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 mast cell tumour can frequently develop other separate mast cell tumours elsewhere on their skin at other times in their life.
Mast cell tumours arise from a special type of cell that typically lives in the skin. These cells are normally involved in inflammatory reactions. Therefore, mast cell tumours can show any of the changes typically associated with inflammation like swelling and redness. In fact, it is common for owners to notice that the lump of a mast cell tumour has been fluctuating in size.
There is a marked variation in the behaviour of mast cell tumours and for this reason a tumour grading scheme is used. This scheme 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. 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.
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.
Mast cell tumours do not have a typical appearance and so any lump in or under the skin should be viewed with suspicion as it could be one. 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.
Diagnosis is typically made by fine needle aspirate. This allows the collection of a small number of cells from a lump that can then be examined under a microscope. Sedation is rarely required and certain tumours, including most mast cell tumours, are very readily identified by this means.
Once a mast cell tumour has been diagnosed a complex series of decisions needs 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.
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 (2-3cm). It is important to note though that approximately 1 or 2 out of 10 of these patients have spread of their tumour before the diagnosis is made. There is little point in successful removal of a single lump in the skin if cancer is left behind in other parts of the body.
Surgical removal of high grade tumours requires much wider margins of normal tissue to be removed. Approximately 8 of every 10 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 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 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; this is defined in part by the particular radiotherapy protocol used. Some animals develop reddening of the skin; others can develop skin ulceration. Skin related side effects can be irritating but tend 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.
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, chemotherapy may be appropriate in the management of metastatic mast cell tumours and some very favourable results can be seen.
Unfortunately, the response to chemotherapy can be unpredictable. While every effort is made to control the patients disease, it is not controlled at the expense of quality of life. The 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.
2009 saw the arrival of the first veterinary designer drug, masitinib. This drug comprises a molecule which is specifically designed to block an enzyme important in the development of a proportion of canine mast cell tumours. Initial reports of its use are very encouraging.
The risks associated with the use of masitinib are limited. In trials a low percentage of cases developed haemolytic anaemia, reason unknown. A relatively high proportion developed low grade gastrointestinal signs (vomiting and diarrhoea) that rarely required treatment modification. Current data suggest that the best indication for masitinib is high grade metastatic or multifocal mast cell tumours.
Complete removal of a mast cell tumour that has not spread from its original site will result in a cure. This is the case for nearly all grade 1 or low grade mast cell tumours and more than three-quarters of grade 2 or intermediate grade tumours. Unfortunately in the majority of cases with grade 3 or high grade tumours this is not possible.
In cases with high grade tumours or cancer that has spread, it is unusual that patients live more than six months from the time of diagnosis, even with treatment. We may find that this changes as we gain more experience with the new designer drugs. A specialist veterinary oncologist would be able to offer the most up to date information in this respect.