By Dial A Vet Veterinary Team
A mast cell tumour can look like an insect bite, a wart or a soft fatty lump, and it can change size from one day to the next. That is exactly why vets ask to test new lumps rather than judge them by eye. Mast cell tumours are the most common skin cancer in dogs, and while many are low grade and cured with surgery, others behave aggressively. Early testing gives your dog the best chance of a simple, complete removal.
Key takeaways
- Any new lump, or a lump that changes, should be checked with a needle sample (fine needle aspirate).
- Mast cell tumours can swell, redden or shrink suddenly, especially after being handled.
- Breeds such as Boxers, Staffordshire Bull Terriers, Labradors, Golden Retrievers, Pugs and Boston Terriers are over-represented.
- Treatment depends on the tumour's grade and whether it has spread, and is decided with your vet.
- Do not squeeze or repeatedly poke a suspicious lump.
What is a mast cell tumour?
Mast cells are immune cells found in the skin and other tissues. They contain granules of histamine and other chemicals involved in allergic reactions. When mast cells grow out of control they form a tumour. Because the tumour cells hold these chemicals, handling the lump can release histamine, causing local redness, swelling and itch, and occasionally stomach ulcers or a drop in blood pressure.
Signs to watch for
- A skin lump anywhere on the body, raised or flat, hairless or haired, firm or soft
- A lump that gets bigger and smaller over days, or becomes red and itchy
- A lump under the skin that feels like a fatty lump
- Bruising or redness around a lump after it has been touched
- Less commonly: vomiting, black tarry stool, reduced appetite or lethargy, which can happen with higher-grade tumours or ones that have spread
Our article I have found a lump on my dog explains how vets approach any new lump.
When to see a vet
Emergency now: collapse, pale gums, repeated vomiting or vomiting blood, or black tarry stool in a dog with a known tumour.
Within a few days: any new lump, or an existing lump that has changed size, shape, colour or texture, or one that keeps flaring up.
Routine: stable, previously tested lumps should still be measured and recorded at check-ups. Photos with a coin or ruler beside them help track change.
How vets diagnose mast cell tumours
A fine needle aspirate is quick, usually done without sedation, and often identifies a mast cell tumour on the spot or after the lab reviews it. However, an aspirate cannot reliably tell how aggressive the tumour is. That requires a tissue sample (biopsy), usually from the surgical removal itself, which a pathologist examines to assign a grade.
Your vet may also recommend staging tests before or after surgery, such as checking nearby lymph nodes, blood tests, and an abdominal ultrasound. This shows whether the cancer has spread and helps plan treatment.
Treatment overview
- Surgery is the main treatment for most skin mast cell tumours. The surgeon aims to remove the lump with a margin of normal tissue around it. The pathology report will say whether the margins look clear.
- Further treatment may be advised for higher-grade tumours, incomplete margins or spread. Options can include a second surgery, radiation therapy, chemotherapy or targeted cancer medicines, often coordinated with a veterinary oncologist.
- Supportive medicines such as antihistamines and stomach protectants are commonly used around surgery or alongside other treatment to reduce the effects of histamine. Your vet will decide whether these suit your dog; never give human antihistamines without vet advice.
Outlook varies a great deal. Many low-grade tumours removed with clear margins never return. High-grade tumours carry a more guarded prognosis, and your vet or oncologist can give you a realistic picture for your dog.
Living with a dog who has had a mast cell tumour
Dogs who have had one mast cell tumour are more likely to develop another. Run your hands over your dog regularly, keep a lump map with dates and sizes, and have new lumps tested promptly. Most dogs recover well from surgery and get straight back to normal life.
Prevention
There is no known way to prevent mast cell tumours. The most useful thing you can do is early detection: regular home checks, prompt aspirates of new lumps, and routine vet examinations, particularly for higher-risk breeds and older dogs.
FAQ
Can a vet tell what a lump is just by feeling it?
No. Mast cell tumours can feel identical to harmless fatty lumps. A needle sample is needed.
Is it safe to wait and see if a lump goes away?
Waiting can let a tumour grow, making surgery larger and margins harder to achieve. It is better to test early.
Will my dog need chemotherapy?
Many dogs with low-grade tumours only need surgery. Additional treatment depends on grade, margins and staging.
Does chemotherapy make dogs very sick?
Veterinary protocols aim to maintain quality of life, and most dogs tolerate treatment well. Your oncologist will explain likely side effects.
Talk with a vet
Found a new lump or noticed a change? A Dial A Vet veterinarian can help you work out how quickly it should be checked and what questions to ask your clinic. Speak with Dial A Vet. Collapse, vomiting blood or black stool needs an in-person emergency vet right away.
References
- Merck Veterinary Manual: Mast cell tumours in animals
- Veterinary Cancer Society: Mast cell tumour owner information
- American College of Veterinary Internal Medicine (ACVIM): Mast cell tumours fact sheet
- Withrow and MacEwen's Small Animal Clinical Oncology (textbook)
- Cornell University College of Veterinary Medicine: Canine cancer information
Disclaimer: Educational only. This page does not replace an examination or advice from your own veterinarian. If you are worried about your pet right now, contact your local vet or nearest emergency animal hospital.