Mast cell tumours (MCT) in dogs

Content by Emma Chandley BVetMed PGCertSAS MRCVS 

Mast cell tumours (MCTs) are one of the most common skin cancers seen in dogs. They develop from mast cells, which are a normal type of immune cell involved in allergic and inflammatory responses. Mast cells contain substances such as histamine and heparin, which are released during inflammation.

When these cells become cancerous, they can form tumours within the skin, and less commonly in deeper tissues. Mast cell tumours can behave very differently from one dog to another. Some remain small and slow-growing for long periods, while others are much more aggressive and capable of spreading to lymph nodes, the liver, spleen or other organs.

One of the more frustrating aspects of mast cell tumours is that they can look very different from one another. Some resemble small harmless skin lumps, while others become swollen, red, ulcerated or itchy.

• Mast cell tumours are among the most common skin tumours in dogs
• They can vary from low grade, slow growing tumours to aggressive cancers
• Lumps may change size due to histamine release within the tumour
• Diagnosis usually involves fine needle aspiration and laboratory analysis
• Surgery is often the main treatment, although some dogs require additional therapy

The exact cause of mast cell tumours isn’t fully understood. Like many cancers, they’re thought to develop through a combination of genetic and environmental influences. Certain breeds appear genetically predisposed, suggesting inherited factors play an important role. 

Mutations involving a protein called KIT are identified in some mast cell tumours and may influence tumour behaviour and growth. Unlike some cancers, there’s no strong evidence linking mast cell tumours to a specific toxin, diet or lifestyle factor. In most cases, there’s nothing an owner has done wrong to cause the condition.

Several breeds are recognised as having an increased risk of mast cell tumours. These breeds include:

• Boxers
• Staffordshire Bull Terriers
• Pugs
• Labrador Retrievers
• Golden Retrievers
• Boston Terriers

Boxers and Pugs can often develop lower-grade tumours, while some other breeds may be more prone to aggressive forms.

Be aware though that mast cell tumours can occur in any breed, including crossbreeds.

The most common sign of a mast cell tumour is a lump in or under the skin. These lumps can vary enormously in appearance. Some are small, soft and slow-growing, while others become inflamed, swollen or ulcerated.

Signs may include:

• A skin lump (which may change in size)
• Redness or swelling around a lump
• Itching or irritation around a lump
• Bleeding or ulceration over a lump
• Hair loss over the lump
• Enlarged nearby lymph nodes

Some mast cell tumours release histamine and other inflammatory chemicals, which can cause the lump to suddenly become larger or more irritated. Occasionally, dogs develop vomiting, reduced appetite or stomach ulcers due to excessive histamine release, although this is less common.

As mast cell tumours can mimic many other types of skin lump, it’s not usually possible to diagnose them accurately just by appearance alone.

Diagnosis

Diagnosis typically begins with a fine needle aspiration biopsy, where a small sample of cells is collected using a needle and then examined under a microscope. Mast cell tumours often have characteristic cells that can be identified relatively easily. Once diagnosed, additional tests may be recommended to assess how aggressive the tumour is and whether it has spread. These investigations can include:

• Full tissue biopsy
• Blood tests
• Ultrasound scanning
• Sampling of local lymph nodes
• CT imaging in selected cases

Tumours are often graded by a laboratory after surgical removal. The grade helps predict how likely the tumour is to behave aggressively or recur.

Treatment depends on several factors, including tumour size, location, grade and whether there is evidence of spread. Surgery is usually the main treatment for localised mast cell tumours. The aim is to remove the tumour completely along with a margin of surrounding healthy tissue. Some areas of the body make this more challenging, particularly the legs, face or groin area. 

Additional treatment may be recommended for higher-grade or incompletely removed tumours. Options can include:

• Chemotherapy
• Radiation therapy
• Targeted anti-cancer drugs
• Antihistamines
• Steroid medication

Some dogs require referral to specialist oncology services depending on the complexity of the case.

Prognosis

The prognosis for mast cell tumours varies considerably. Many low-grade tumours that are completely removed carry an excellent prognosis. Some dogs go on to have no further problems following complete surgical removal. Higher-grade tumours behave more aggressively and may recur or spread despite treatment.

Factors influencing prognosis include:

• Tumour grade
• Tumour location
• Whether complete surgical removal was achieved
• Evidence of spread
• Presence of specific genetic mutations

Early diagnosis generally improves treatment options and outcomes.

“One of the reasons mast cell tumours can be difficult is that they don’t always look particularly alarming at first. In practice, I’ve seen some very small, innocent looking lumps that turn out to be mast cell tumours, while some larger dramatic looking masses are actually benign. This is why vets often recommend sampling skin lumps earlier rather than simply ‘keeping an eye on them’.

“It’s a very stressful time for owners when they are told their dog has cancer, but it’s important to remember mast cell tumours vary hugely in behaviour. Many dogs with low-grade tumours do extremely well after surgery with no further problems.”

Emma Chandley BVetMed PGCertSAS MRCVS

When getting a quote for Petgevity Lifetime Plus dog insurance, tell us about your dog’s mast cell tumour, including whether they’ve received treatment or surgery in the last two years. Be sure to also declare any other conditions they’ve had during that time*.

It’s important to list your dog’s conditions when taking out pet insurance, so you increase your chances of a successful claim in future.  

Find out more about our dog insurance for pre-existing medical conditions.