Mast Cell Tumors

Mast cell tumors, or mastocytomas (MCTs), are found in a dog’s skin or other tissues of the body and account for the most common of canine cancerous tumors. They are the accumulation of mast cells, components of a dog’s immune system that normally help mediate allergic and inflammatory reactions in your dog. What causes these mast cells to cluster into sometimes-malignant tumors is still unknown.  What is known is that the same enzymes and chemicals released by mast cells that are vital to normal bodily function become very damaging to the body when persistently released in excess by MCTs. Mast cell tumors can arise anywhere on a dog’s body and vary widely in their size, shape, appearance and texture. Although MCTs are found in all ages and breeds of dogs, they most commonly appear in older dogs and the brachycephalic breeds such as Boxers, Bulldogs, and Boston Terriers.

Signs and Symptoms

Symptoms are variable, depending on the location of the tumor and the degree to which it has developed and/or spread. MCTs most commonly are seen as slow-growing solitary lumps or masses in or underneath the skin. The hair on the skin may be lost, and lesion can develop superficial ulceration especially in aggressive cases. The release of chemicals (histamine, heparin and others) from the MCT can cause local symptoms such as itching, but also systemic problems including gastric ulcers, internal bleeding, and a range of allergic manifestations. Some dogs may also exhibit other warning signs, including decreased appetite, increased respiratory rate, vomiting, diarrhea, abdominal pain, dark feces, itchiness, lethargy, anorexia, irregular heart rhythm and blood pressure, coughing, labored breathing, various bleeding disorders, delayed wound healing, and/or enlarged lymph nodes.

Whenever your pet is showing signs of a health issue your first step is to contact your primary care veterinarian. If it is indicated that your pet may suffer from mast cell tumors or another serious condition, a veterinary specialist is available at an ExpertVet certified hospital.


Mast-cell tumors do not have a specific, standard appearance, and there is no way to definitively identify them without specialized testing. ANY LUMP ON YOUR DOG COULD POTENTIALLY BE A MCT and as such a fine-needle biopsy or aspiration is typically performed first. This is a simple and painless procedure. The veterinarian inserts a thin needle into the tumor, sometimes with the guidance of an ultrasound machine, and collects fluid from within the mass. If the result indicates the presence of an MCT, the tumor may then be removed for a more in-depth biopsy. Your dog’s lymph nodes may also be aspirated for diagnostic purposes.

The following staging tests are often performed for patients with MCT: CBC (complete blood count-- will show the levels of red and white blood cells, as well as platelets); buffy coat, and a serum chemistry profile. Your veterinarian may also want to test a sample of your dog’s urine or bone marrow, obtain radiographs (X-rays) and perform an ultrasound of the tumor. The combination of these tests helps the veterinarian assess the overall health of your dog and determine the stage of disease (see below) of the MCT, which will influence and guide treatment recommendations.

Evaluation of the Body Systems

Both the histologic (anatomy of cells and tissues) grade and the clinical stage of disease influence the prognosis and treatment of MCTs. The ‘grade’ defines assessment of how well-differentiated the cells are and therefore how aggressively malignant the cancer appears to be. Grades are classified on a scale of I-IV, (more recently I-II) where grade I represents the least aggressive tumor. 

Once the grade of the MCT is classified, the surgically excised and examined tumor is ‘staged’ to determine the degree to which the tumor has spread (metastasis).  Stages range from I-IV, where stage I represents a tumor with little to no spread. Several staging systems have been described:

Stage I: One tumor in the skin, with no lymph-node involvement
Stage II: One tumor in the skin, with lymph-node involvement
Stage III: Multiple large, deep-skin tumors, with or without lymph-node involvement
Stage IV: One or more tumors with metastasis in the skin, with lymph- node involvement