Mast Cell Tumors (MCT, Mastocytoma) in Dogs


Well-differentiated Mast Cell Tumors

  • 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 Mast Cell Tumors

  • 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 Mast Cell Tumors

    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.
  • In-depth Information on Diagnosis

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

  • Your veterinarian will take a complete medical history and perform a thorough physical examination of your dog.
  • A fine needle aspirate and cytologic evaluation of the mass may be performed. In this test, your veterinarian uses a regular syringe and needle are used collect some cells from the mass, which are examined under the microscope or may be submitted to a laboratory for evaluation by a veterinary pathologist. This procedure helps differentiate a non-neoplastic inflammatory process from a neoplastic process (a tumor) and will help determine the primary cell type of the tumor.
  • A complete blood cell count (CBC or hemogram) may be performed to evaluate for anemia, infection, or low platelet count. It is a useful screening testing to evaluate your pet’s general health. In very rare instances, circulating mast cells are observed in pets with systemic mast cell disease.
  • A biochemical profile may be performed to assess your pet’s general health and to evaluate the effect of a mast cell tumor on other body organs such as the liver and spleen.
  • A urinalysis may be performed to evaluate your pet’s general health, assess kidney function and check for the presence of urinary infection. Urine may be collected by catching some of your pet’s urine in a cup as he voids normally, by passing a urinary catheter, or by inserting a needle through the abdominal wall into the urinary bladder to remove a sample of urine.
  • A buffy coat blood test may be performed to look for rare circulating mast cells. In this test, white blood cells in a sample of blood are concentrated by a technique called centrifugation and the veterinary pathologist evaluates a stained smear of these cells microscopically for any circulating mast cells.
  • X-rays of the abdomen may be taken to evaluate for enlargement of abdominal organs such as the liver and spleen. If abnormalities are noted in the biochemical profile, your veterinarian may recommend taking abdominal X-rays to look for spread of the cancer.
  • An abdominal ultrasound (or sonogram) may be recommended to evaluate organs such as the liver, spleen, kidneys, and lymph nodes which may be affected by spread of a mast cell tumor. You may be referred to a veterinary specialist for this procedure. During the test, a probe placed on the skin of the abdomen transmits sounds waves that are reflected from organs inside the abdomen generating an image on a monitor. Ultrasound allows evaluation of the internal structure of the organ as well as its shape and size and can allow identification of masses within organs that may represent spread of a neoplastic process.
  • A fine needle aspirate of the bone marrow may be recommended to evaluate for spread of mast cell disease. A syringe and special bone marrow needle are used to aspirate a small sample of marrow from the shoulder or hip and the aspirated material is examined microscopically by a veterinary pathologist. This procedure is performed using sedation and local anesthesia to prevent pain.
  • A fine needle aspirate of the spleen may be recommended to evaluate for spread of mast cell disease. A regular syringe and needle are used in conjunction with abdominal ultrasound to obtain a small specimen of cells from the spleen. The aspirated material is examined microscopically by a veterinary pathologist to evaluate for the presence of malignant mast cells. Monitoring the procedure by ultrasound minimizes risk for the patient. This procedure may be performed under sedation so that your dog does not move inadvertently during collection of the aspirate.
  • Biopsy and histopathology should be performed on samples of any tumors or lymph nodes removed from your pet at the time of surgery. Your veterinarian will send surgically removed samples to a laboratory where a pathologist will determine whether the process is inflammatory or neoplastic and, if a tumor, the cell type involved and whether the tumor is benign or malignant. The pathologist also will assess whether or not the tumor appears to have been completely removed. Wide surgical excision of mast cell tumors is important because some tumor cells can extend into the surrounding tissues without being apparent to the naked eye. The pathologist also will grade the tumor.

    Mast cell tumors are graded as follows:

    Grade I (well-differentiated)

    Grade II (moderately-differentiated)

    Grade III (poorly-differentiated)

  • Grading is very important in proper diagnosis and management of mast cell tumors because these tumors have variable biologic behavior. For example, your veterinarian may recommend cautious surveillance for a dog with a Grade I mast cell tumor that appears to have been completely removed but may recommend referral to a veterinary oncologist for a dog with a Grade III mast cell tumor.
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