What is a mast cell tumour (MCT)?
Mast cells are normal cells found in the body. They operate in both inflammatory and allergic mechanisms.
MCTs are the most common malignant skin tumours in dogs and up to 20% of skin masses in dogs are MCTs. They are usually found as individual masses but some dogs may present with multiple tumours. MCTs can range from low grade (almost benign) to high grade in nature (highly malignant with a high rate of spread, or metastasis).
MCTs can occur in any age of dog, but they are more common in older pets. Certain breeds have shown to have an increased risk of developing MCTs: Beagles, Shar Peis, Boston terriers, English bulldogs, Pugs, Labrador, Cocker Spaniels, Schnauzers, Staffordshire Bull Terriers, Rhodesian Ridgebacks, Golden Retrievers, Weimaraners and Boxers. The good news is that in these dogs, even though they are more likely to develop MCTs compared to other breeds, they tend to develop tumours with less aggressive behaviours.
How can I tell if my dog has an MCT?
You should regularly examine your dog every month by simply running your hands over their body, visually checking and feeling the surface of the skin. You should ensure that you check the neck, the flanks, limbs, face and feet. Look for any areas of changed fur direction, fur loss, unexplained redness or swelling, or a thickening or mass beneath the skin.
MCTs can have many different appearances. When they are within the skin, they may be raised, firm, hairless, and sometimes reddened or swollen. When they are just below the skin surface they may be a soft and sometimes mobile mass that can feel just like a fatty tumour. The size of MCTs can vary enormously, from a few millimetres to 20-30cm in diameter in extreme cases. The majority, however, are 2-3cm in diameter.
What causes MCTs?
The exact cause for the development of an MCT is currently not fully understood. Importantly it is not something that you have done, or conversely, something you have failed to do. As stated, some breeds are predisposed and so there is definitely a genetic factor at play in some dogs. We also see more MCTs in overweight dogs compared to underweight dogs, giving owners another reason to keep their pet at a healthy weight.
How is MCT diagnosed?
An investigation of an MCT in a dog may involve performing a variety of tests, which will enable our oncology clinicians to determine the extent of the disease – a process called staging. This is particularly relevant if the MCT is suspected of being a high-grade tumour – and so more likely to display aggressive behaviour. During this process, your dog will be looked after by one of the specialised oncology nursing team.
Blood work and urinalysis
Taking blood and urine samples are routine procedures. It provides us with invaluable information regarding the general health of the patient, which in turn enables us to develop individual anaesthetic protocols as necessary. It may also provide us with information about the MCT itself, for example, is it affecting the liver, or is there evidence of cancer side-effects for example stomach ulceration and bleeding.
Fine needle aspirate (FNA)
A very fine needle is placed into the mass to remove a sample of the cells, which are then put onto a glass slide and examined under a microscope to identify mast cells. This is usually performed without the need for sedation. It would also be typical for the local lymph gland/node to be sampled, as this would be the first place that metastasis would be seen.
X-rays of the abdomen might identify if the tumour has spread to other parts of the body.
This is an additional imaging technique which we use to identify if the tumour has spread to other areas of the body, especially the abdominal organs and lymph nodes.
MCTs typically spread to lymph nodes first, before progressing to more distant sites such as liver and spleen. Evidence is growing that removing a metastatic lymph node may even help to cure some aggressive MCTs if they have not spread further. It is not always obvious which lymph node drains the MCT (the ‘sentinel’ node), raising the possibility that the incorrect node is sampled or worse, removed.
We recently invested in new equipment to offer contrast-enhanced ultrasound sentinel lymph node mapping. Sentinel lymph node mapping means the correct lymph node is identified for that tumour, at that time, in that patient. The technique consists of injecting a benign contrast agent into the tissues around the tumour which travels in the lymph system to only the draining lymph node(s) allowing them to be identified by ultrasound. This confirms which lymph nodes should be investigated more closely. Non-draining lymph nodes appear unaffected on ultrasound and can be ignored.
AURA is one of only a few places in Europe able to routinely perform this minimally-invasive test as part of our MCT management. Using our advanced ultrasound machine, we are able to identify which lymph nodes are draining the tumours, and to take ultrasound-guided tissue samples at the same time if indicated. Samples are often taken from the lymph nodes, liver and spleen. Depending on what other tests your dog may be having, most of the time we can perform this with your dog conscious and one of the oncology nursing team will be with them and taking good care of them during the procedures.
