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Mast Cell Tumours in Dogs

What are Mast Cell Tumours?

Mast cell tumours are malignant and aggressive cancers of cells called “mast cells.” Mast cells are found in the dermis and subcutaneous tissues of the skin of dogs and cats. Mast cell tumours are the most common malignant skin tumour of dogs and the second most common malignant skin tumour of cats.

Who can get a Mast Cell Tumour?

Any dog or cat can develop a mast cell tumour. There are, however, some breeds of dogs and cats that show a predisposition to developing mast cell tumours.

Dogs showing a breed predisposition to mast cell tumours include…

  • Boxer
  • Boston Terrier
  • Labradors
  • Beagles
  • Schnauzer

Cats showing a breed predisposition to mast cell tumours include

  • Siamese cats

What do Mast Cell Tumours look like?

Mast cell tumours have an extremely varied appearance, and can appear as small, hairless or pigmented regions of the skin, through to large, ulcerated masses.

The many and varied appearances of mast cell tumours. If your dog develops a mass or lump of any type, you should have it evaluated for the presence of a tumour – the earlier you detect cancer, the better the chances are that treatment may be curative

What symptoms do these tumours cause?

Mast cell tumours may cause no symptoms apart from the presence of a lump in the skin of the patient. However, mast cells also can release a chemical called histamine that can cause vomiting, diarrhoea, skin redness and swelling and loss of appetite. In addition, malignant mast cell tumours can spread to other organs in the body, including the liver and spleen. Internal haemorrhage/bleeding or fluid accumulations can result from these organs in advanced stages of mast cell tumour disease, causing symptoms of acute depression and lethargy, pale gums and rapid respiration.

How are Mast Cell Tumours diagnosed?

If your dog or cat has a mass in or on the skin, you should have the mass evaluated by your veterinarian. Mast cell tumours are best diagnosed by looking at cells within the mass. Once a diagnosis of mast cell tumour is made, the tumour should be biopsied, in order to establish how malignant the tumour is. This step is VERY important – as it can tell your vet how best to manage the tumour to get the best results from treatment.

A biopsy result will generally give the tumour what is called a “grade” from 1-3, with grade 1 tumours being relatively benign, and grade 3 tumours being highly malignant. The grading system used is outlined in the following table.

Typical cytology specimen from a canine mast cell tumour – with cells of varying size, containing many histamine granules

Grading System for Canine Mast Cell Tumours

Grade Properties Metastatic Rate
Grade 1 Low grade, slower growing, better differentiated cells 5%
Grade 2 Intermediate grade, moderate growth rate, many cells undifferentiated 15%
Grade 3 High grade, highly malignant, undifferentiated cells with rapid growth 90%

Following grading of the tumour, assessment of lymph nodes, an ultrasound of the abdomen and occasionally chest radiographs (x-rays) are generally performed to determine the presence of tumour spread (metastasis) prior to any surgery to remove the skin tumour.

How are Mast Cell Tumours treated?

Mast cell tumours are managed usually using a combination of surgery and chemotherapy. Occasionally, radiation therapy may be used to reduce the size of a tumour prior to surgery, or to assist in removal of cancer cells if surgery is not completely successful in removing all of the cancer.

Surgery offers the best chance of curing patients with grade 1 and 2 mast cell tumours, and needs to be planned carefully to achieve the best outcome. In order to remove all of the cancer cells in a mast cell tumour, normal tissue must be removed for 2-3 cm around the site of the edge of the tumour, and into muscle tissue deep to the tumour. If mast cell tumours are removed with a smaller surgery, there will almost certainly be cancer cells left in your pet – and the surgery will not be as successful.

This is an intra-operative photograph of a mast cell tumour removal. Note the wide surgical margins surrounding the tumour – at least 2-3 cm is currently recommended – in order to maximise the chances of complete tumour removal at surgery.

If the cancer has been incompletely removed at surgery, or if the cancer has metastasised (spread), post-operative chemotherapy provides the best chance for treatment. There are a number of different drugs that can be used to treat mast cell tumours – each with different side effects and success rates. It is important to discuss the available chemotherapy treatments with your vet so that you can be aware of the potential side effects and success rates for each drug.

What is the Prognosis?

As mentioned above, the grade of the mast cell tumour is predictive of survival and metastatic rate. A summary of available scientific literature on this information is presented below…

Mast Cell Tumour Grade Metastatic Rate Survival Rate with Surgery Survival Rate with Surgery + Chemotherapy
Grade 1 5% 95% alive at 1 year83% alive at 2 years Not required if surgery removes entire tumour
Grade 2 15% 90% alive at 1 year44% alive at 2 years Median survival 1400 days; 85% complete cure
Grade 3 90% 20% alive at 1 year6% alive at 2 years  

Regardless of the protocol of treatment selected for your pet, you should visit your vet every 3 months after surgery +/- chemotherapy for reassessment of the skin, lymph nodes and abdominal examination by ultrasound to allow early detection of further mast cell tumours or metastasis.

About the Author

Dr. Philip Judge

BVSc MVS PG Cert Vet Stud MACVSc (VECC; Medicine of Dogs)

Director: Vet Education Pty Ltd
Consultant in Veterinary Emergency and Critical Care
Internationally renowned lecturer and published author
Dedicated to providing you with innovative and exciting online learning!


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