What is it?Mast cells are a type of white blood cell. As part of the immune system they are involved in stimulating inflammation through the release of chemical substances, especially histamine. This can be helpful in defence against bacterial infection and parasites, but mast cells can also be involved in allergic reactions, releasing these substances in response to non-harmful triggers such as pollen, or certain drugs, causing unwanted inflammation.
Mast cell tumours are one of the most common skin tumours seen in dogs. These tumours occur when mast cells begin to multiply uncontrollably, due to mutations (changes or mistakes) in the genes which are involved in cell growth and multiplication.
Why is it important?Studies have shown that between 7 and 21% of skin tumours found in dogs are mast cell tumours. They vary from being essentially benign growths, to highly aggressive malignant tumours, which can spread to lymph nodes and internal organs, most commonly the liver and spleen.
What's the risk?Whilst they are seen occasionally in younger animals, they develop most commonly in middle-aged to older animals. Male and female dogs are equally likely to be affected. Mast cell tumours can develop in any breed of dog, but certain breeds, including boxers, labrador retrievers, pugs, weimaraners, golden retrievers and staffordshire bull terriers are predisposed.
What happens to the dog?
Mast cell tumours have been labelled as the 'great pretender' as they can vary widely in appearance. Most commonly they occur as a raised nodule within the skin and can vary in size. As they are formed of mast cells which can release histamine, the overlying skin can sometimes appear red and inflamed, and is sometimes ulcerated. Some mast cell tumours grow very slowly over time, whereas others appear and increase in size rapidly. Faster growing tumours are more likely to be malignant.
In the early stages, and in cases of benign tumours, the dog would not be unwell. However, more aggressive tumours, particularly if they have spread, can make the dog unwell with signs including but not limited to lethargy, weight loss, reduced appetite and vomiting.
How do you know what is going on?
If your vet is concerned about the possibility of a growth being a mast cell tumour, they will need to take a sample of the lump. They may opt to perform a fine needle aspirate, or FNA. This is a test which involves inserting a needle into the lump to obtain a sample of the cells within it. The sample is squirted onto a slide using a syringe, then the sample is stained and analysed to try to establish what sort of growth the lump may be, as there are many other different types of skin lumps dogs can develop. This is a non-invasive way of obtaining a sample, but does not give a diagnosis in all cases. In some instances your vet may wish to take a surgical biopsy of the mass, which requires some form of anaesthesia.
The vet may also run blood tests to check organ function and blood cell counts, as these can sometimes be abnormal in dogs with mast cell tumours. As there is the potential for tumour spread, tests may be performed to assess whether spread has occurred. This may include taking samples of lymph nodes, particularly if they are enlarged, usually by FNA, and/or performing imaging, usually an ultrasound scan, to assess internal organs for signs of tumour spread.
What can be done?
In general, surgery is the best way to treat mast cell tumours. Because cancerous cells can extend into the surrounding normal skin and tissues, it is best to remove an area of normal tissue around the tumour, to reduce the risk of the tumour recurring. In general, the aim is to remove 2cm of normal skin around the mass and a layer underneath the mass. In some areas of the body, such as on the limbs, or in very small dogs, it may not be possible to remove this amount of skin and close the wound, so a smaller margin may be taken.
The tumour can be sent to be analysed under a microscope. It can then be given a grade, and it can be determined whether there are any tumour cells close to the edges of where the surgery has been performed. Low grade tumours which have been completely removed have a good prognosis and are unlikely to recur. If the tumour is high grade and/or cancer cells have not all been completely removed by surgery, your vet may recommend further treatment. This may include radiotherapy or chemotherapy to kill any remaining cancer cells. Chemotherapy for animals is different to that for people, and whilst there is still a risk of side effects, much lower drug doses are used which mean severe side effects are much less likely to occur. This may also be an option for treatment if the tumour cannot be removed by surgery, because of the location or large size of the tumour, or if the animal is not a good candidate for a general anaesthetic due to other health problems.