Mast Cell Tumours in Dogs: Grades, Treatment and Prognosis

Mast cell tumours (MCTs) are the single most common skin cancer diagnosed in dogs, accounting for approximately 20 per cent of all skin tumours. Mast cells are a normal part of the immune system — they contain granules packed with histamine, heparin, and other inflammatory chemicals that play a role in allergic responses and wound healing. When these cells become cancerous, they can form tumours that range from slow-growing and relatively benign to highly aggressive and life-threatening.

"Mast cell tumours are sometimes called the great imitators because they can look like almost anything — a harmless fatty lump, an insect bite, or a wart. Any new or changing lump on your dog deserves a veterinary check, no matter how innocent it appears." — Dr. Sarah Chen, DVM

MCTs can appear anywhere on the body, though they are most commonly found on the trunk, limbs, and around the genital area. They may present as a single lump or, less commonly, as multiple masses. The behaviour of mast cell tumours varies enormously — some remain localised for months or years, while others spread rapidly to lymph nodes, liver, spleen, and bone marrow. This unpredictability is precisely why early investigation is so critical. If you notice any new lump on your dog, your first step should always be a veterinary examination — our guide on how often your pet needs vet visits explains why regular check-ups are especially important for catching changes early.

Understanding what mast cell tumours are and how they behave empowers you to act quickly, advocate for appropriate diagnostics, and make informed treatment decisions alongside your veterinary team.

One of the most challenging aspects of mast cell tumours is their wildly variable appearance. Unlike some cancers that have a distinctive look, MCTs are true chameleons. They can present as small, firm, raised nodules on the skin's surface, flat pink patches, or large, ulcerated masses. Some feel well-defined under the skin, while others have indistinct borders that blend into surrounding tissue. Colour ranges from skin-toned to red, pink, or even purplish.

A hallmark feature of mast cell tumours is something veterinarians call Darier's sign — when the tumour is touched or manipulated, the mast cells release histamine from their granules, causing the surrounding area to become red, swollen, and itchy. This local reaction can cause the tumour to fluctuate in size, sometimes appearing larger after being handled and then shrinking again. Owners frequently describe lumps that seem to grow and shrink over days or weeks, which can falsely reassure them that the mass is harmless.

Some MCTs cause systemic effects due to histamine release, including gastrointestinal ulceration, vomiting, decreased appetite, and in severe cases, anaphylactic-like reactions. Dogs with high-grade tumours may also show signs of bruising around the mass due to heparin release, which interferes with normal blood clotting. If your dog shows signs of discomfort or pain alongside a skin lump, this warrants an urgent veterinary visit rather than a wait-and-see approach.

The bottom line: there is no way to determine whether a lump is a mast cell tumour — or any other type of cancer — simply by looking at it or feeling it. Every new lump should be investigated with diagnostics.

Once a mast cell tumour is diagnosed, the most important next step is grading — a microscopic assessment of how aggressive the tumour cells appear. Grading is the single strongest predictor of how the tumour will behave and guides every subsequent treatment decision. Two grading systems are used in veterinary oncology.

The Patnaik system, developed in 1984, divides MCTs into three grades. Grade I tumours are well-differentiated — the cells look relatively normal, the tumour is confined to the skin, and the prognosis is excellent with surgery alone. Grade II is the most common and most controversial category, encompassing a wide range of tumour behaviours from nearly benign to moderately aggressive. Grade III tumours are poorly differentiated with highly abnormal cells, a high rate of cell division (mitotic index), and a significantly worse prognosis — they frequently metastasise to distant organs.

Because Grade II covers such a broad spectrum, the Kiupel two-tier system was introduced in 2011 to improve consistency. This system classifies MCTs as either low grade or high grade based on specific cellular criteria: the number of mitotic figures, the presence of multinucleated cells, bizarre nuclei, and karyomegaly (abnormally large nuclei). Studies have shown that the Kiupel system has better inter-pathologist agreement and more reliably predicts outcome — low-grade tumours carry a median survival time exceeding two years, while high-grade tumours have a median survival of around four months without treatment.

Your veterinary oncologist will typically report both grading systems along with the mitotic index — the number of dividing cells per 10 high-power microscope fields. A mitotic index of 5 or fewer is generally favourable, while a count above 5 raises concern for more aggressive behaviour regardless of the overall grade.

The diagnostic journey for a suspected mast cell tumour typically begins with a fine needle aspirate (FNA) — a quick, minimally invasive procedure where your vet inserts a thin needle into the lump and collects a small sample of cells. Mast cells have distinctive granules that stain a characteristic purple with Romanowsky-type stains, making FNA highly accurate for identifying MCTs. Most vets can perform this in-house within minutes, often without sedation.

