Mast cell tumors make up roughly 16–21% of all canine skin tumors. While that number sounds alarming, understanding how these tumors 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 breaks it all down here for dog owners—sharing what you need to know in clear, practical terms.
Dr. Charney is a co-founder of Boundary Bay Veterinary Specialty Hospital and a board-certified specialist in both medical oncology (DACVIM) and radiation oncology (DACVR), with over 20 years of clinical experience in treating cancer in pets.
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 tumors. This abnormal growth is called a mast cell tumor (MCT).
Which Dogs Are at Risk?
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 tumor growth.
There’s no known link between MCTs and a dog’s sex.
Signs & Symptoms of Mast Cell Tumors
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 tumor.
- 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 tumor’s grade (how aggressive it is).
If there’s concern that the tumor 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 Tumor Grading
Not all mast cell tumors 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.
Tumor 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 tumor 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 tumors, 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 tumors showed inconsistent behavior, 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 tumor.
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 Tumors
Treatment is based on tumor grade, location, and whether it has spread. Common options include:
- Surgery: Often the first-line treatment for localized tumors. Wide surgical margins (2 cm or more) help prevent recurrence. Surgery may be curative for low-grade tumors.
- Radiation Therapy: Used when tumor removal isn’t possible due to location, or when surgery doesn’t achieve clean margins. Often paired with surgery for intermediate-grade tumors.
- Chemotherapy: Recommended for high-grade tumors or tumors that have metastasized. Common drugs include vinblastine, lomustine (CCNU), and prednisone. Chemotherapy helps slow progression and improve quality of life, but is not always curative.
- Targeted Therapy: Some tumors have mutations in the c-KIT gene that make them more aggressive. Palladia (toceranib phosphate) blocks this pathway and may shrink tumors; it works best in c-KIT positive tumors but may benefit others as well.
- Supportive Therapy: Helps manage symptoms from histamine release. Options include antihistamines (e.g., Benadryl), antacids (e.g., Pepcid or omeprazole), and pain and anti-inflammatory medications.
Prognosis: What to Expect
- Low-grade tumors: Many dogs do very well with surgery alone and may live for years post-treatment.
- High-grade tumors: 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 tumors go on to live happy, fulfilling lives.
At BBVSH, 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 MCT and you have questions about next steps, we’re here to help.
