Mast Cell Tumours in Dogs
Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS
Table of Contents
Mast cell tumours are one of the most common skin tumours in dogs. They typically occur in middle aged dogs, with an average age at diagnosis of 8 years old. They can affect any dog however some breeds are predisposed to mast cell tumours, including Boxers, Bulldogs, Boston terriers, Labradors and Golden Retrievers.
Mast cell tumours are all considered malignant but can have very variable behaviour. They range from low grade (very slow to grow and/or spread) to high grade (often highly aggressive). Most mast cell tumours fall in the middle, sometimes referred to as intermediate grade tumours. These can sit at either end of the spectrum, so a complete diagnosis and considered treatment plan is essential for successful management.
Mast cell tumours can be cutaneous (within or on the skin), or sub-cutaneous (sitting under the skin). They can occur anywhere on the body; however, the trunk is the most common site followed by the limbs. Mast cell tumours are often inflamed and hairless, but do not always look the same. Mast cell tumours can often look like other types of tumours, especially fatty lumps, so it is important to test any new masses.
Mast cell tumours produce a molecule called histamine, which is most involved in allergic reactions. As a result, mast cell tumours are often associated with swelling and inflammation. If a mast cell tumour is traumatised, this releases histamine and the tumour can appear to suddenly grow, before settling down after a few days. Uncommonly, chronic release of histamine can cause gastrointestinal ulceration.
Most mast cell tumours can be diagnosed using a fine needle aspirate (FNA). This involves taking a sample of cells from the tumour with a needle and can be performed easily in a consultation. The main limitation of an FNA is that it cannot tell us what grade the tumour is. A solid tissue biopsy must be taken surgically to grade a mast cell tumour. Sometimes, an FNA can be non-diagnostic if insufficient cells are retrieved. In this case, a biopsy is required for diagnosis.
Higher grade mast cell tumours are more likely to metastasise (spread) around the body. Imaging and sampling of the liver, spleen, lymph nodes and sometimes lungs is recommended before treatment is planned. This is often performed at the same time as a tissue biopsy.
Over 50% of mast cell tumours fall into the intermediate zone, between low and high grade. This sometimes makes it difficult to predict their behaviour and plan the most appropriate treatment. Specific tests, known as a “mast cell tumour prognostic profile” can be performed on a biopsy sample at the lab. These extra tests take a few weeks to process but look at several specific genetic markers within the cells to gauge the tumours malignancy and therefore behaviour.
Surgery is the mainstay of mast cell tumour treatment. For tumours with no identifiable metastases, surgery is often curative. When surgically removing a mast cell tumour, it is recommended to remove at least 2cm of tissue around the visible mass in all directions. This is to try and ensure all tumour cells are removed. Occasionally, this may be challenging due to the location of the tumour, and reconstructive surgery or referral to a specialist surgeon may be required. After surgery, the excised tissue is sent to the lab to confirm that all cancer cells have been removed, known as clear margins. Sometimes, despite large margins, tumour cells may not all be removed. If this is the case, further surgery or medical treatment may be recommended.
If the tumour is high grade, has known metastases, or is in a location that would make surgery challenging, other treatments should be considered instead or as well as surgery.
Chemotherapy for mast cell tumours can take two forms. Injectable protocols are most often used to treat tumours where surgery is not possible, or to try and reduce metastases after surgical removal of the primary tumour. Oral chemotherapy is also available, and often used instead of surgery. This involves a medication known as a tyrosine kinase inhibitor (TKI) and works by blocking a pathway essential for tumour growth. TKI drugs are most effective if the mast cell tumour displays a specific genetic mutation, identified in the “mast cell tumour prognostic profile”. TKIs will therefore not work in all patients, but for some they can lead to complete resolution of the primary tumour.
Radiotherapy is the application of radiation to cancer cells to kill them. Radiotherapy is mostly used for mast cell tumours after surgery, where complete margins aren’t achievable due to local anatomy. Radiotherapy can have good success rates but is only available in certain specialist centres due to the complexity of the equipment required.
Intra-lesional treatment for canine mast cell tumours is the newest treatment option available. It is used for tumours that are not suitable for surgery, such as on the limbs. The drug is injected directly into the tumour and causes the body to attack the cancer cells and destroy them. Not all mast cell tumours are suitable for this treatment, but of those that are, up to 75% can show a complete response with just one injection.
Prognosis for mast cell tumours is strongly related to tumour grade, with higher grade tumours having a more guarded prognosis. Tumours that are very large, or growing very quickly, also have a more cautious prognosis. The prognosis for low grade, or low-end intermediate grade tumours is very good, especially if they are in an area where surgical resection is straightforward. Irrespective of tumour grade, if no metastases can be identified and surgical resection is achieved, there is an excellent chance of achieving a cure.
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Page last reviewed: 16th January 2024
Next review due: 16th January 2026