Mast Cell Tumours in dogs

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Mast Cell Tumours in dogs

Mast Cells Tumours (MCTs) account for around 1 in 5 skin tumours, which makes them the most common form of malignant skin tumour in dogs. Some are described as low grade, which means they act like a benign tumour: they don’t tend to spread and pose very little risk to the dog. In comparison, the high grade mast cell tumours are very aggressive, and often spread around the body, which can, unfortunately, be fatal.

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Signs of Mast Cell Tumours

  • MCTs can have many different appearances: changed direction of hair, hair loss, unexplained redness or swelling, or a thickening or mass beneath the skin.
  • Masses vary from 2-30 cm in diameter
  • A rapid change in size, shape or colour of a mass
  • Vomiting or black, tarry stools
  • Abdominal discomfort

Cause of Mast Cell Tumours

Mast cells contain histamine granules, which are an important part of normal inflammatory and allergy mechanisms in all animals. When mast cells undergo malignant transformation, a Mast Cell Tumour forms. The exact reason that malignant transformation occurs is unknown, but genetics and obesity may play a role.

Malignant mast cells contain an abnormally high concentration of histamine. Histamine is released from the cells when they are stimulated, for example by itching. This is the reason that some tumours change rapidly in size, shape or colour.

Mast Cell Tumours can affect animals of any age, but most commonly affect middle-aged to older animals, between 7.5-9 years of age. Certain breeds are predisposed to this type of tumour, including Pugs, Golden Retrievers, Boxers and Shar-Pei’s. The behaviour of the Mast Cell Tumour will typically be very different, depending on the breed affected.

What can you do to help your dog?

Being vigilant and regularly checking your dog from nose to tail will help you to spot any abnormalities early. Check for lumps, bumps, thickening of the skin, areas of hair loss, red patches or other lesions. Read more about doing a simple exam of your pet here.

Treatment of Mast Cell Tumours

As with most tumours, Mast Cell Tumours can only be definitively diagnosed by taking a sample to analyse under the microscope. Due to histamine release, it is possible for swelling to occur after sampling, as the cells are disturbed.

If a Mast Cell Tumour is diagnosed, the most common first step is for the mass to be removed. The mass will need to be surgically excised under general anaesthetic. Wide margins (several cms) of normal skin around, as well as tissue below the mass, are removed to ensure that no abnormal tissue or cells are left behind. This will mean that the surgical site will often be substantially larger than the original mass. This surgery will occasionally need to be performed by a specialist if the tumour is in a complex anatomical location.

A section of removed tissue is then sent to a lab and checked under the microscope to ensure that the margins of the tumour are sufficient. The mass can also be sent for a prognostic panel. This is a series of tests that establishes the grade of the tumour: how aggressive it is, and what the risk of spread is.

If the Mast Cell Tumour is thought to be high grade (aggressive), or the mass was not able to be completely removed, further treatment may be required. This is usually in the form of chemotherapy, or a second surgical procedure, to remove more tissue.

In some cases, for example where a Mast Cell Tumour is deemed to be aggressive, your vet may recommend further imaging to look for evidence of tumour spread (metastases). This may include an ultrasound scan, x-rays or CT scanning. Your vet’s recommendation will be based on all the clinical information to date, which will determine the most appropriate approach for your pet.

What is the long term prognosis?

Prognosis is closely linked to the tumour grade. For low grade tumours complete surgical removal is considered curative. Unfortunately, for high grade tumours most have already metastasised by the time of diagnosis. Therefore, these are the cases that may benefit from adjunct chemotherapy. The success of chemotherapy will be dependent on the extent of metastasis and the response to treatment.

When is it time to visit a vet?

  • If you notice any of the signs mentioned above that could indicate a Mast Cell Tumour.

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Published: 04/03/2020
Last updated: 23/03/2021

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