While FNA confirms the tumour type, it cannot reliably grade the tumour. For grading, a tissue biopsy is needed — either an incisional biopsy (a small piece of the tumour) or, more commonly, an excisional biopsy where the entire tumour is surgically removed and sent for histopathology. The pathologist evaluates cellular morphology, mitotic index, tumour margins (whether cancer cells extend to the edges of the removed tissue), and invasiveness into surrounding structures.

Staging determines whether the cancer has spread beyond the original site. Standard staging for MCTs includes aspiration of regional lymph nodes (even if they feel normal — MCTs can spread microscopically), abdominal ultrasound to evaluate the liver and spleen, and blood work including a buffy coat smear to check for circulating mast cells. Some oncologists also recommend chest X-rays, though MCTs rarely metastasise to the lungs. For high-grade tumours, bone marrow aspiration may be recommended to check for marrow involvement.

Staging is particularly important for Grade II (Patnaik) or high-grade (Kiupel) tumours because the results directly influence whether additional treatments like chemotherapy or radiation are recommended beyond surgery. Complete staging can feel costly, but understanding the full picture is essential for making the best treatment decisions. Our guide on whether pet insurance is worth it explains how coverage can help manage the financial impact of cancer diagnostics and treatment.

Low-grade / Grade I MCTs: Surgery is typically curative when performed with adequate margins — most oncologists recommend at least 2 cm lateral margins and one fascial plane deep. When these margins are achieved, recurrence rates are below 5 per cent, and additional treatment is rarely needed. If clean margins cannot be obtained due to tumour location (e.g., on the lower leg or face), follow-up radiation therapy can achieve local control rates exceeding 90 per cent.

Grade II (Patnaik) with favourable markers: Surgery with wide margins remains the primary treatment. The decision to add chemotherapy depends on the Kiupel grade, mitotic index, c-KIT mutation status, and lymph node involvement. If the tumour is Kiupel low-grade with a low mitotic index and clean margins, monitoring alone may be appropriate. Regular rechecks every three months for the first year are essential.

High-grade / Grade III MCTs: These tumours require multimodal treatment. Surgery is performed when possible, followed by chemotherapy — typically vinblastine and prednisolone over several months. For dogs with c-KIT mutations, targeted therapy with toceranib phosphate (Palladia) or masitinib (Masivet) has shown significant benefit. These tyrosine kinase inhibitors block the abnormal growth signals driving the cancer. Radiation therapy may be added for incomplete surgical margins or inoperable tumours.

Palladia (toceranib) deserves special mention — it was the first drug approved specifically for canine cancer and has been a game-changer for dogs with aggressive MCTs. Side effects can include diarrhoea, decreased appetite, and occasional lameness, but most dogs tolerate it well with appropriate dose adjustments. Your oncologist will monitor blood work regularly during treatment.

Certain breeds carry a significantly higher risk of developing mast cell tumours, suggesting a strong genetic component. Boxers are the most commonly affected breed — however, their MCTs tend to be predominantly low-grade with an excellent prognosis. Pugs also develop MCTs frequently and often present with multiple tumours, though these are typically low-grade as well. Boston Terriers, Bulldogs (English and French), Staffordshire Bull Terriers, and Weimaraners are also overrepresented in MCT statistics.

Labrador Retrievers and Golden Retrievers develop MCTs at a moderate rate, and their tumours are more likely to be intermediate or high-grade compared to brachycephalic breeds. Shar-Peis are at particularly high risk for aggressive, high-grade MCTs that can be challenging to treat. Knowing your dog's breed predisposition helps you stay vigilant — owners of high-risk breeds should perform monthly at-home lump checks and report any new masses promptly.

Overall prognosis depends heavily on grade and stage at diagnosis. Low-grade MCTs treated with complete surgical excision carry a cure rate exceeding 90 per cent, with most dogs living a normal lifespan. High-grade MCTs are more sobering — even with aggressive multimodal treatment, median survival times range from six months to one year. However, a subset of dogs with high-grade tumours do respond well to chemotherapy and targeted therapy, achieving survival times well beyond the median.

Early detection remains the single most impactful factor in outcomes. Regular veterinary check-ups, prompt investigation of new lumps, and awareness of your breed's risk profile give your dog the best chance of a favourable outcome. If your dog is diagnosed with a mast cell tumour, consulting a board-certified veterinary oncologist ensures access to the most current treatment protocols and clinical trials.