Mast Cell Tumors (Mastocytoma) in Dogs

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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

    Diagnosis

    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

    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.

  • 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.

  • 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 pet.

  • 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.

  • Depending on their grade of differentiation, mast cell tumors often can be effectively treated. Treatment for mast cell tumors may include one or more of the following:

  • Surgery. In this effective method of treatment for many mast cell tumors, wide surgical excision removes one to two inches of normal-appearing tissue in all three directions around Grade I or II mast cell tumors. This may be all that is necessary for effective treatment.

    In dogs with especially large tumors or tumors located in regions of the body that are difficult to manage surgically, referral to a specialist in veterinary surgery may be advisable.

    The surgeon also should remove any enlarged lymph nodes in the area of the tumor because the tumor may have spread to these local lymph nodes. If any of the removed lymph nodes contains tumor cells, follow-up chemotherapy will be needed.

    Dogs with incompletely excised tumors should be referred to a veterinary oncologist for consultation on additional treatment options. Some Grade II and all Grade III tumors behave in a malignant fashion and may require chemotherapy to manage the disease.

    For dogs with Grade I mast cell tumors, complete surgical excision carries an excellent prognosis with more than 90 percent of affected dogs alive without disease after 4 years.

    For dogs with Grade II mast cell tumors, there is a wide spectrum of biological behavior, but approximately 50 percent of affected dogs survive for 4 years without tumor recurrence.

    For dogs with Grade III mast cell tumors, the prognosis is poor with surgical excision alone, and less than 10 percent of affected animals survive for 4 years. It often is impossible to completely excise Grade III tumors.

  • Radiation therapy. A beam of radiation is directed at the tumor or the region of the body from which the tumor has been removed surgically. Your pet will likely be referred to a specialty clinic or university because this specialized therapy is available only at selected locations. Radiation therapy may be recommended to shrink large tumors that initially cannot be treated by surgical excision alone. Surgical excision of such tumors may be feasible after they have shrunken in size in response to radiation therapy.

    Radiation therapy also may be recommended after surgery if the removal of the tumor was thought to have been incomplete based on the pathology report that tumor cells were observed in specimen margins. Radiation therapy can be a very effective treatment for incompletely excised Grade I and II tumors with control rates approaching 90 to 95 percent at five years. Radiation therapy may also be used in conjunction with surgery and chemotherapy to treat animals with Grade III mast cell tumors. In this situation, the life span of affected animals may be prolonged and their quality of life improved.

  • Chemotherapy may be recommended for animals with mast cell tumors that already have metastasized or tumors that have a high potential to metastasize (Grade III mast cell tumors). Your veterinarian likely will refer you to a specialist in veterinary oncology if your pet requires chemotherapy. Many different chemotherapy drugs have been used to treat pets with mast cell tumors including vinblastine, lomustine (CCNU), prednisone and cyclophosphamide.

    Drugs used in chemotherapy have potentially serious toxic effects and should only be administered by veterinarians experienced in their use.

    The success of chemotherapy in treating pets with malignant mast cell tumors has been mixed and overall chemotherapy should be viewed as palliative, which means it will improve your pet's quality of life and prolonging survival.

    Mast cell tumors contain several chemical mediators of inflammation that can cause adverse reactions in your pet either in association with the tumor itself or when the tumor cells die during chemotherapy and release these inflammatory mediators. Potential adverse effects include stomach ulceration, generalized itchiness and, in unusual instances, massive release of inflammatory mediators with a potentially fatal allergic reaction called anaphylaxis. Additional drugs to counter the effects of the inflammatory mediators released by mast cell tumors may be prescribed for your pet. These drugs include anti-histamines such as diphenhydramine, H2-blocking drugs such as cimetidine, ranitidine or famotidine and ulcer-coating agents such as sucralfate.

    Optimal treatment for your pet requires a combination of home and professional veterinary care. Follow-up can be crucial. Administer as directed any prescribed medications and call your veterinarian if you have difficulty administering them.

    Restrict your pet's activity during the time he is recovering from surgery, usually 10 to 14 days, to allow for proper healing of the surgery site.

    Your pet may be placed on a 7- to 14-day course of antibiotics to prevent infection from developing at the site of tumor removal.

    Your veterinarian may prescribe a short course of anti-inflammatory analgesic medications. In cases in which radical surgery was necessary, your veterinarian may prescribe narcotic analgesic medications. These medications may be given by mouth or in the form of a patch applied to the shaved skin. The patch is placed on your pet's shaved skin and the narcotic is absorbed slowly over several days, delivering constant pain relief.

    Your dog usually will need to have sutures removed from the skin 14 to 21 days after surgery after healing has occurred.

    The veterinary pathologist's biopsy report will help your veterinarian (often in consultation with a veterinary oncologist) decide if your dog needs additional treatment such as chemotherapy or radiation therapy.

    Even if no follow-up treatment is recommended, your pet should be evaluated on a regular basis for recurrence or spread of the mast cell tumor. Follow-up examinations are recommended every 2 to 3 months for the first year and then every 6 months thereafter for dogs with mast cell tumor. Your veterinarian will take a complete history and perform a thorough physical examination during re-evaluations, and may also perform buffy coat smears to check for circulating mast cells.

    You should examine your pet routinely for signs of recurrence at the site of tumor removal or for new masses. Unfortunately, dogs that develop one mast cell tumor may have a tendency to develop others during their lifetime. If you detect any skin masses, contact your veterinarian for a re-evaluation visit.

    Signs to watch for that may indicate spread of the mast cell tumor may include decreased activity, decreased appetite, vomiting or diarrhea. If you note any of these signs, contact your veterinarian to schedule a re-evaluation visit.

    If your pet receives either radiation therapy or chemotherapy, your veterinary oncologist will instruct you on proper follow-up.

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