What is a mast cell tumour?

Mast cell tumours are common in dogs, accounting for about one skin tumour in every five. Although this form of cancer can affect dogs of any age, it usually appears in middle-aged and senior dogs.

Mast cells are specialized blood cells found in all tissues of the body, with higher numbers in the skin, respiratory and gastrointestinal tract. These cells are part of the body’s immune system and help it to do things such as heal wounds and respond to inflammation and allergens. Mast cell tumours can form when mast cells replicate in higher than normal numbers. While some mast cell tumours are benign, most are malignant.

The typical mast cell tumour arises in the skin, most often on a dog’s trunk, limbs or in between the toes. If the tumour spreads it’s usually to the lymph nodes, spleen and liver. Mast cell tumours vary widely in their appearance. They may show up as a raised lump on the skin and may be red or swollen or feel like a bump below the skin’s surface. In some cases, mast cell tumours may be present for months without growing much while others can appear suddenly and grow very quickly.

Mast cell tumours that appear on the nail bed, genital areas and muzzle and oral cavity tend to be the most malignant. And dogs with mast cell tumours that originate in tissue of the liver or spleen are not expected to survive for long after diagnosis.

Causes

The causes of most animal cancers including mast cell tumours is unknown. Mast cell tumours appear in some breeds more than others including brachycephalic (flat-faced) breeds such as Boxers, English Bulldogs and Boston Terriers.

Some veterinarians suggest that their may be a link between the development of mast cell tumours and a golden/red coat colour and/or with over-stimulation of the immune system in dogs with allergies or other inflammatory conditions. Also, environmental factors and viruses may also play a roll in the development of mast cell tumours.

Symptoms

Symptoms of this form of cancer will usually depend on the location of the mast cell tumour and whether or not the cancer has spread:

  • Mast cell tumours of the skin may present as a lump or bump on the surface of the skin or just below the skin’s surface.
  • Lumps are usually found on a dog’s trunk, hind limbs and between the toes. They may also appear on the head or neck.
  • Lumps vary in appearance: some may be red, swollen, ulcerated and may even change in size from day to day. (Dogs may or may not show any other symptoms other than the unexplained lump.)
  • Other symptoms may include loss of appetite, vomiting, diarrhea, abdominal pain, dark or black feces, itchiness, lethargy, coughing, laboured breathing, slow wound healing and enlarged lymph nodes. 

Diagnosis

Diagnosis is often made by aspirating (collecting cells and fluid with a needle) the tumour to determine whether it’s cancerous or not. A tissue biopsy is usually performed to grade the tumour, which helps determine the course of treatment and the dog’s likelihood of recovery.

Grade I: The mast cell tumour is benign (about half of all mast cell tumours). Grade I tumours can often be cured with surgery alone.
Grade II: The mast cell tumour is unpredictable, it may or may not be malignant.
Grade III: Accounts for 25% of all mast cell tumours. The mast cell tumour is malignant (very invasive and aggressive).

Along with the grade, the stage or extent of tumour spread is also determined to further help vets decide on the best course of treatment.

Stages

Stage I: one tumour confined to the skin with no signs of spread
Stage II: one tumour confined to the skin but with signs of spread to local lymph nodes
Stage III: multiple tumours or large, deep tumours, with or without lymph node involvement
Stage IV: any tumour with distant spread (beyond lymph nodes)

Some other tests will likely include basic blood work to help determine which chemotherapy drugs, if any, can be used; a bone marrow tap to see if there are any malignant mast cells in the marrow; lymph node aspiration to find out if the tumour has spread to the lymph nodes; and x-rays to determine if the cancer has spread to the spleen and/or abdominal lymph nodes.

Treatment and prognosis

Conventional treatment for dogs with mast cell tumours is a combination of surgery, radiation therapy and/or chemotherapy. This combination will depend on the location of the tumour and whether or not the cancer has spread and to what extent.

Surgery is the most common form of treatment for this type of cancer. Mast cell tumours are very invasive so it’s important that during surgery, a wide margin of healthy tissue surrounding the tumour is also removed to ensure all the cancerous cells have been eliminated.

After surgery a biopsy of the tumour is conducted to check whether or not there are still any signs of cancer in the margin. Radiation therapy is often used after surgery to kill any remaining cancerous cells around the surgical site. Radiation therapy is most effective on localized tumours and can help, along with surgery, to offer a cure for dogs with Grade I or II mast cell tumours.

There are times when surgery is not an option such as when the tumor is too large, in a difficult location or the pet is an anesthetic risk. Radiation therapy is used as an option in place of surgery in order to shrink tumours that cannot be easily operated on. While radiation therapy is very effective at controlling mast cell tumours, it can be expensive and the dog will need to be anesthetized for each treatment.

Immunotherapy is another option with certain restrictions. The US Food and Drug Administration has recently approved an intratumoral (injected directly into the tumour) injection product for specific Mast Cell tumours. The drug activates a protein that spreads throughout the treated tumour and disintegrates tumor cells. It is intended for use in those tumours that have not spread or are in difficult locations under the skin.

In high-risk cases, where the cancer has spread to the local lymph nodes or other areas, chemotherapy may be used to help treat the cancer. For those dogs where the cancer has spread or the tumour originates in the spleen, liver or gastrointestinal tract, the prognosis is very poor.

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This page has been reviewed by our Panel of Experts for accuracy. Our Panel of Experts is comprised of practitioners with varying specialties and perspectives. As such, the views expressed here may not be shared by all members of our Panel.

The content on this website is for informational purposes only and is not intended to be a substitute for professional veterinary medical advice, diagnosis or treatment.