Overview
Mast cell tumours are one of the most common types of skin cancer in dogs, but can also affect internal organs like the spleen or liver. They form from mast cells — a type of white blood cell which plays a role in helping your dog deal with allergies or injuries. They can have lots of different appearances (including appearing like insect bites or harmless lumps). In any case, they can be serious.
Even though mast cells usually help protect your dog, cancerous ones can release chemicals like histamine in unpredictable ways. This can lead to swelling, irritation, or other symptoms (even in areas away from the tumour). Sometimes, dogs can get more than one mast cell tumour at the same time.
What are the symptoms of mast cell tumours in dogs?
Mast cell tumours can look quite different from dog to dog. Some feel like firm nodules, others like soft lumps or sore patches. They can appear on or just under the skin. They can change size quickly, sometimes from one day to the next, because the tumour releases chemicals like histamine when touched or irritated. This can cause sudden swelling, redness, or itching, often triggered by your dog licking, scratching, or rubbing the area. These tumours can appear anywhere, but they’re often found on the limbs, chest, or belly. Many dogs seem well at first, and owners often spot the lump by chance during cuddles or grooming.
While local swelling is common, some dogs show other signs when the tumour’s chemicals affect areas like the stomach. These can include:
Low energy or tiredness
In rare cases, a sudden and large release of these chemicals can cause a severe allergic reaction leading to breathing difficulties, sudden illness, or even collapse.
Read more: Allergies in dogs
Which dogs are at higher risk of mast cell tumours?
Mast cell tumours can affect any dog, but some breeds are more at risk. These include Boxers, Boston Terriers, Pugs, Bulldogs, Labrador Retrievers, Beagles, Golden Retrievers, and Schnauzers. Larger breeds like Bullmastiffs and Bernese Mountain Dogs may also be more prone.
Most cases are diagnosed in middle-aged to older dogs, usually between eight and 10 years old, though younger dogs can be affected too. Unlike some other cancers, mast cell tumours don’t seem to be more common in one sex.
Genetics play a big role. Changes in certain genes (especially one called c-KIT, which helps control how cells grow) can lead to abnormal mast cell growth. Some breeds are more likely to have these changes, which may explain why they’re more prone to mast cell tumours.
How are dog mast cell tumours diagnosed?
Diagnosis usually starts with a physical exam and a chat about any changes you’ve noticed in your dog. Because mast cell tumours often look like harmless lumps (such as cysts or warts) your vet will always recommend further tests to confirm what it is.
The first step is often a fine-needle aspiration (FNA). This uses a small needle to collect a few cells from the lump, which are then looked at under a microscope. It’s quick, usually doesn’t need sedation, and can often be done during a routine appointment. Cells from mast cell tumours have a distinct grainy appearance under the microscope, which helps confirm the diagnosis. However, an FNA can’t tell how aggressive the tumour is.
To learn more, your vet may recommend a biopsy — either by removing part of the lump or the whole thing. This sample is then sent to a lab to check how the cells are behaving and how likely the tumour is to come back or spread. Mast cell tumours vary from benign (low grade) to highly aggressive (high grade), and a biopsy is an important method of distinguishing this.
Your vet might also suggest staging tests to see if the cancer has spread. These could include blood tests, lymph node checks, X-rays, or an ultrasound scan. Together with the biopsy, these results help guide treatment and give a clearer picture of your dog’s outlook.
What treatments are there for dog mast cell tumours?
Treatment for mast cell tumours depends on a few things (like the tumour’s grade, where it is, whether it’s spread, and how much can be removed). Your vet will tailor a plan to suit your dog’s individual needs. Here are the most common types of treatment your vet is likely to suggest.
Surgery
Surgery is often the first step, especially for low-grade (more benign) tumours that haven’t spread. The aim is to remove the lump along with a margin of healthy tissue to lower the chance of it coming back. If the edges are too close or cancer cells are found near them, your vet may suggest a second surgery or other treatments. Many dogs with low-grade MCTs are successfully treated this way.
