Mast Cell Tumours in Dogs: A 2026 Guide to Treatment Options.

What is a mast cell tumour?

Mast cells are part of the immune system. They live in connective tissue and the skin, and they're the cells responsible for releasing histamine, the chemical behind allergic reactions, itching, and swelling. A mast cell tumour (often abbreviated MCT) is what happens when those cells become cancerous and start dividing uncontrollably, almost always forming a lump on or under the skin.

MCTs are the most common skin tumour in dogs. They account for roughly 16 to 21 percent of all canine skin cancers, depending on which case series you look at. Certain breeds are over-represented, including Boxers, Boston Terriers, English Bulldogs, Labrador Retrievers, Golden Retrievers, Pugs, Shar-Peis, and Rhodesian Ridgebacks. That doesn't mean other breeds don't develop them; it just means a vet seeing a new skin mass on a middle-aged Boxer is going to keep MCT high on the differential list.

The single most important thing to understand about MCTs is that they are wildly variable. One dog's MCT is a small, low-grade nodule that surgery cures completely. Another dog's MCT is a high-grade tumour that has already spread by the time it's diagnosed. Treatment recommendations depend almost entirely on which kind your dog has.

How vets diagnose mast cell tumours

Diagnosis usually starts with a fine needle aspirate (FNA). Your vet inserts a small needle into the mass, draws a few cells out, and looks at them under a microscope. Mast cells have a very distinctive appearance, studded with dark granules that stain a characteristic purple, so cytology alone can often confirm an MCT diagnosis in minutes, right there in the clinic.

What cytology can't reliably tell you is the grade, which is what predicts how the tumour will behave. For that, your vet needs a tissue sample. This typically means either a biopsy (taking a piece of the tumour for examination) or, more commonly, removing the whole tumour and sending it to a veterinary pathologist for full examination.

Once an MCT is confirmed, your vet may also recommend staging tests to look for spread:

• Bloodwork and a urinalysis to check organ function before considering chemotherapy.
• Abdominal ultrasound, particularly to examine the spleen and liver, the most common internal sites of MCT spread.
• Lymph node aspirates: even normal-feeling lymph nodes near the tumour are sometimes sampled, because MCTs can spread to lymph nodes that aren't obviously enlarged.
• Buffy coat smear or bone marrow aspirate in select high-risk cases.

Grading: Patnaik and Kiupel

Two grading systems are in routine use in 2026. Most veterinary pathology reports include both.

The Patnaik system dates from 1984 and divides MCTs into Grade I (well-differentiated, behave benignly), Grade II (intermediate, where most MCTs fall and prognosis is the hardest to predict), and Grade III (poorly differentiated, behave aggressively).

The newer Kiupel two-tier system, published in 2011, simplifies the picture into just low grade and high grade. It was designed specifically to address the prognostic ambiguity of Patnaik Grade II, many of which turn out to be low-grade and behave well, while some behave aggressively. Studies have shown the Kiupel system has better prognostic agreement between pathologists.

Why the grades matter: a Kiupel low-grade MCT that's completely excised carries a very good prognosis, with many dogs living out their normal lifespan. A Kiupel high-grade MCT has roughly a 50 percent one-year survival rate even with treatment, and almost always warrants further therapy beyond surgery.

Other factors that go into prognosis include the mitotic index (how many tumour cells are actively dividing), KIT mutation status (more on this below under Palladia), Ki-67 and AgNOR proliferation markers, and tumour location.

Treatment: surgery

For the majority of MCTs, surgery is the foundation of treatment. The goal is wide complete excision, removing the tumour with a margin of healthy tissue around and beneath it, to ensure no cancer cells are left behind.

For years the convention was 3 cm lateral margins and one fascial plane deep. More recent research has shown that for many Kiupel low-grade MCTs, smaller 2 cm lateral margins are sufficient as long as the surgical margins come back clean on pathology. For high-grade tumours, oncologists typically advise wider margins and may recommend post-operative radiation if margins are narrow.

If the tumour is in a location that doesn't allow for wide excision, such as a leg, a paw, the face, or near the anus, your vet may discuss marginal excision plus adjuvant therapy (radiation or chemotherapy), or referral to a board-certified veterinary surgeon for advanced reconstruction.

Treatment: radiation

Radiation therapy is most commonly used as adjuvant treatment after surgery when complete excision wasn't possible, either because the tumour was in a difficult location or because the post-operative biopsy revealed dirty margins. For incompletely excised Kiupel low-grade MCTs, definitive-intent radiation provides excellent long-term control, with most dogs achieving durable remission.

Radiation is also sometimes used in palliative settings, particularly for non-resectable tumours that are causing local symptoms.

The practical considerations are real: radiation requires a referral to a facility with a linear accelerator, and definitive-intent protocols typically involve 15 to 20 daily anaesthetised treatments over three to four weeks. Cost is significant. Side effects are usually mild and localised but can include skin irritation that takes weeks to heal.

Treatment: chemotherapy

Chemotherapy comes into the picture for higher-grade MCTs, MCTs that have spread to lymph nodes or distant sites, MCTs with high mitotic indices, or MCTs in locations associated with worse outcomes (the muzzle, the inguinal region).

