Mast Cell Tumor (Mastocytoma) in Dogs

Key takeaways

Mast cells are part of the immune system. Uncontrolled growth of these cells leads to mastocytoma, or mast cell tumors (MCTs), which are the most common type of skin tumor in dogs.

• These tumors commonly present as a lump in or under the skin

• Other forms of MCT also exist, occurring most often in the spleen and liver

• MCTs may release histamine and other substances into the surrounding tissue, often leading to redness and irritation of the mass

• Diagnosis is almost always by cytology with microscopic examination

• Surgical excision is the treatment of choice

• MCTs often recur, and radiation or chemotherapy may be recommended depending on the tumor grade

• Prognosis depends on the size and grade of the tumor and whether or not it has spread to other areas of the body

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A closer look: Mast Cell Tumor (Mastocytoma) in Dogs

Mast cells originate in bone marrow and are a normal component of a healthy immune system. When these cells have uncontrolled growth, they develop into mastocytoma or mast-cell tumors. MCTs are the most common form of skin tumors in dogs. MCTs also have a visceral (within the larger body cavity) form, which is more severe and less common.

Growths that get suddenly larger and then smaller over a twenty-four-hour period are likely to be mast-cell tumors, and this phenomenon is referred to as Darier's sign. Rapidly changing skin growths may suddenly release histamines and lead to anaphylaxis, which is life threatening. Sudden changes in skin growths require urgent medical attention. A diagnosis of MCT can have a big impact on pet parents and pet quality of life. Treatment is aggressive and often time consuming, regardless of how far the cancer has progressed. Palliative care or humane euthanasia may be the best option in many cases.

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Risk factors

MCTs are common tumors in dogs that have been called “the great pretenders,” in that they may resemble something as harmless as an insect bite, wart, or allergic reaction. This is why any abnormalities of the skin should be evaluated by a veterinarian. Prognosis ranges from complete remission to fatal.

In some cases, MCTs will break down and release histamines and other compounds into the surrounding tissues. Histamines are associated with allergic responses. If a large amount of histamine is released into the body all at once, it can lead to anaphylaxis, which is life threatening. Dogs showing signs of anaphylaxis, including difficulty breathing or loss of consciousness, require emergency veterinary attention.

Certain breeds may be predisposed to developing them, and breed may also influence outcome.

Possible causes

MCTs occur when uncontrolled growth of mast cells develops into a lump or tumor. The underlying cause of MCTs is unknown. As with most forms of cancer, genetic and environmental factors are suggested.

Main symptoms

The main symptom of cutaneous (skin) MCT is a raised bump in or under the dog’s skin. This may be red, itchy, or neither. This, as well as bruising in the area surrounding the lump, are due to chemical release from the mast cells. If massive degranulation occurs, symptoms like those seen with the visceral form can result

Testing and diagnosis

When presented with a possible MCT, veterinarians begin with a physical exam and blood work. To confirm diagnosis, cytology is performed. Abdominal ultrasound may also aid in diagnosis by helping to localize the mass.

Steps to Recovery

Surgery is the first-choice treatment for mast-cell tumors, with wide surgical margins required. What this means is a large amount of the surrounding tissue must also be removed to help prevent recurrence. Depending on the location of the tumor, it may not be possible to get wide enough margins during the procedure. In these cases, recurrence is more likely.

The MCT can be graded only after it is removed. Prior to surgery, the veterinarian may recommend medical management. This may include steroids, antihistamines, and histamine blockers to help reduce the inflammation and associated side effects of these tumors prior to surgery.

Incompletely removed tumors can be treated with a course of radiation therapy for long-term control. Additional therapy options include chemotherapy and steroids. Chemotherapy is often recommended for dogs with a tumor that is too large to remove surgically, has spread to a lymph node, spread to other places, or has a high likelihood of metastasis.

Steroids deserve a special mention because they seem to be directly toxic to mast cells and can lead to a brief remission even when used alone.

Chances for recovery depend upon the grade of the tumor, whether or not complete surgical removal is possible, whether the cancer has spread to other tissues or organs, and the type of therapy that is pursued. In cases of complete surgical removal of low-grade tumors, the prognosis is typically good. Some dogs may develop other MCTs in the future (in the same area or elsewhere on the body). High-grade tumors have a lower survival time and increased rate of mortality.


Since there is no single identifiable cause, MCTs are impossible to prevent. However, dogs that have had one mast-cell tumor in their lives are at risk for developing another mast-cell tumor, so careful monitoring is recommended. Mast-cell tumors are not contagious.

Is Mast Cell Tumor (Mastocytoma) in Dogs common?

Mast-cell tumors are common in dogs, accounting for up to 20% of all skin tumors. MCTs are usually discovered in middle age, but can occur at any age. The visceral form is less common.

Typical Treatment

  • Surgical removal
  • Medications (antihistamines, steroids)
  • Chemotherapy
  • Radiation