Mast Cell Tumours in Dogs: What Every Owner Should Know

What Are Mast Cell Tumours?

Mast cell tumours (MCTs) are the most common skin cancer in dogs. They can appear anywhere on the body — most often as lumps on the skin or just beneath it.

Mast cells are part of your dog's immune system. They normally release histamine during allergic reactions. When these cells become cancerous, they grow out of control and form tumours.

Not every lump is an MCT. But any new skin mass should be checked by a vet. A fine needle aspirate — a quick procedure where the vet draws cells from the lump with a needle — can confirm whether it's a mast cell tumour.

Why Grading Matters More Than Anything

The single most important thing to know about mast cell tumours is this: grade determines everything.

Low-grade MCTs can often be cured with surgery alone. If the surgeon removes the tumour with clean margins — meaning no cancer cells at the edge of the tissue removed — many dogs never see it come back.

High-grade MCTs behave differently. They're aggressive. They recur after surgery. They spread to lymph nodes, the spleen, and other organs. The prognosis is much more serious.

Your vet will send the removed tumour to a pathologist, who grades it based on how abnormal the cells look under a microscope. Older systems used three grades (I, II, III). Newer systems use two (low-grade, high-grade). The principle is the same: grade tells you how the tumour will behave.

Current Treatment Options

Surgery is the first-line treatment for most MCTs. Wide surgical margins are critical — the surgeon removes the tumour plus a margin of healthy tissue around it to catch any stray cancer cells.

If the tumour is in a location where wide margins aren't possible (like a paw or the face), radiation therapy can be added after surgery to kill remaining cells. Not every vet clinic offers radiation — it requires specialised equipment and is typically available only at referral hospitals.

Palladia (toceranib) is a targeted therapy approved for canine mast cell tumours. It's a pill given at home. Palladia blocks specific proteins — including KIT, a driver in many MCTs. It doesn't cure the cancer, but it can slow progression and buy time. Response rates vary. Some dogs respond well; others don't.

Chemotherapy is sometimes used for high-grade MCTs or when the cancer has spread. Vinblastine and lomustine are the most common drugs. Dogs tolerate chemo much better than humans do — veterinary protocols use lower doses and aim for quality of life, not maximum tumour kill.

Prognosis depends entirely on grade, location, and whether the cancer has spread. Low-grade MCTs removed with clean margins have an excellent prognosis. High-grade MCTs carry a guarded to poor prognosis even with aggressive treatment.

The Genetics Behind the Tumour

Mast cell tumours aren't all the same at the genetic level. A study of canine MCTs found mutations in 87 different genes. One gene — GNB1 — was mutated in 17.3% of cases. The KIT gene, which Palladia targets, is mutated in a significant portion of MCTs.

This genetic variation explains why two dogs with "the same" cancer can respond so differently to the same treatment. The mutations driving one dog's tumour may be completely different from the mutations driving another dog's tumour.

Current treatments don't account for this. Surgery removes the bulk tumour. Palladia targets KIT — but what if your dog's tumour doesn't have a KIT mutation? Chemotherapy kills dividing cells indiscriminately.

What Personalised Medicine Means for MCT

In human cancer medicine, personalised mRNA vaccines are in Phase III trials. These vaccines are built from the patient's specific tumour mutations — not a one-size-fits-all formula.

The same science is being developed for dogs. A personalised mRNA vaccine targets the exact mutations in your dog's specific tumour. Each vaccine is different because each tumour is different.

The approach works like this: a biopsy is sequenced to identify which mutations are driving the tumour. Those mutations create abnormal proteins that the immune system can recognise. The vaccine trains T cells — the body's cancer-killing immune cells — to find and destroy any cell carrying those specific mutations.

In human melanoma trials, Moderna's personalised vaccine reduced recurrence risk by 49% at five years when given after surgery. The post-surgical setting — where the immune system only needs to mop up small amounts of residual disease — is where personalised vaccines work best.

For dogs with high-grade MCTs removed surgically, this approach could address the problem conventional treatments can't solve: microscopic cancer cells left behind that grow into recurrence months later.

Novectis Labs, based in Zürich, is developing personalised mRNA cancer vaccines for dogs. The Founding Patient Programme offers early access to a limited number of dogs in exchange for contributing anonymised treatment data.

This isn't a guaranteed cure. It's science proven in human trials, now being applied to dogs. Each case contributes data that improves the approach for future patients.