Mast Cell Tumours in Dogs

Information for Pet Owners

Mast cell tumours are the most common skin tumour in dogs. They are generally solitary and can occur anywhere on the skin surface. Mast cell tumours can also occur in internal organs but this is a less common, more aggressive presentation.

Mast cell tumours of the skin have a variable appearance. They usually present as a raised skin lump and may be red, ulcerated and/or fluctuate in size due to local histamine release from the mast cells.

Mast cell tumours can be diagnosed by fine needle aspirate which is a very small needle biopsy that can be performed without sedation or anaesthesia in cooperative animals. Treatment requires surgical removal of the mass with a margin of normal tissue (2-3cm) both around and deep to the tumour. Once the tumour has been removed the pathology report can provide prognostic information.

The pathologist will report the grade of the tumour as low, intermediate or high. Low grade tumours carry the best prognosis and can often be cured with surgery. High grade tumours are less likely to be curable and chemotherapy is recommended after surgery to improve outcome. Chemotherapy is generally very well tolerated. For further information click here.

The prognosis for intermediate grade mast cell tumours fits somewhere in the middle of low and high grade tumours and, in these cases, we ask the pathologist to report other information such as Kiupel grade, Ki67, c-kit mutation and mitotic index to help guide ongoing treatment options and provide prognostic information. In some cases of intermediate and high grade mast cell tumours radiation therapy may also be recommended. This is most common when the pathologist reports that tumour cells have been left behind after surgery (dirty margins) and when a revision surgery is not possible.

Dogs with intermediate or high grade mast cell tumours should also have the lymph node that drains the tumour area sampled by fine needle aspirate and the spleen and liver similarly sampled. In some cases these tumours can spread and this will alter the treatment options and prognosis. Discussing the results of these tests and pathology findings with an oncologist is recommended as mast cell tumours can vary widely in behaviour. In addition there are new medications called 'tyrosine kinase inhibitors' that are given orally as tablets at home and may be an alternative to chemotherapy in certain cases. For further information tailored to your pet please ask your vet to submit a request for advice on your behalf.