Canine Cutaneous Mast Cell Tumours
Mast cell tumours are common tumours of the skin in dogs. Whilst many mast cell tumours can be cured by appropriate management, dogs that get one can frequently develop other separate mast cell tumours elsewhere on their skin at other times in their life.

What is a Mast Cell Tumour?
Mast cell tumours arise from a special type of cell that typically lives in the skin. Their job is to orchestrate inflammatory reactions. Therefore, mast cell tumours can show changes typically associated with inflammation like swelling and redness. In fact, owners often comment that the lump in question has been fluctuating in size prior to their referral appointment.

There is a marked variation in the degree of severity of a mast cell tumour and for this reason a tumour grading scheme is used. This categorises mast cell tumours into three groups usually described as grades 1-3; sometimes the terms well-differentiated, intermediately differentiated and poorly differentiated are used. The tumour grade significantly influences the treatment decision-making process. Many of these tumours can be cured but only by appropriate intervention and there is great merit in finding out what grade a mast cell tumour is before definitive treatment is planned.

The higher grade a tumour is, the more likely it is to infiltrate into the normal looking body tissues around the tumour and the more likely it is to spread through the body via the blood or lymph systems. Mast cell tumours do not have a typical appearance and so any lump in or under the skin could be one. Diagnosis is typically made by fine needle aspirate; this allows the acquisition of a small number of cells from a lump that can then be examined under a microscope. Sedation is rarely required and certain tumours, like mast cell tumours, are very readily identified by this means.

Patient Evaluation
Once a mast cell tumour has been diagnosed a complex web of decisions need to be made. Ideally the grade of the tumour is determined first. This requires a biopsy to be obtained and submitted to a pathology laboratory.

Sometimes mast cell tumours are not identified until after surgery to remove what had appeared to be an innocuous skin lump. The pathology laboratory describes the presence of a mast cell tumour and they will usually define the tumour grade and comment on the degree of invasiveness of the tumour. Under these circumstances further treatment is often necessary as the surgery has rarely been adequate to completely remove the cancer.

In the initial assessment of any patient with a lump that is considered likely to be cancerous, the local lymph nodes are examined by palpation. If they are enlarged, samples need to be obtained to determine whether there is evidence of tumour spread.

Treatments
Low grade mast cell tumours can be cured in almost all cases. Surgical removal with appropriate margins of apparently normal tissue (1cm) is appropriate.

Intermediate grade tumours require wider margins of excision, 2-3cm is appropriate in most cases, with a complete unadulterated layer of protective fibrous tissue removed to prevent spread through the underside of the tumour. It is important to note though that approximately 15% of these patients have spread of their tumour before the diagnosis is made. There is clearly limited merit in successful removal of a single lump in the skin if cancer is left behind in other parts of the body.

High grade mast cell tumours tend to look bad from the start. They can be big, red and frequently discharging serum or blood with no apparent border between the normal and the cancerous tissues. Surgical removal of these tumours requires much wider margins of normal tissue to be removed with associated protective fibrous layers as for the intermediate grade tumours. Approximately 80% of these tumours have spread before the time of diagnosis and therefore the role of surgery in their management is limited.

Other treatments do exist for canine cutaneous mast cell tumours. As a rule, they are less effective than surgery in that they do not cure the disease. However, there are instances when these treatments are more appropriate.

Radiotherapy
Radiotherapy or radiation therapy has a particular role to play in the management of mast cell tumours at sites that are not amenable to surgery, for instance the lower limb or around the face. Radiotherapy can dramatically affect the further progression of mast cell tumours, irrespective of tumour grade, with many cases failing to worsen for many months or even years after treatment. For this reason radiotherapy is also used in the post-operative management of some mast cell tumours, when there is a strong suspicion of incomplete removal. By and large the side effects of radiotherapy are minimal; a small number of cases develop reddening of the skin that can be irritating but this tends to be short-lived. More serious effects can develop in the long term and for this reason radiotherapy is undertaken less freely in younger patients.

Chemotherapy
While there are countless descriptions of the use of chemotherapy in the management of canine cutaneous mast cell tumours in the veterinary literature, many of the successes reported have probably failed to recognise that a number of the cases described were already cured by the earlier administration of appropriate surgery. As a result, the stated responses to chemotherapy are likely to be overoptimistic.

Having said that, we do use chemotherapy when appropriate in the management of metastatic mast cell tumours and in this particular context we have recorded some very favourable results. Unfortunately, the response to chemotherapy can be unpredictable. While every effort is made to control the patient’s disease, it is not controlled at the expense of quality of life. Our overriding philosophy in the management of all canine and feline cancer patients is the promotion of quality of life first and length of life second.

Gerry Polton

If you are concerned about the health of your pet you should contact your veterinary surgeon.