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Mast cell tumours in Dogs

Mast cell tumours (MCT) are the most common malignant skin cancer in dogs. These tumours originate from an uncontrolled growth of cancerous mast cells, a type of white blood cell that plays a vital role in the immune system’s response. MCT is considered a malignant and locally invasive tumour.

The exact cause of MCT in dogs is still unknown. Any dog can develop the disease; however, there is a well-known predisposition in certain breeds, such as Boxers, Bulldogs, and Boston Terriers. Other risk factors have also been identified, including exposure to certain chemicals or toxins and immune system dysfunction.

MCT can appear as raised skin lumps or masses, which can vary in size, colour, and texture. They are usually called ‘the great pretenders’ as they may resemble any other lump (insect bites, warts, allergic reactions, or other types of skin lumps). That’s why any abnormalities of the skin that you notice should be evaluated by your veterinarian.
The tumours may be solitary or multiple, and while MCTs occur mostly on the skin, they can also be found in any other organ. They may change in size over time, getting bigger or smaller, but they don’t go away.
Common signs of the disease include:

  • Skin masses that may be firm, ulcerated, or itchy
  • Swelling and inflammation around the tumour site
  • Gastrointestinal symptoms such as vomiting, diarrhoea, and loss of appetite in advanced cases
  • General signs of illness, including lethargy and weight loss

Most of the lumps look the same on the outside, but not all of them need to be surgically removed, so it’s hard to reach a diagnosis only by looking at the lump. To determine whether a skin mass is an MCT or something else, your veterinarian will perform a thorough physical examination and may recommend further testing, including:

  • Fine-needle aspiration (FNA): A sample of the mass is collected using a thin needle, and the cells obtained are examined under a microscope to identify the presence of mast cells. This examination is usually enough to diagnose MCT, but it does not provide further information regarding the grade of the tumour.
  • Biopsy: In some cases, a surgical biopsy may be necessary to obtain a larger tissue sample for a more accurate diagnosis and grading of the tumour.

Staging the tumour involves further tests to determine if the cancer has spread to internal organs (metastasis).

  • Chest x-rays are typically obtained to evaluate lymph nodes in the chest. Any enlarged lymph nodes are aspirated (FNA) to check for metastasis.
  • Ultrasound of the abdomen is performed to evaluate the spleen and liver. The spleen and liver are frequently aspirated as well to look for mast cells.
  • Blood work and urine sampling usually provide crucial information regarding your dog’s overall health and their ability to handle certain treatment options.

There are three grades of MCT:

  • Grade 1: The least aggressive type of MCT that only occasionally metastasizes to other parts of the body. Surgery can often be curative, with no further treatment generally required.
  • Grade 2: This type of MCT tends to be more aggressive than Grade 1 as they extend more into surrounding tissues. They may also metastasize to the local lymph nodes, spleen, liver, or bone marrow.
  • Grade 3: This type of MCT accounts for about 25% of all MCT cases and tends to invade the skin and underlying tissue, with high rates of metastasis. As a result, it is unlikely to be controlled by surgery alone and carries a poorer prognosis overall.

The treatment of MCT in dogs depends on several factors, including the tumour’s size, location, grade, and stage, as well as the overall health of the dog.
Common treatment options include:

  • Surgery: Surgical removal of the tumour is often the primary treatment approach. The extent of the surgery will depend on the tumour’s size and location, but it usually involves removing wide margins of tissue around the lump to increase the likelihood of removing the entire tumour (it is not possible to see the true margins of the tumour with the naked eye). Complete removal is not always possible due to the size or location of the lump. Therefore, cancerous cells can be left behind sometimes. This would be checked by a biopsy of the removed tissue after the surgery. If remaining cells are found, additional treatment might be required, which may include further surgery, chemotherapy, or radiation.
  • Radiation Therapy: Radiation may be considered in cases where complete surgical removal is not possible or when there is a high risk of tumour regrowth. This is a local treatment only, so it won’t be suitable in cases of metastasis.
  • – Chemotherapy: This treatment is frequently used to shrink large tumours prior to surgery or in cases of aggressive or metastatic MCT to target cancer cells throughout the body. Chemotherapy is generally well tolerated in pets.

The prognosis for MCT in dogs can vary widely depending on various factors, including the tumour’s grade, stage, and the success of treatment. In cases of complete surgical removal of low-grade tumours, the prognosis is typically good, and surgical excision might even be curative, although some animals might develop other MCTs in the future. In this case, regular follow-up visits and monitoring are essential to detect any recurrence or spread of the tumour. Unfortunately, high-grade tumours have an increased chance of mortality, with a median survival time of about 4 months.

MCTs can be a concerning diagnosis, but with early detection, proper diagnosis, and appropriate treatment, the prognosis for many dogs can be improved. If you notice any unusual lumps or changes in your dog’s skin, it’s important to have them examined by a veterinarian as soon as possible for a proper diagnosis and treatment.