A mast cell tumour (MCT) is the most common malignant skin tumour in dogs. In fact, it is estimated that up to 20% of skin masses in dogs are actually MCTs. They are normally found in the body and are most often found on your dog’s skin either on the surface or underneath. As they are involved in allergic and inflammatory conditions, they are also known to be found in other sites like around the eyes, the mouth, throat, and urethra.
Mast cell tumours are compiled of multiple mast cells which are white blood cells that play a pivotal role in your dog’s allergic responses. When irritated by an allergen, mast cells undergo a process called degranulation, releasing histamine which causes typical allergic responses like sneezing, itchiness, and runny nose. Sometimes, however, the chemicals and compounds released can enter the bloodstream and cause problems elsewhere in the body. This can allow tumours to spread to other tissues and organs throughout the body. That is why it’s important to identify the tumours early on before full malignant transformation.
Symptoms of mast cell tumours in dogs
As with many other conditions, some dogs may be more prone to developing mast cell tumours over others. Most commonly Labradors, Staffordshire Bull Terriers, Shar Peis and Boxers are known to show MCTs. The tumours can be found at any age but are more common in older dogs.
Below are some symptoms of mast cell tumours in dogs to look out for.
- Individual or multiple masses anywhere in or just below the skin.
- Appearance can vary from raised, hairless nodules to reddened, swollen, ulcerated masses.
- Size can vary from just a few millimetres to 20-30cm in diameter. The majority are approximately 2-3cm in diameter.
- Inflammation and swelling of the skin around a MCT, with nodules or hives forming nearby.
- Occasionally, the dog may have a loss of appetite, vomiting, abdominal pain, diarrhoea, weight loss, bloody or black faeces due to the systemic effect of the mast cells.
Diagnosis of mast cell tumours in dogs
There are many tests and methods vets use to identify and diagnose as mast cell tumour. If you find a suspicious lump or if your dog exhibits some of the other symptoms mentioned above like vomiting or weight loss, it’s a good idea to take your pet to your nearest veterinary clinic as soon as possible.
If the vet does indeed find a lump that raises suspicion, they may decide to conduct further tests such as:
- Routine blood tests – which can be used to gain information about the general health of your pet for an anaesthetic and to assess any tumour side effects on the liver or stomach.
- A fine needle aspirate – where a fine needle is placed into the mass to remove a small sample of the cells. These can then be examined under a microscope to identify mast cells. This is usually performed during your consultation and without the need for sedation, depending on the location of the mass and the temperament of the dog. Lymph node fine needle aspirate samples may also be collected to check for any spread of cells.
- A biopsy – can be useful prior to surgical removal in order to evaluate the grade of the tumour. However, it may be avoided if a high-grade MCT is suspected as interference with the tumour can trigger an inflammatory response which may make your dog unwell.
- Ultrasound and X-rays – of the abdomen and chest will help to identify if the tumour has spread to other parts of the body. This is known as tumour ‘staging’.
- CT scans – can be used to identify spread of the tumour within the body and to examine the precise location and size of the MCT for detailed surgical planning.
How to treat mast cell tumours in dogs
The treatment for MCT is ideally surgical removal. Depending on the location and nature of the MCT, complete surgical removal with appropriate margins (removal of a border of healthy tissue and normal cells around the tumour) can often result in a surgical cure (i.e. no further treatment is required).
Once removed the mass will be sent for analysis at the laboratory (histopathology). A pathologist will examine the tissue margins to confirm whether they are free of mast cell cancer cells.
In some cases, local lymph nodes will also be removed to be assessed for signs of metastasis (spread) to determine the extent of the disease.
Radiotherapy may be indicated if the surgery has not removed all of the cancer cells.
Chemotherapy may be used for high-grade MCTs, to reduce the size of an MCT prior to surgery or where there is evidence of spread to lymph nodes. This may lead to remission but is unlikely to be curative.
Oral medications (Masitinib and toceranib) are available for patients with inoperable tumours, or for use following incomplete tumour removal. They can reduce the size of MCTs and delay tumour progression (spread and growth) by around 5 months.
Prednisolone can be used to protect the body against the inflammatory process caused by the mast cells and may result in temporary remission of the MCT.
Life expectancy of dogs with mast cell tumours
The prognosis following diagnosis of a MCT is highly variable. It is dependent on many factors including how aggressive the tumour is (grade) and whether or not there is evidence of spread (stage). With early detection and treatment, the life expectancy is improved.
Low-grade MCTs that are completely removed with margins can be cured (no regrowth), requiring no further treatment. The prognosis is also very good for incompletely removed low-grade MCTs followed by radiation therapy, with 90-95% of dogs having no recurrence within 3 years.
High-grade MCTs are associated with aggressive tumour behaviour and sadly most dogs die within 4-6 months of surgery. This is a result of tumour spread and the effect of the associated inflammatory reaction on the dog’s system. Chemotherapy may extend this life expectancy by several months.
Some dogs will have multiple tumours, either at the same time or as unrelated benign growths over months or years. These may vary in grade and should always be removed.
Dogs who have been diagnosed with an MCT, even one which has been cured, should be closely monitored for any new skin lesions on a regular basis. This should be done for the rest of the dog’s life as nearly half of dogs with MCT will develop others during their lifetime.