Computed tomography (CT)
Our CT machine is operated by our advanced imaging team; they are trained to use this specialised equipment to produce detailed images of the tumour and the surrounding tissue. The images created allow our oncology clinicians to understand in better detail the true extent of the tumour, as well as look for more subtle evidence of spread, compared to what can be seen on x-rays or ultrasound.
How are MCTs treated?
A typical MCT is fundamentally a surgically treated disease. Depending on the location and nature of the MCT, complete surgical removal with appropriate tissue margins can often result in a surgical cure (non-regrowth) with no further treatment of any description needed.
The MCT once removed would be sent for histopathology, where a pathologist will examine the tissue margins and confirm if they are clean and ‘free from cancer cells’. Many factors go into how much normal tissue needs to be removed around the MCT, for instance, the size of the tumour, whether it is in the skin or under the skin, the recent behaviour of the tumour, and the appearance of the cells under the microscope.
If the local lymph node was also suspicious for metastasis, it is likely our surgeons will also remove one or more and submit these to the pathologist too to determine the true extent of disease. The results of the pathology tests help our oncology clinicians decide whether additional therapies are needed, such as radiation therapy or chemotherapy.
Radiation is sometimes indicated in the treatment of an MCT when surgery was unsuccessful at removing all of the cancer cells. Radiation is delivered by a linear accelerator to the area of the scar and local lymph nodes to kill the remaining mast cells present. The number of treatments can vary, but usually, it is up to 18 doses, one dose per day on a Monday-Friday regime. Each dose needs to be delivered under a short general anaesthetic.
When MCTs are high grade, or there is evidence of spread to lymph nodes or other tissues, then the oncology team will discuss starting chemotherapy with you to try and push the disease into remission. This will keep the patient comfortable (whilst living with cancer) for as long as possible but is unlikely to cure the MCT. Our oncology nursing team are specially trained to deliver different types of chemotherapy to our patients under the supervision of the oncology clinicians. Most protocols for MCT involve daily oral tablets and intravenous injections every 1-2 weeks for a few months. Our goal with anti-cancer drugs is that our animal patients are not aware they are receiving chemotherapy, i.e. they live normal lives at home. Side-effects occur in less than half the patients and they tend to be transient and self-limiting, for example, loose stools for a day or two, or inappetence and nausea for a day or two. Most dogs on MCT protocols experience few to no complications.
What is the prognosis of MCT?
The prognosis following a diagnosis of MCT can vary between dogs and it is strongly linked to how aggressive the MCT is (the grade) and whether or not there is evidence of spread away from the primary tumour (the stage). The success of MCT therapy often depends on the ability of your surgeon to achieve clean surgical margins and it is recommended you seek our recognised surgical oncology clinicians, available at AURA. Prognosis can be improved if we can work as a team to identify, diagnose and treat MCTs as early as possible.
Are there any new advances in the management of MCT?
Increasing frustration among oncologists and histopathologists has led to a search for more reliable prognostic indicators for mast cell tumours than the histological grading schemes in current use. In recent years the indicators of rate of proliferation, Ki-67 and AgNOR count have been shown to exhibit prognostic significance. Unfortunately both of these techniques have been described in multiple ways and there is an issue of comparability between users. The consequence of this is that one person’s published scoring scheme may not apply to another individual if they do not follow exactly the same technique. Other recent developments include demonstration of the prognostic significance of mitotic index and identification of a specific mutation in a cell surface protein called c-KIT which is involved in the transduction of growth factor signals. We really need a large multicentre study to attempt to validate these various findings and to put them into some sort of useful clinical context.
Publications by AURA clinicians
Quentin Fournier, Florence Thierry, Maurizio Longo, Alexandra Malbon, Paola Cazzini, Jocelyn Bisson,, Samantha Woods, Tiziana Liuti, Spela Bavcar, Contrast-enhanced ultrasound for sentinel lymph node mapping in the routine staging of canine mast cell tumours: A feasibility study, Vet Comp Oncol 2021 Sep;19(3):451-462. doi: 10.1111/vco.12647