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Mast Cell Tumors (Mastocytoma) in Dogs

By: Dr. Jeffrey Philibert

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Mast cell tumors, also called mastocytomas, arise most commonly in the skin. They develop from a normal component of body tissues called the mast cell that play a role in the process of tissue repair by releasing inflammatory mediators.

Malignant mast cell tumors can spread to the lymph nodes, spleen, liver and bone marrow. Mast cell tumors are among the most common tumors of dogs, accounting for approximately 20 percent of all skin tumors. The cause of mast cell tumors is unknown.

Dogs that develop mast cell tumors often are older (usually 8 to 9 years of age), although they can occur in dogs of all ages.

What to Watch For

  • Round, raised masses in the skin
  • Lack of appetite
  • Vomiting
  • Abdominal pain
  • Black tarry stools due to bleeding in the upper intestinal tract


    Diagnostic tests are needed to recognize mast cell tumors and exclude other diseases that may cause similar symptoms. Tests may include:

  • A complete medical history and thorough physical examination

  • Needle aspiration of the mass and any enlarged lymph nodes. The aspirated material is expelled onto a glass slide and submitted to the laboratory for cytologic evaluation by a veterinary pathologist.

  • A complete blood cell count (hemogram or CBC) to evaluate for anemia, low platelet count, or signs of inflammation. Rarely, circulating mast cells are found in animals with systemic mast cell disease.

  • A serum biochemistry profile to evaluate your dog's general health and to assess any effects of the mast cell tumor on other body systems

  • A urinalysis to evaluate kidney function and check for urinary infection

  • A buffy coat smear of blood to look for rare circulating mast cells

  • X-rays of the abdomen to evaluate liver and spleen size

  • Abdominal ultrasound, if further evaluation of the liver and spleen is needed to detect masses and abnormal tissue densities within the spleen or liver; X-rays usually only show generalized enlargement.

  • A fine needle aspirate and cytologic evaluation of the bone marrow, if widespread mast cell disease is suspected

  • A fine needle aspirate and cytologic evaluation of the spleen, if splenic mastocytoma is suspected.

  • Surgical removal of a suspicious skin tumor followed by histopathologic evaluation by a veterinary pathologist (excisional biopsy); this may be curative for small, well-differentiated mast cell tumors of skin provided a wide surgical excision is performed.


    Treatment for mast cell tumors may include one or more of the following:

  • Wide surgical excision (removal) of a well-differentiated mast cell tumor of the skin
  • Radiation therapy for local control of a less well-differentiated skin tumor
  • Chemotherapy in selected cases

    Home Care and Prevention

    Your dog should be examined by a veterinarian if you notice any mass on his skin. A fine needle aspirate or surgical biopsy can be performed to identify the nature of the mass. Most skin tumors in dogs are benign, but early detection and treatment of malignant tumors, especially mast cell tumors, can dramatically affect your pet's long-term prognosis.

    No method of protection against development of mast cell tumors can be recommended because the cause of these tumors is unknown. Careful monitoring of your pet is important, especially if you have one of the breeds known to be at increased risk. If you notice a lump in your pet's skin, the mass should be examined by a veterinarian as soon as possible.

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