You’re petting your dog and feel a lump. It might seem harmless—or it might be something more. One of the most common skin cancers in dogs is the mast cell tumour (MCT), a diagnosis that can be overwhelming without clear information and a plan.
Mast cell tumors make up roughly 16–21% of all canine skin tumours. While that number sounds alarming, understanding how these tumours behave, how they’re diagnosed, and how they’re treated can help you make informed, confident decisions for your dog’s care.
Dr. Sarah Charney is co-founder of Boundary Bay Veterinary Specialty Hospital and a double-board-certified specialist in both medical oncology and radiation oncology, with over 20 years of clinical experience in treating cancer in pets. In this blog, Dr. Charney breaks it all down for dog owners—how to spot mast cell tumours and what to do about it.
What Are Mast Cells?
Mast cells are immune cells involved in allergic reactions and inflammation. Found throughout the body—in the skin, respiratory system, and intestines—they store and release histamines and enzymes to help the body respond to allergens or injury.
When mast cells begin to multiply uncontrollably, they form tumours. This abnormal growth is called a mast cell tumour (MCT).
Which Dogs Are at Risk for Mast Cell Tumours?
Mast Cell Tumours (MCTs) can affect dogs of any breed, age, or gender, but certain risk factors make some dogs more likely to develop them:
- Age: Most common in older dogs (average age ~9 years), but younger dogs can also be affected.
- Breed: Boxers, Boston Terriers, Labradors, Beagles, and Schnauzers have higher risk.
- Genetics: Mutations in the c-KIT gene may drive tumour growth.
There’s no known link between MCTs and a dog’s sex.
Signs & Symptoms of Mast Cell Tumours (MCTs)
MCTs are sometimes called the “great imitators” because they can resemble many other types of skin lesions. They may appear as:
- A lump or bump on or under the skin
- A red, raised, or swollen area that may be itchy
- An ulcerated or bleeding mass
- A soft, rubbery growth
Some MCTs cause systemic symptoms due to histamine release:
- Vomiting or poor appetite (from stomach ulcers)
- Swelling and inflammation around the tumour
- Diarrhea or black, tarry stool (from internal bleeding)
A distinctive reaction called Darier’s Sign occurs when touching the mass causes redness and swelling in the surrounding skin.
How Are MCTs Diagnosed?
Diagnosis typically begins with:
- Fine Needle Aspiration (FNA): A simple, minimally invasive test where cells are extracted and examined under a microscope.
- Biopsy: If MCT is confirmed, a tissue sample may be collected to determine the tumour’s grade (how aggressive it is).
If there’s concern that the tumour may have spread, your veterinarian may recommend staging tests, such as:
- Lymph node aspiration
- Abdominal ultrasound or chest X-rays
- Bloodwork to assess overall health
Understanding Mast Cell Tumour Grading in Pets
Not all mast cell tumours behave the same way. Some are slow-growing and relatively benign, while others are aggressive and more likely to spread. That’s why grading is an important part of diagnosing MCTs.
Tumour grading is based on how the cancer cells look under a microscope, including their growth pattern, cell shape, rate of division, and likelihood of invasion or spread. The goal is to predict how aggressively the tumour might behave and guide treatment planning.
Traditional 3-Tier Grading System
Veterinarians have traditionally used a 3-tier system to classify MCTs:
- Grade 1 (Low Grade): Least aggressive; often cured with surgery.
- Grade 2 (Intermediate Grade): Can behave unpredictably—some act like low-grade tumours, while others behave more aggressively.
- Grade 3 (High Grade): Most aggressive; likely to spread to lymph nodes, liver, spleen, or bone marrow.
Newer 2-Tier Grading System
Because Grade 2 tumours showed inconsistent behaviour, a newer 2-tier system was developed to simplify classification and improve treatment decisions. This system groups MCTs into:
- Low Grade: Less likely to spread and typically associated with a better prognosis.
- High Grade: More likely to spread and requires more aggressive treatment.
Veterinarians may also assess the mitotic index (MI)—which measures how quickly cells are dividing. A higher MI indicates a faster-growing, more aggressive tumour.
Grading helps your veterinary team determine the most appropriate treatment plan and give you a more accurate understanding of what to expect.
Treatment Options for Canine Mast Cell Tumours
Treatment is based on tumour grade, location, and whether it has spread. Common options include:
- Surgery
- Often the first-line treatment for localised tumours.
- Wide surgical margins (2 cm or more) help prevent recurrence.
- Surgery may be curative for low-grade tumours.
- Radiation Therapy
- Used when tumour removal isn’t possible due to location.
- Used when surgery doesn’t achieve clean margins.
- Often paired with surgery for intermediate-grade tumours.
- Chemotherapy
- Recommended for high-grade tumours or tumours that have metastasised.
- Common drugs include vinblastine, lomustine (CCNU), and prednisone.
- Helps slow progression and improve quality of life, but is not always curative.
- Targeted Therapy
- Some tumours have mutations in the c-KIT gene that make them more aggressive.
- Palladia (toceranib phosphate) blocks this pathway and may shrink tumours.
- Works best in c-KIT positive tumours but may benefit others as well.
- Supportive Therapy
- Helps manage symptoms from histamine release.
- Antihistamines (e.g., Benadryl).
- Antacids (e.g., Pepcid or omeprazole).
- Pain and anti-inflammatory medications.
Prognosis: What to Expect After Your Dog is Diagnosed with a Mast Cell Tumour
- Low-grade tumours: Many dogs do very well with surgery alone and may live for years post-treatment.
- High-grade tumours: More aggressive and prone to spreading; treatment can extend survival but prognosis is more guarded.
- Metastatic MCTs: Can shorten life expectancy, but with the right care, many dogs still enjoy months—or even years—of good quality life.
Final Thoughts: What Pet Owners Should Know
- Early detection saves lives. Don’t wait—have new lumps or bumps checked by your veterinarian.
- Every MCT is different. Some are easily treatable; others are more complex.
- Treatment should be personalized. Work with your vet or a veterinary oncologist to determine the best plan for your dog.
With prompt diagnosis and the right care, many dogs with mast cell tumours go on to live happy, fulfilling lives.
At Boundary Bay Veterinary Specialty Hospital in Surrey, our board-certified oncologists are here to guide you through diagnosis, treatment, and long-term care with compassion and expertise. If your dog has been diagnosed with an mast cell tumour and you have questions about next steps, we’re here to help.
*Adapted from a presentation by Dr. Sarah Charney, DVM, DACVIM (Oncology), DACVR (Radiation Oncology), Co-owner, BBVSH
