Dog Mast Cell Tumor: Diagnosis and Treatment

Mast cell tumor (MCT) is a type of blood cell cancer. These cells reside in the connective tissue, especially in the nerves and vessels closest to the external surfaces, including the lungs, mouth, nose, and skin. The primary function of mast cells is to defend the body against parasitic infestations and also promote the formation of new blood vessels.

How common is Mast Cell Tumor in Dogs?

MCT is the most common skin cancer in dogs and can also affect other areas of the body, including the liver, bone marrow, spleen, and gastrointestinal tract. Retriever breeds and the flat-faced dog breeds (brachycephalic) are the most predisposed to this type of cancer, including Boxers, Bulldogs, Pugs, and Boston Terriers.

These tumors are graded in terms of the presence of inflammation, the location, and how well they are differentiated. When they form on the skin, they vary in appearance and can appear as bumps on or under the skin, swollen, red, or ulcerated.

Mast cells have granules containing substances which can be released into the bloodstream leading to systemic problems such as bleeding, stomach ulceration, and redness. This can also result in life-threatening conditions, including a dangerous drop in blood pressure. The biological behavior of these tumors can also vary. Some of them can remain in the skin for months without changing in size, while others can form and begin to grow big within a short time.

Veterinarian with dog in examination room

Signs and Symptoms of Mast Cell Tumors

These tumors do not have any characteristic form. They can appear as big, small, soft, raised, flat, or firm. They can also be present on any part of the body, especially the legs, head, chest, and abdomen. One of the most interesting features of these tumors is how fast they change in size. Should you notice a new lump on your dog, report to your vet immediately. Some of the symptoms of these tumors include:

  • Rapid growth of lumps

  • The tumor may appear to fluctuate

  • The lumps may resemble an insect bite or an allergic reaction

  • It can appear on or under the skin for days or months

  • Single skin mass or multiple masses in the body

  • The lymph nodes could be enlarged around the area of the lump

  • The masses could be itchy or inflamed

  • Enlarged spleen and liver as a sign of widespread cancer

  • Loss of appetite

  • Diarrhea

  • Vomiting

These symptoms are also characterized by the stage of cancer in the following manner:

  • Stage 1 contains a single tumor that does not contain metastasis

  • Stage 2 has a single tumor containing metastasis in the surrounding nodes

  • Stage 3 has multiple tumors

  • Stage 4 has a tumor containing metastasis to one organ. It may also be characterized by a widespread mast cell in the blood

The causes of mast cell tumor are unknown.


In the initial evaluation of your dog, the vet will need the full history of your pet, including his health and the history of the symptoms. Whether your dog has given up any food or drinks. The history given will lead your vet in identifying the organs likely to have been affected by cancer.

A biopsy of the suspicions mass may be taken for definitive identification of the grade of the cells, followed by other diagnostics to determine the extent of the tumor. The vet may also examine a sample from the lymph node, the bone marrow, the spleen, or the kidney. Ultrasound images and an X-ray of the abdomen and the chest may also be done to determine the location and the stage of the tumor. Urine sampling and a complete blood count may also be required. The most preliminary test will include an examination of the cells from the tumor.

Prognosis and Treatment

The biopsy taken should determine the right therapy, such as chemotherapy, radiation, or surgery. The grade, completeness of the surgical margins, and the stage of the cancer are the most important factors needed for decision making. Low-grade tumors have low spreading chances, while high-grade tumors can be aggressive with very high metastasis incidences.

Most mast cell tumors are treated through surgical excision. This is especially effective in low-grade cases. If after the surgery, the histopathology report shows that the surgical margins are not clean, radiation therapy may be conducted.  Low-grade cases that are removed completely with adequate margins do not require additional therapy. However, dogs that present high-grade cancer or those with a clear spread of the tumors to the lymph nodes require systemic therapy and could also need extra treatment, also known as multi-modality therapy. Other strong drugs can be used for MCT, including steroids, RTK-inhibitors, or traditional chemotherapy.

If your dog is found to have MCT, he may be kept under supportive medication during the course of treatment. These may include a steroid and prednisone that may kill cancerous mast cells, antacids, and antihistamines. Although prednisone is a drug that works against the cancer cells, it can lead to certain side effects such as increased urination, thirst, appetite, panting, vomiting, and nausea. You may even find traces of blood in his urination. It can also cause bleeding and gastrointestinal irritation. You will find blood in its diarrhea and vomit. Antacids may be prescribed along this drug to prevent or lessen some of the side effects.

Dogs that have had a mast cell tumor before are at a higher risk of developing additional tumors. Detecting these cancer tumors early when they are still small can increase the chances of successful treatment. Tumors that spread quickly and those that form in other parts of the body apart from the skin tend to have a poor prognosis. The treatment of choice is to surgically remove these tumors and the surrounding tissue.

The Management

Your vet may choose to microscopically evaluate new masses and the lymph nodes regularly to aid in detecting the masses early before they graduate to grade 2, 3, and 4. The vet may also require regular full blood count when your dog is undergoing chemotherapy. Because your dog will be exposed to poor immunity, it is important that you boost it with a proper diet and protect him from communicable diseases during his treatment period.

For many pets, including dogs, cancer is an unfortunate reality. Fortunately, veterinary oncology today, is armed with advanced measures of treating the condition. The key to fighting cancer is early detection, diagnosis, and the start of treatment.