Hey there, pet parents. Today we're going to talk about the most common type of skin cancer in dogs: the Mast Cell Tumor. These tumors are known as "The Great Pretenders" because they can look like anything. They can be a small, innocent-looking bump, a fatty lump, or an angry, ulcerated sore. Any lump or bump on your dog could be a mast cell tumor, which is why it's so important to have every new lump checked by your vet.
What are Mast Cells?
Mast cells are a type of immune cell that are normally involved in allergic reactions. They are found in the skin and other tissues, and they contain granules filled with potent chemicals like histamine. When a mast cell is triggered (e.g., by an allergen), it "degranulates," releasing all these chemicals. This is what causes the redness, swelling, and itching of an allergic reaction.
A mast cell tumor is a cancerous proliferation of these mast cells.
Spotting the Signs: The Great Pretender
As mentioned, a mast cell tumor can have almost any appearance.
- •It can be a raised, hairless lump.
- •It can be a soft, fatty-feeling lump under the skin.
- •It can be an angry, red, ulcerated mass.
- •They are most commonly found on the trunk of the body, but they can appear anywhere.
One of the classic signs of a mast cell tumor is something called Darier's sign. This is when the tumor becomes red and swollen after it has been agitated or handled. This is because the physical manipulation has caused the mast cells to degranulate and release histamine. You might notice that the lump seems to change in size.
Breeds at Risk
There is a strong genetic component to mast cell tumors. Breeds that are at a high risk include:
- •Boxers
- •Boston Terriers
- •Bulldogs
- •Pugs
- •Other brachycephalic (flat-faced) breeds.
Diagnosis and Grading: The Key to Prognosis
If your vet finds a suspicious lump, the first step is a Fine Needle Aspirate (FNA). Your vet will use a small needle to get a sample of cells from the lump and will look at them under a microscope. Mast cells have very characteristic purple granules, so they are usually easy to identify.
Once a mast cell tumor is diagnosed, the next and most important step is to determine the grade of the tumor. The grade is a measure of how aggressive the cancer is, and it is the single most important factor in determining the prognosis. The grade can only be determined by a pathologist looking at a biopsy (a surgical tissue sample) of the tumor.
- •Grade I: These are low-grade, benign-behaving tumors. They are very unlikely to spread (metastasize).
- •Grade II: These are intermediate-grade tumors. Their behavior is unpredictable. Some will be cured with surgery alone, while others may spread.
- •Grade III: These are high-grade, aggressive tumors. They have a high rate of metastasis to the lymph nodes, spleen, and liver.
Treatment: Surgery and Beyond
Surgical Removal
The cornerstone of treatment for a mast cell tumor is aggressive surgical removal. Because these tumors can have microscopic "tentacles" that extend out into the surrounding tissue, the surgeon needs to take wide margins around the tumor. The goal is to get at least 2-3 cm of clean, normal tissue around the tumor in all directions.
The removed tumor is then sent to the pathologist to be graded and to make sure the surgeon got "clean margins" (meaning no cancer cells were left behind).
- •For a Grade I or a low-grade Grade II tumor with clean margins: Surgery is curative! No further treatment is needed.
- •For a high-grade Grade II or a Grade III tumor, or a tumor with "dirty" margins: Further treatment is needed.
Further Treatment Options
- •Re-operation: If the margins were not clean, a second surgery to remove more tissue may be recommended.
- •Radiation Therapy: If a second surgery is not possible, radiation therapy can be used to "clean up" the remaining cancer cells in the area.
- •Chemotherapy: For high-grade tumors that have a high risk of spreading, chemotherapy is recommended. The most common chemotherapy protocol for mast cell tumors uses drugs like vinblastine and prednisone. There is also a targeted "chemo" pill called toceranib (Palladia) that can be effective.
Before any treatment, your vet or oncologist will recommend staging tests (bloodwork, abdominal ultrasound, aspirate of the local lymph node) to check for any evidence of spread.
The prognosis for a mast cell tumor is entirely dependent on the grade. For a low-grade tumor that is removed with clean margins, the prognosis is excellent. For a high-grade tumor, the prognosis is much more guarded.
The key takeaway is that any new lump or bump on your dog should be checked by your vet. The sooner a mast cell tumor is diagnosed and removed, the better the prognosis.
Sources & Further Reading:
- •American College of Veterinary Surgeons - Mast Cell Tumors
- 1VCA Animal Hospitals - Mast Cell Tumors in Dogs