 2-3cm margin as before, and thus will require a larger surgical “dose”. If there is room for this surgery to be done without causing major problems with healing,
then this can again result in a cure.

If further surgery is not an option, or if a grade 3 tumour is diagnosed, then chemotherapys the treatment of choice. Chemotherapy can significantly increase the quality and quantity of life for these patients. It is effective in many cases at reducing or removing the remaining microscopic disease before it begins to cause problems. Staging of these dogs, to show where else in the body the disease may be occurring, is useful for prognosis. Experience in this clinic has shown marked increases in survival times with the use of chemotherapy, with some patients still enjoying life one year after a diagnosis of grade 3 MCT.

Systemic mast cell disease

This disease can be regarded as advanced grade 3 disease, and sadly we still have little success against it. Thankfully it is rare, and tends to be associated with high grade skin MCTs.

Summary

MCT can appear in many different forms, and their behaviour is very difficult to predict until their grade is known (and even then they can still surprise us). Many MCTs can be cured by surgery, and those that cannot will usually benefit from appropriate chemotherapy.

The first part of treatment is diagnosis, usually with a fine needle aspirate and cytology. Once MCT has been confirmed, surgical removal can be planned, and then histology will reveal the grade of tumour. In some situations biopsy will be recommended first, to grade the tumour before definitive removal. Once the grade of tumour is known, further treatment (if necessary) can be planned to ensure maximum quality and quantity of life for your pet.

If you have any questions, please contact your local Vets4Pets practice.