Chemotherapy
This is usually used for high-grade or spreading tumours. It involves medication (these may be tablets, injections, or a combination of both), aiming to target cancer cells both in the original lump but also that may have spread elsewhere in the body. Chemotherapy can be used on its own or after surgery to reduce the risk of recurrence.
Targeted therapies
Some MCTs have a genetic change in a gene called c-KIT. This gene is supposed to send signals to either encourage or stop cell growth. If it mutates, sometimes it won’t send the ‘stop’ signals, leading to tumours. In these cases, targeted drugs can block that signal and help slow the cancer down. These may be used when standard chemotherapy isn’t suitable or hasn’t worked.
A newer treatment for some skin-based tumours is called tigilanol tiglate. It’s injected straight into the tumour and works by breaking it down from the inside. It can be a good option when surgery isn’t possible.
Radiotherapy
Radiotherapy can be used if the tumour can’t be fully removed or is in a tricky spot (like the face or limbs). It’s also useful when cancer cells are left behind after surgery. Radiation can help control the tumour and improve long-term results when combined with other treatments.
Read more: My dog has been referred to a specialist — what's next?
How do I help my dog recover from a mast cell tumour?
While your dog is undergoing cancer treatment, there are ways you can help them feel comfortable, manage their side effects, and make recovery as smooth as possible. Because mast cell tumours can cause inflammation throughout the body, supportive care often includes medications such as:
Antihistamines to counteract histamine release
Steroids to reduce swelling and slow tumour growth
Stomach protectants to help prevent ulcers
Pain relief or anti-nausea medicines to ease discomfort from treatment
Your vet might also recommend a buster collar to stop your dog licking or scratching the tumour or surgery site, which can help prevent swelling or infection.
After surgery, most dogs need a couple of weeks of rest while they heal. Keep an eye on the incision site each day and follow your vet’s advice, including check-ups to spot any complications or signs the tumour might return. Give your vet a call if you notice swelling, redness, discharge, or changes in your dog’s energy or appetite.
Even after treatment, mast cell tumours can sometimes return, or new ones may appear. That’s why regular skin checks and six-monthly health checks are often recommended for dogs with a history of mast cell tumours. The long-term outlook depends on how aggressive the tumour is and how early it was treated. Many dogs with low-grade mast cell tumours that are fully removed go on to live a full, active life. Others may still enjoy a good quality of life with ongoing care and support.
Can mast cell tumours in dogs be prevented?
There’s no guaranteed way to prevent mast cell tumours. They’re thought to develop from a mix of genetic and environmental factors, rather than from any one thing like diet or exposure to a specific chemical. Some breeds are simply more prone than others.
Early detection is the most important thing. Finding a tumour when it’s small and hasn’t spread gives your vet the best chance to treat it (often with surgery alone). If your dog has had a mast cell tumour before, lifelong monitoring is essential, as new ones can appear over time.
One of the best ways to spot problems early is with regular at-home checks. When grooming or cuddling your dog, gently run your hands over their body — particularly their limbs, chest, and belly — to feel for any new lumps. If you spot any lumps, keep an eye on any changes in size, texture, or firmness. Don’t just assume an old lump is harmless just because it’s been there a while. If your dog has any new lumps, or changes to old ones, seeing your vet is the best thing to do.
FAQs about mast cell tumours in dogs
How do I know if a lump on my dog is cancerous?
You can’t tell just by looking or feeling. Mast cell tumours can mimic harmless lumps like warts or cysts. The only way to know for sure is to ask your vet to examine it and possibly take a small sample for testing.
Can a mast cell tumour shrink on its own?
Sometimes a tumour may seem to shrink temporarily, especially if it's been irritated or treated with medication, but this doesn’t mean the tumour has gone away. Any shrinking lump should still be checked by your vet.
Do mast cell tumours hurt my dog?
Some MCTs may cause discomfort or itching, especially if they release histamine or become inflamed. Others might not cause any visible pain. Always consult your vet if your dog seems bothered by a lump.
