Mast Cell Tumours in Dogs: Causes, Symptoms & Treatment

Mast cell tumours are among the most common malignant skin cancers in dogs, accounting for approximately 20-25% of all canine skin tumours. These tumours can appear in a wide range of forms, and although often found as a single growth, they may sometimes develop in multiple areas. While mast cell tumours can be benign, they are mostly malignant and can cause various whole-body symptoms as they progress. Fortunately, detection usually occurs at the skin surface, making surgical removal a viable treatment option in many cases.

This comprehensive article explores what mast cell tumours are, which dogs are at higher risk, symptoms to watch for, diagnosis methods, treatment options, and prognosis to help pet owners understand and manage this condition responsibly.

What Are Mast Cells and How Do Tumours Form?

Mast cells originate in the bone marrow and are present throughout the body’s connective tissues as key components of the immune system. They play essential roles in inflammatory and allergic reactions by releasing bioactive compounds such as histamine, serotonin, and heparin. These substances are beneficial in normal amounts, supporting the body's defence mechanisms.

However, mast cell tumours arise when these cells undergo mutations, causing uncontrolled growth and excessive release of these compounds. This abnormal release can disrupt normal bodily functions, leading to complications such as swelling, inflammation, and bleeding disorders.

Breeds Prone to Mast Cell Tumours

While any dog breed can develop mast cell tumours, some breeds show a higher predisposition. Among the breeds with increased risk are retriever types like Labrador Retriever puppies and Golden Retriever puppies, as well as brachycephalic and muscular breeds like the Boxer, Bulldog, and Staffordshire Bull Terrier (Staffordshire Bull Terrier puppies).

Other breeds at higher risk include the Basset Hound, Pug (Pug puppies), Shar Pei (Shar Pei puppies), and Beagle (Beagle puppies).

Common Locations of Mast Cell Tumours

Mast cell tumours typically develop on the skin’s surface, most frequently on the trunk and body, but they can also appear on limbs and extremities. In rarer cases, tumours may develop internally in organs such as the liver, spleen, gastrointestinal tract, or bone marrow, where visual detection is impossible, posing diagnostic challenges.

Signs and Symptoms to Watch For

The primary indicator of a mast cell tumour is the presence of a lump or growth on the skin, which may change shape, size, or texture day to day. Sometimes, the dog may scratch, lick, or bite at the tumour site due to itchiness or discomfort (pruritus).

Other systemic symptoms particularly if the tumour affects internal organs include loss of appetite, vomiting (possibly with blood), diarrhoea, dark or bloody stools, abdominal pain or cramps, and lethargy. Due to histamine release from the tumour, bleeding and clotting irregularities or delayed wound healing can occur.

Additional signs may include enlarged lymph nodes near the tumour, irregular heartbeat, laboured breathing, coughing, and skin redness or swelling around the tumour site.

How Are Mast Cell Tumours Diagnosed?

Diagnosis usually starts with a veterinary examination including fine needle aspiration (FNA) of the suspected tumour to collect cells for microscopic evaluation. FNA is quick, minimally invasive, and helps confirm the tumour type.

For more detailed information, a biopsy may be performed, especially for larger, irregular, or rapidly growing tumours, to determine tumour grade, which predicts behaviour and aggressiveness.

Further diagnostic tools include lymph node aspirates, complete blood count, biochemistry panels, urinalysis, X-rays, and abdominal ultrasounds, particularly when internal spread is suspected.

Treatment Options for Mast Cell Tumours

Surgical removal remains the primary treatment for mast cell tumours, ideally excising the tumour with a wide margin of apparently healthy tissue to reduce recurrence risk. The surgeon must balance tumour removal with preserving enough skin and tissue to close the wound safely.

Radiotherapy may be used pre-operatively to shrink large or poorly defined tumours for easier removal, or post-operatively to eliminate residual microscopic disease. In cases where surgery is not feasible or the tumour is highly aggressive, chemotherapy and/or radiotherapy can be recommended as alternative or adjunct treatments.

Careful communication with your vet or veterinary oncologist is essential for choosing appropriate therapies tailored to your dog’s specific condition.

Prognosis and Long-Term Outlook

The prognosis depends greatly on tumour size, growth rate, location, grade, and the overall health of the dog. Tumours that grow quickly, especially those increasing more than 1 cm per week, tend to have a poorer prognosis, with successful long-term treatment achieved in about 25% of such cases.

Conversely, approximately half of dogs undergoing complete tumour removal are considered cured. If no recurrence occurs within six months, chances of a lasting remission are high.

It is important to note that dogs who have had a mast cell tumour are more susceptible to developing further tumours in other areas at some stage, necessitating ongoing vigilance and regular veterinary check-ups.

Questions About Mast Cell Tumours in Dogs

What causes mast cell tumours in dogs?

Quick Answer: Mast cell tumours arise from mutations in mast cells, which disrupt their normal functions and cause uncontrolled growth.

Detailed Explanation: Like many cancers, mast cell tumours start when the DNA inside mast cells mutates, leading to abnormal cellular behaviour. These mutations cause mast cells to proliferate uncontrollably and release excessive amounts of chemicals like histamine, which aggravate symptoms and can cause secondary health issues. Genetic predisposition in certain breeds also plays a significant role in risk.

Can mast cell tumours be prevented?

Quick Answer: There is currently no known way to prevent mast cell tumours, but responsible breeding and regular vet checks can help early detection.

Detailed Explanation: While genetic factors contribute, environmental risks are not well established. Ensuring dogs come from reputable breeders who screen for genetic health issues can reduce risk. Regular veterinary examinations allow early detection and intervention, improving treatment outcomes significantly.

How are mast cell tumours graded and why does it matter?

Quick Answer: Tumours are graded by examining tumour tissue under a microscope to assess aggressiveness and likelihood of spreading.

Detailed Explanation: Grading involves classifying tumours as low, intermediate, or high grade based on cellular features such as mitotic rate and tissue invasion. Low-grade tumours usually have a better prognosis with slower growth and lower recurrence risk. High-grade tumours tend to be more aggressive and may spread or regrow after treatment, impacting treatment plans and prognosis.

Summary and Next Steps

Mast cell tumours in dogs are a common and varied form of skin cancer that require attentive care. Early diagnosis and treatment tailored to tumour grade and location offer the best chance of cure or long-term control. Pet owners of at-risk breeds should maintain regular vet check-ups and promptly investigate any suspicious skin lumps.

For further assistance, consult your vet for diagnostic evaluation and advice on the best treatment options to support your dog’s health and quality of life.