The most commonly used protocols in 2026 are:

• Vinblastine and prednisone: a well-tolerated outpatient protocol. Vinblastine is given intravenously every 1 to 3 weeks; prednisone is given orally at home. Often used as first-line systemic therapy.
• CCNU (lomustine): an oral chemotherapy given every three weeks. CCNU can suppress white blood cell counts and occasionally cause liver injury, so monitoring is important, but it's effective and convenient.
• Combination protocols: alternating or combining vinblastine, CCNU, and prednisone. Some oncologists also include cyclophosphamide.

Most owners are pleasantly surprised by chemotherapy in dogs: the dose intensity is much lower than in human oncology, and the goal is good quality of life rather than cure-at-all-costs. Significant nausea or severe immunosuppression is uncommon with the protocols above.

Treatment: targeted therapy (Palladia / toceranib)

This is the targeted-therapy story that has played out over the last fifteen years. Roughly 25 to 30 percent of canine MCTs carry a mutation in the c-KIT gene, which drives uncontrolled mast cell proliferation through a receptor tyrosine kinase. Toceranib phosphate, marketed by Zoetis as Palladia, is a small-molecule inhibitor of that receptor and several others.

Palladia was the first cancer drug formally approved by the FDA for use in dogs (2009), and it remains the standard targeted therapy for MCTs in 2026. It's typically reserved for non-resectable MCTs, recurrent MCTs, or MCTs that have spread. Some oncologists also test for the c-KIT mutation before starting Palladia, because mutant-positive tumours tend to respond better.

Side effects are real and require monitoring: gastrointestinal upset, decreased appetite, protein loss through the kidneys, and changes in blood pressure. A close working relationship with your veterinary oncologist matters.

What's new: personalized mRNA cancer vaccines

The most significant development in cancer therapy this decade isn't a new drug. It's an entirely new modality. In humans, personalized neoantigen vaccines have moved from theoretical to late-stage clinical trials. Moderna and Merck's mRNA-4157, given in combination with Keytruda, lowered the risk of melanoma recurrence or death by about 44 percent in a randomised phase 2b trial. BioNTech's autogene cevumeran is producing durable T-cell responses in pancreatic cancer patients. Dozens of late-stage human programmes are running.

The underlying idea is simple. Every tumour contains mutations that produce neoantigens, bits of protein that exist only on the cancer cells, never on healthy tissue. If you can identify those neoantigens from sequencing data, you can design an mRNA vaccine that teaches the immune system to recognise and attack any cell carrying them. The vaccine is genuinely personalised. It's only useful for the patient whose tumour it was designed from.

The same approach works in dogs. The species reference is different (the canine genome, not the human one), the MHC alleles are different (DLA rather than HLA), and the codon table is different, but the pipeline structure is identical. In 2025, Australian data analyst Paul Conyngham used this approach on his rescue dog Rosie, whose aggressive mast cell tumours had recurred after surgery and chemotherapy. Her vaccine was synthesised at UNSW's RNA Institute and administered by a licensed veterinary oncologist. Her tumours shrank by approximately 75 percent in the months that followed.

Personalized mRNA cancer vaccines for dogs are an emerging modality, not a standard of care. They should not replace surgery, chemotherapy, radiation, or targeted therapy where those treatments are indicated. Where they are most interesting today is as adjuvant therapy after surgical resection, adding a layer of immune memory aimed at any residual cancer cells the surgery may have missed.

Questions to ask your veterinary oncologist

If your dog has been diagnosed with a mast cell tumour, here are the questions to bring to your oncologist appointment:

1. What grade is the tumour, on both the Patnaik and Kiupel systems? What's the mitotic index?
2. Have we tested for c-KIT mutations? If positive, does that change your treatment recommendation?
3. Did staging show any spread (lymph nodes, spleen, liver, bone marrow)?
4. What does the recommended treatment look like: surgery alone, surgery plus radiation, surgery plus chemotherapy, Palladia, or some combination?
5. What's the realistic prognosis with the recommended treatment? What's the prognosis without it?
6. If we proceed with surgery, what margins will you take, and what will we do if the post-op pathology shows dirty margins?
7. Are there experimental options we should know about, such as clinical trials, autologous tumour vaccines, or personalized mRNA vaccines? Are any of them a reasonable fit for our case?
8. What does follow-up look like? How often will we re-check, and what are we watching for?

Where RosieVaccine fits in

RosieVaccine is a bioinformatics design service for veterinary oncologists. We take a tumour biopsy and a matched blood sample, identify the mutations that make your dog's specific cancer different from their healthy tissue, and design a codon-optimised mRNA vaccine targeting those mutations. The vaccine is manufactured by our CDMO partner and shipped back to your veterinary oncologist for administration.

We are in closed beta and we accept a small number of pilot cases per quarter. Mast cell tumours are one of our primary focus areas. If you are a pet owner whose dog has been diagnosed with an MCT, the right way to learn whether a personalized mRNA vaccine could be part of their treatment is to discuss it with your veterinary oncologist, and have them request a pilot case through the form on our home page.