Mast Cell Tumors in Dogs: Warning Signs, Diagnosis, and Treatment

Most lumps on a dog turn out to be nothing much. That is true, and it is worth saying first, because the honest headline from Morris Animal Foundation's 2023 white paper on mast cell tumours is that this particular lump is common and sometimes serious, and that the difference between those two outcomes often comes down to how early it was looked at. Mast cell tumours account for 16% to 21% of all tumours found in the skin of dogs. They are, in other words, one of the most likely explanations for a new bump — which is exactly why the sensible response is a vet appointment rather than a panic.

What a mast cell actually is

A mast cell is a type of white blood cell and part of the immune system. Most of them sit in the places where your dog's body meets the outside world: the skin, and the lining of the gut. They are among the first responders to invading organisms and they help wounds heal, which makes them useful. They also contain histamine, the chemical behind itching, swelling and severe allergic reactions, which makes them capable of causing trouble when they go wrong.

A mast cell tumour is a growth of these cells. The histamine content is why these tumours behave differently from a simple lump of tissue: the MSD Veterinary Manual notes that up to 25% of affected dogs develop stomach ulceration from the chemicals released by the tumour cells. That is also why some of these lumps are itchy, or change size from week to week.

No two look the same, which is why testing matters

This is the single most useful thing an owner can know, and the white paper leads with it. Mast cell tumours take many different forms — from a slow-growing, isolated mass to ulcerated, itchy, spread-out lesions. There is no characteristic appearance you can learn to spot from photographs. They can also arise from deeper tissue under the skin, and these subcutaneous mast cell tumours are, the paper says, often mistaken for lipomas, the harmless fatty lumps that most dogs eventually collect.

That last point deserves emphasis, because "oh, it's just a fatty lump" is one of the most commonly repeated sentences in dog ownership and one of the least reliable. A soft, squishy lump under the skin can absolutely be a fatty lump. It can also be something else. The only way to tell is to look at the cells.

• Male and female dogs are equally likely to develop mast cell tumours.
• Several breeds are predisposed, including flat-faced (brachycephalic) breeds such as Boston terriers, boxers, pugs and bulldogs, along with golden retrievers and Labrador retrievers.
• Any breed can develop them. A predisposition list is a nudge to be attentive, not an exemption for everyone else.
• Up to 40% of dogs that have one mast cell tumour will develop another during their lifetime, so a dog with a history needs ongoing checks rather than a single all-clear.

Here is the part that should lower the barrier to going in. The white paper states that the majority of mast cell tumours are diagnosed by fine needle aspirate — a simple procedure that any veterinarian can perform, using a needle to draw a small sample of cells from the lump for examination under a microscope. It is quick, usually done in the consulting room, and generally does not require anaesthesia. If the result confirms a mast cell tumour, further tests may follow to check whether it has spread, which shapes both treatment and prognosis.

Grading: why your vet's language may have changed

If you have been through this before with a previous dog, you may remember tumours being called grade 1, 2 or 3. That three-tier system was the standard for years. The white paper explains that roughly a decade and a half ago, Dr Matti Kiupel of Michigan State University and colleagues around the world proposed a simpler two-tier system: low grade and high grade.

The reason for the change is practical rather than academic. Grading is how pathologists predict how a tumour is likely to behave, and the two-tier system produces better agreement between different pathologists looking at the same biopsy. Better agreement means more reliable predictions, which means you and your vet can make treatment decisions on firmer ground. Subcutaneous mast cell tumours are handled separately and are not described using the usual grading systems, because they behave differently.

Treatment, and what the numbers honestly say

Surgery is the primary treatment. The white paper stresses that complete removal is essential to preventing recurrence, which is why your vet may refer you to a specialist surgeon rather than operating themselves — this is not a sign that something has gone wrong, but that margins matter. For higher-grade tumours, tumours that have come back at a previous surgery site, or more spread-out tumours, chemotherapy or radiation therapy may be added. A veterinary oncologist tailors this to the individual dog.

The outcomes genuinely diverge, and there is no kind way to blur it. The paper says that small, isolated, lower-grade tumours have an excellent prognosis when caught early, and that mast cell tumours can be successfully treated and cured in many cases. Subcutaneous ones are slow to spread and have only an 8% chance of returning after surgical removal. High-grade tumours are a different story: the paper reports a very poor prognosis, with most dogs dying of their disease in under a year even with aggressive chemotherapy, and an average survival time of less than four months.

Read those two paragraphs together rather than separately. The gap between them is the argument for getting a lump checked in week one rather than month six.

Where the research is going

Morris Animal Foundation reports investing nearly $850,000 across 15 research projects on mast cell tumours over nearly two decades, covering basic tumour biology, genetics and new treatments. One completed study identified new genetic targets in hard-to-treat high-grade tumours, and the team is now screening more tumours as a step toward a diagnostic test. Another study in progress is testing a drug derived from the feverfew plant against high-grade tumours.

The Foundation's Golden Retriever Lifetime Study, which follows more than 3,000 dogs across their lives, treats mast cell tumours as one of its primary cancers of interest. As of April 2023 the cohort had recorded 107 low-grade cases, 29 high-grade cases and 22 subcutaneous cases — the kind of long-term, real-world data that is scarce in veterinary oncology and that may eventually explain why one dog's tumour turns aggressive and another's does not.

This article is general information and does not replace diagnosis or treatment advice from a licensed veterinarian. If you have found a lump on your dog, the next step is an appointment, not more reading.