Can my dog live a normal life after a mast cell tumour diagnosis?
Yes, many dogs live happy, active lives after treatment — especially if the tumour is caught early and removed completely. Even dogs with more aggressive tumours can enjoy a good quality of life with the right care and monitoring.
Should I worry about every lump I find?
Not every lump is cancerous, but every new or changing lump is worth checking. Getting it looked at early can make treatment easier and improve your dog’s outcome if it is something more serious.
How fast do mast cell tumours in dogs grow?
Some MCTs grow slowly, while others can change rapidly (even within hours or days) due to swelling or chemical release. Fast growth doesn't always mean a lump is serious, so all lumps should be assessed — although a rapidly growing lump is generally of highest concern.
Can mast cell tumours spread to other parts of the body?
Yes, some tumours (especially high-grade ones) can spread to the lymph nodes, liver, spleen, or bone marrow. This is why staging tests are often recommended after diagnosis.
When should I stop treating mast cell tumours in my dog?
This is a personal decision that depends on your dog’s quality of life, how the tumour is behaving, and how well treatment is working. If your dog is in pain, off their food, or struggling day to day despite treatment, it may be time to shift focus from cure to comfort. Mast cell tumours vary — some are low-grade and easy to remove, while others are more aggressive and can return. Your vet can talk you through the options and help you decide what’s best for your dog’s wellbeing.
Are there any studies into dog mast cell tumours?
Ongoing research is helping vets understand mast cell tumours better — how they behave, how they spread, and how treatment can be tailored to each dog.
Recent stats on dog mast cell tumours
A 2025 UK study looked at nearly 200 dogs with mast cell tumours on or under the skin. These tumours made up around 16 to 20% of all skin tumours seen in dogs. Most dogs were diagnosed at around seven and a half years old, with Labradors, Boxers, and Staffordshire Bull Terriers more commonly affected. Tumours that grew quickly were linked to shorter survival. The study also gave some insights into just how strongly connected c-kit mutations are with mast cell tumours — one particular mutation was found to make it 23 times more likely that tumours would return after surgery.
Mast cell tumours in dogs vary widely
This Austrian study from 2021 showed just how unpredictable mast cell tumours can be. Even some low-grade tumours behaved aggressively over time. While some lumps grew slowly, others ulcerated or spread to internal organs. The research also flagged up a symptom called Darier’s sign. This is when a lump becomes red or itchy after being touched, and the study suggested it could be an early warning sign for owners.
Mast cell tumours often spread to the lymph nodes
A Brazilian study from 2023 found that when mast cell tumours in dogs spread, over 75% of the time it spread to their lymph nodes. The outlook varied by tumour grade. About 25% of dogs with intermediate-grade tumours passed away from the disease, and this rose to 67% in high-grade cases. The study did have some good news too: dogs with c-kit mutations responded well to targeted treatments, which helped improve both survival and quality of life.
Surgery makes a big difference
A 2025 Malaysian study found that the average survival time of dogs with high-grade tumours was around 10 months. Surgery made a big difference, boosting survival sevenfold compared to dogs who didn’t have surgery. The chances of the tumour coming back were much lower when all cancer cells were fully removed (26% vs. 58%).
Mast cell tumour surgery might not need wide margins
This American study conducted in 2020 questioned the need for wide surgical margins (where quite a bit of additional tissue around the tumour is removed along with it) in every case. In over 90% of low- to intermediate-risk tumours, removing a 2cm margin (or one equal to the tumour’s size) was enough. The study suggested that adopting this approach could lead to better outcomes, with less pain and fewer complications after surgery.
A 2020 review of several studies backed this up. For tumours under 4cm, a 2cm margin was usually sufficient. Since many tumours aren’t graded until after removal, this supports using smaller, more targeted surgeries with good results. More research will be needed to know for sure.
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Medically reviewed by Lizzie Youens, BSc(Hons) BVSc MRCVS
Published on 16/01/2026 • Reviewed on 16/01/2026