Mast Cell Tumors (Mastocytoma) in Cats

Overview of Feline Mast Cell Tumors

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.

Mast cell tumors rarely affect the skin of cats, although it is the most common site for dogs. Malignant mast cell tumors in cats usually involve the intestine or spleen, and the spleen can become dramatically enlarged. Malignant mast cell tumors can spread to the lymph nodes, spleen, liver and bone marrow.

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.

Among cats, the Siamese breed may be more commonly affected. Males and females are affected equally.

Mast cell tumors in the skin of cats usually are benign and can be managed effectively by simple surgical excision. Some mast cell tumors in young Siamese cats may even spontaneously regress in time without any treatment.

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 of Mast Cell Tumors in Cats

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 cat’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 of Mast Cell Tumors in Cats

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 cat 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 cats 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.

In-depth Information on Mast Cell Tumors in Cats

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 play 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 rarely affect the skin of cats as they do in dogs. Malignant mast cell tumors in cats usually involve the intestine or spleen. The spleen of a cat with a splenic mast cell tumor can become dramatically enlarged.

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 unproved. Among cats, the Siamese breed may be more commonly affected. 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.

Mast cell tumors in the skin of cats usually are benign and can be managed effectively by simple surgical excision. Some mast cell tumors in young Siamese cats may even spontaneously regress in time without any treatment.

Diagnosis In-depth

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 cat with a Grade I mast cell tumor that appears to have been completely removed but may recommend referral to a veterinary oncologist for a cat with a Grade III mast cell tumor.

Treatment In-depth

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.
• 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.

Follow-up Care for Cats with Mast Cell Tumors

Optimal treatment for your cat 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 cat’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 cat 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 cat’s shaved skin and the narcotic is absorbed slowly over several days, delivering constant pain relief.

Your cat 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 cat needs additional treatment such as chemotherapy or radiation therapy.

Even if no follow-up treatment is recommended, your cat 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 cats 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, cats 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 cat receives either radiation therapy or chemotherapy, your veterinary oncologist will instruct you on proper follow-up.