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Mast Cell Tumors (Mastocytoma) in Dogs

By: Dr. Jeffrey Philibert

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

Mast cell tumors vary greatly in their biological behavior. Some mast cell tumors remain localized for extended periods of time, but others invade local areas causing much inflammation, and they eventually metastasize or spread to distant sites in the body. 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.

Mast cell tumors occur as one of three types:

  • Well-differentiated
  • Moderately-differentiated
  • Poorly-differentiated

    This classification refers to how closely the mast cells of the tumor resemble normal mast cells and ultimately to the biological behavior of the tumor or its tendency to remain localized or spread throughout the body.

    Well-differentiated

  • The cells of well-differentiated mast cell tumors closely resemble normal mast cells.

  • Well-differentiated mast cell tumors tend to remain localized and have benign biological behavior.

    Poorly-differentiated

  • Poorly-differentiated mast cell tumors can be difficult to identify as mast cells without special stains.

  • Poorly-differentiated mast cell tumors tend to spread through the body and have malignant biological behavior.

    Moderately-differentiated

    This type is between well and poorly differentiated.

    The cause of mast cell tumors is unknown. Mast cell tumors have been transmitted experimentally using tumor extracts suggesting possible viral origin, but this hypothesis remains unproven. Dogs that develop mast cell tumors often are older (usually 8 to 9 years of age), but mast cell tumors can occur in dogs of all ages. Breeds that seem predisposed include boxers, Boston terriers, bull terriers, bullmastiffs, English setters and golden retrievers. Males and females are affected equally.

    Mast cell tumors generally respond well to treatment. Well-differentiated mast cell tumors in the skin often can be removed successfully by wide surgical excision. The less common, poorly-differentiated mast cell tumors can cause severe swelling and inflammation locally and tend to spread throughout the body. These aggressive tumors are much more difficult to treat effectively and often result in death. Mast cell tumors that occur in areas around the mouth, anus and genitals tend to be more aggressive and have a worse overall prognosis than those occurring elsewhere in the skin.

    Appearance can vary but mast cell tumors are usually round, raised masses in the skin. They can be covered with hair or hairless, and the affected skin may be reddened and ulcerated and may bleed. Mast cell tumors have the unique characteristic of growing and shrinking in size rapidly over short periods of time due to the release of inflammatory chemicals from the mast cells of the tumor with minor trauma. In animals with mast cell tumors of the spleen or intestinal tract, clinical symptoms of inappetence, vomiting, abdominal pain, and black tarry stools can be seen.

    Many tumors, both benign and malignant, can affect the skin of dogs. The most common are:

  • Mast cell tumors, sebaceous gland tumors, sweat gland tumors, soft tissue sarcomas (tumors of connective tissue), and melanoma in dogs

  • Some skin infections can resemble mast cell tumors because both may cause raised, reddened, ulcerated or bleeding mass-like lesions in the skin. Infectious skin disorders tend to produce many lesions and affect extensive regions of skin, often the abdomen and other thinly-haired regions.

  • Cutaneous lymphosarcoma can also result in multiple, raised, reddened, mass-like lesions in the skin.

  • Careful physical examination and fine needle aspirates of suspicious lesions are needed to distinguish these diseases.

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