Animal Cancer Update – Mast cell tumours in dogs

Mast cell tumours are the most common cancers of the skin and underlying (subcutaneous) tissue in dogs, despite being rare in people. Whilst the genetic abnormalities that occur within the cancer are well characterised, the causes are unknown. Breeds such as Boxers, Labrador retrievers and Staffordshire bull terriers are at much higher risk than other dogs. Almost all of these cancers begin in or under the skin, but rarely can originate in the stomach or intestine. The tumours themselves generally appear as a lump in the skin, and may appear reddened or ulcerated. The tumours vary greatly in their degree of malignancy; some will never be more than a small lump in the skin whilst others will grow rapidly and spread elsewhere in the body.

Initially, a diagnosis must be made. A sample of tissue will need to be sent to the laboratory to determine how aggressive the tumour is. Generally, the cancer will be given a grade, where grade I tumours are low grade and generally harmless, and grade III tumours are high grade and spread rapidly. All of them however tend to burrow deeply into the surrounding tissue such that the visible mass is no more than the ‘tip of the iceberg’. The surgeon therefore will take as much tissue as possible around the cancer to decrease the odds of leaving any microscopic cancer behind. Often, only surgery will be necessary to effect a cure. For tumours of grade II or III severity, and especially if the location prevents aggressive surgery from being performed (e.g. the leg or face), further treatment may be helpful – both radiation and chemotherapy have been shown to be effective. The majority of dogs with mast cell tumours can be cured, though the chance reduces the higher the grade or stage.

The medical treatment itself is usually intravenous injections, which are quick and largely painless. Most dogs will receive 8 or 10 treatments over 12 to 16 weeks.  The drugs most commonly used are Vinblastine and lomustine. A new medication called imatinib is available which can cause dramatic reduction in the tumour. It is unfortunately quite expensive and should be used over a long period of time.

Radiation therapy protocols range from a few doses at weekly intervals with palliative intent, to 3 times per week over a month and are generally done under a general anesthetic.