Canine Mast Cell Tumours (cutaneous)
Introduction
Mast cells are normal cells that are found in all areas of the body although there are more of them in certain locations. Their role is in co-ordinating inflammation. Within the cell there are small granules which contain a chemical called histamine which is important within the normal inflammatory response. It is not fully known what causes normal mast cells to stop responding to the normal control mechanisms of the body and become cancerous (i.e. turn into MCT), but there are likely to be genetic factors involved – as some breeds are predisposed to MCT (including Boxers, Pugs, Staffordshire Bull terriers and Bostons). In some case a genetic mutation is present which is involved in driving the cancerous change. This can be used as a target for some drugs in the treatment of MCT (see below).
Mast cell tumours (MCT) are the most common form of canine skin tumours. Although MCT can be seen in other areas too, this is less common. The advice here is specific to MCT in the skin, although there is a degree of overlap between MCT seen in the skin and in other locations.
What to look out for
There are no specific features of MCT. They all have vastly differing appearances so any skin lump could be MCT. Sometimes they can be small, innocuous looking lumps that are present for a long time before diagnosis, others will change size and shape rapidly. They can be red, itchy or even ulcerated on the surface. Some of these changing signs are caused by the histamine inside the mast cells. It is common for approximately 25% of dogs with MCT to have more than one.
Any age dog can have MCT but it is more common to see them in middle aged to older dogs. They are most commonly located on the neck and flanks but can appear anywhere. In some areas the MCT behave more aggressively and treatment recommendations for these may differ.
Diagnosis
There are a number of factors that will determine diagnostic and therapeutic plans. Discussion with a vet is recommended to address the specifics of each case. The more common approaches are discussed below.
Aspirates
Often MCT are diagnosed on an aspirate. This is also sometimes called a “fine needle aspirate” or referred to as “cytology” which just means looking at cells under a microscope. Aspirates are usually performed with the dog fully conscious. A needle and syringe are used to harvest some cells from the lump. This has the advantage of being less expensive, less invasive and getting quicker results back from the lab, but occasionally these samples do not give all the answers. They also do not give any information about the grade of MCT (more about this below).
Biopsy
Is a minor procedure usually done under sedation or anaesthesia to obtain a small piece of tissue. This is slightly more invasive and therefore has more cost associated with it than an aspirate but provides more information. One extra piece of information is the grade of the MCT. This provides us with information about the biological behaviour which helps determine the prognosis and can influence treatment options and recommendations.
It may be possible to plan treatment from aspirates alone, but there are situations when biopsy will be recommended. There are many factors that influence this decision such as location of MCT, how the MCT presented and previous treatment. There is no one single approach to MCT and a discussion with your specialist vet is important.
Staging
The behaviour of MCT is very variable between different MCTs. The grade, as mentioned above, will in part dictate this. All have the ability to spread (metastasis) but this is uncommon with low grade MCTs. Staging is a way of trying to establish if MCT has already spread. Depending on many factors, staging may be recommended either at the outset of treatment or following biopsy results or even after surgery. The specialist will discuss this with you in more detail. Staging involves aspirates of local glands (also called lymph nodes) which are the most common places MCT will spread to. Other procedures that may be performed include imaging (e.g. scans or X-rays).
Treatment
There are a number of options for treatment of MCT. Treatment options will depend on many factors and are usually bespoke for an individual case and therefore are best discussed with your specialist vet.
For low and intermediate grade tumours, surgery is often the mainstay of treatment but this will also be influenced by location. Sometimes this is relatively straightforward but in some situations can be very complex requiring a high level of expertise to reconstruct the skin. A discussion with your specialist vet is important to understand the specifics for each case. Once removed the MCT will be sent to a lab for analysis to confirm the diagnosis and most importantly to establish that it has been completely removed as this affects outcome and follow up recommendations.
Surgery can be part of the treatment for high grade MCT but this will usually only be one aspect of treatment in this situation. Other treatment options include anticancer medications. There are a variety of medications available to help. Some medications are designed for MCT and can target specific mutated genes (if present), others are more commonly used drugs such as anti-histamines. The choice will vary on the specific situation, and is best discussed with your specialist vet. These medications can be used prior to surgery, as a planned adjunct to surgery and/or post operatively especially if the MCT is incompletely removed.
Radiotherapy is also used for management of MCT in cases where location prohibits surgical removal or as a follow up to surgical excision. Radiotherapy can, in the correct situations, be very effective at preventing the progression of MCTs.
Prognosis (outcome)
As there are many aspects to MCT the prognosis is very variable. For low grade MCT which can be completely removed the prognosis can be extremely favourable, with many cases even cured. However for higher grade MCT the prognosis is less favourable as these behave in a much more aggressive way with many having already spread at the time of diagnosis. There are many facets to the management of MCT and these are best discussed with your specialist vet to provide a bespoke plan that is most suitable for you and your dog.