Mast Cell Tumours in Pets

What are mast cell tumours?

Mast cells are normally found throughout the body and are often involved in functions such as inflammatory and allergic reactions. Mast cell tumours (MCT) are the cancerous proliferation of mast cells at a particular site/s of the body. At present, no one knows exactly what causes normal mast cells to transform and develop into malignant tumours.

MCTs are one of the three most common types of skin cancer in dogs, and is also a common type of skin cancer seen in cats, although at a lower frequency than dogs¹.

Although MCTs are usually associated with the skin, they can also occasionally develop in the internal organs, such as in the gastrointestinal tract or the spleen. Depending on the location, MCTs that develop in non-skin regions are termed disseminated or systemic mastocytosis or mast cell leukaemia.

Breed predispositions

While any dog or cat has the potential to develop MCTs, some of the following canine and feline breeds are thought to be more susceptible to developing these tumours. Do note that this is not an exhaustive list:

Dogs

Australian cattle dogs
Beagles
Boxers
Bullmastiffs
Cocker spaniels
Dachshunds
English bulldogs
Labrador & Golden Retrievers
Pugs
(some) Terrier breeds Bull, Boston, Fox, Staffordshire
Rhodesian Ridgebacks
Schnauzers
Shar-peis
Weimaraners

Cats

Siamese

What are the clinical signs of mast cell tumours?

It is not possible to diagnose a MCT simply by just its appearance, because it can resemble many other benign or cancerous skin lumps.

A MCT on the skin may also present in many different ways. It may appear as a firm or soft lump attached to or located just beneath the skin. The mass may fluctuate in size and show signs of redness, swelling, bruising, hair loss and/or ulceration, or the skin overlying the tumour may just appear normal. Affected pets may have a single tumour or several masses in different locations on the body.

The non-skin (visceral) form of MCTs that occurs on internal organs may present in different and often vague ways, with affected pets showing a variety of vague clinical signs, for example loss of appetite, lethargy, vomiting, diarrhoea, abdominal enlargement or difficulty breathing.

Diagnosis and staging of mast cell tumours¹

Your veterinarian may recommend a number of tests to help determine if a mass is a mast cell tumour. These tests may also help to stage aggressiveness of the MCT. Staging is the process of determining the extent of metastasis (tumour spread) throughout the body.

Fine-needle aspirate - A small needle is inserted into the mass and cells are extracted for examination under a microscope.
Punch or wedge biopsy - Occasionally, the fine-needle aspiration does not yield enough cells to give a firm conclusion on the type of mass in question, and your veterinarian may recommend a biopsy.
Lymph node aspiration - A needle is inserted into a lymph node located near to the mass, and the cells are extracted for microscopic examination to ascertain if the tumour cells have started to spread to the lymph nodes which, then have the potential to spread to the rest of the body.
Radiographs (x-rays) - Chest and/or abdominal radiographs may help identify masses and indicate if the tumour has metastasised (spread) within the body.
Ultrasound examination - This may help further characterise and give a more detailed view of the architecture of the mass/es and may help identify if the tumour has metastasised within the body or abdominal cavity.
Blood and urine tests - These tests may provide your veterinarian with an assessment of your pet's general health, and may sometimes provide clues about possible tumour spread to other organs in the body.
Histopathology - After the mass has been surgically removed, it will be sent away to the laboratory for thorough analysis. The MCT will then be assigned a Grade from I to III, with Grade I tumours being the least aggressive and least likely to spread; Grade III being the most aggressive and has a higher chance of spreading to distant sites in the body.

While these may seem like a long list of diagnostic tests, not all patients with MCT will be subjected to the full range of tests as every patient is different. Your veterinarian will be able to tailor a diagnostic plan specifically for your pet.

Treatment and prognosis

Depending on whether the MCT is limited to the skin or if other organs are involved, treatment may include but are not limited to¹:

Surgery - When possible, the surgical removal of isolated MCTs is the treatment of choice, and is likely curative if surgical margins are clear based on histopathology.
Radiation therapy
Chemotherapy
Other medications (e.g. tyrosine kinase inhibitors such as Mastinib / Palladia)

The outcome and survival rate of pets with mast cell tumours depends on many factors. If there is one tumour that is limited to the skin, is a low grade MCT with no evidence of metastasis, and is completely excised at time of surgery, then the prognosis can be very good.

For more aggressive MCTs, systemic mastocytosis or those with suspicious evidence of spread to elsewhere to the body, the prognosis may not be as good. Your veterinarian may recommend a referral to a specialist oncologist for guidance about all available treatment options.

As early diagnosis and treatment can vastly improve the prognosis, it is important that you have your veterinarian to examine and/or investigate any new lumps or bumps, or changes to existing masses on your fur friend.

The above information is provided for educational purposes only and not intended or implied to be a substitute for professional veterinary medical advice, diagnosis or treatment; and should not be relied on solely as veterinary advice. Please do not hestitate to contact us at (06) 3588675 if you have any concerns about suspicious lumps and bumps on your pet's body.

References
¹ van Nimwegen S. and Kirpensteijn K. Chapter 82: Specific disorders (Section V: Skin and Reconstruction). In: Tobias KM and Johnson A (eds). Pp. 1303 1339. Saunders, Canada, 2012.

Parts of this information sheet have also been adapted from the Vetlearn Compendium Care Guides.

Published by Cahill Animal Hospital on 13 August 2016

The team at Cahill Animal Hospital is here to provide you and your pet with the best possible medical, surgical and supportive care. Our motto "We care as much as you do" is a very important part of our day to day work. We are committed to providing you and your pet with the best options for care.