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Mast Cell Tumors

Mast cell tumors are the most common form of skin tumors in dogs. It is important to have your veterinarian evaluate any lumps that develop on your pet’s skin.

Canine Mast Cell Tumors

It is not known why certain dogs develop mast cell tumors and others do not. There is some speculation that mast cell tumors may develop secondary to skin disease/inflammation, but this is not always the case. Mast cells are normal cells in the body that aid in incidences of inflammation/infection.

There are three grades of mast cell tumors (I, II, and III), and this is very predictive of how the tumor behaves clinically. Often mast cell tumors can be cured with surgery alone. If the tumor cannot be removed entirely with surgery, then we will recommend radiation therapy postoperatively. Radiation therapy would begin approximately 2 weeks postoperatively, as the surgery site needs a chance to heal. Grade I and II mast cell tumors are regularly cured with surgery alone and have a very low chance of spreading to other areas of the body. For grade III mast cell tumors, chemotherapy is always recommended because grade III mast cell tumors have a 50% chance of spreading to other areas of the body (usually the spleen, liver, and/or bone marrow). If there is any evidence that a grade II mast cell tumor has spread (metastasized), chemotherapy is also recommended. The final reason that chemotherapy would be recommended is if the tumor is in a negative prognostic location (ear, muzzle, mouth, mucocutaneous junction, or digits; some studies say the inguinal/prepucal region as well).

Ten percent of dogs that have mast cell tumors will develop additional mast cell tumors in the future, so please monitor your dog frequently for any new masses. If you ever notice a new mass, please contact your veterinarian so that a fine needle aspirate can be performed.


Blood sampling (complete blood count, serum chemistry profile, and a buffy coat analysis) and a urinalysis will typically be performed to ensure that your pet is generally healthy overall. An abdominal ultrasound may be performed to ensure that the tumor has not spread to the liver, spleen, or lymph nodes in the abdomen. Fine needle aspirates of the liver/spleen may also be performed to look for spread of the tumor. A fine needle aspirate will be performed of the lymph node that drains the tumor site. Cytology of this lymph node will tell us if the tumor has spread to the local lymph node and if chemotherapy will be needed. A bone marrow aspirate may also be performed; this is to look for spread of the tumor into the bone marrow. Your pet will be given an injection for pain before the procedure, and they will be under light anesthesia during the procedure. Your pet’s oncologist will ask that you do not feed your pet the morning of the bone marrow aspirate; however, water is allowed until 7am. Eating the morning of the procedure will increase their risk of vomiting and potentially aspirating under anesthesia. Most dogs are not lame from the bone marrow aspirate, but your pet may be sleepy from the anesthesia, so please keep them confined to a small area and avoid steps or walking the evening that the procedure was performed.


Surgery may be recommended in hopes of removing the entire mast cell tumor. The sample will be submitted to a pathologist so that they can evaluate the surgical margins. If the tumor is unable to be removed entirely with surgery, then radiation therapy will be recommended after surgery. Mast cell tumors are very responsive to radiation therapy, and most dogs (75 to 80%) are disease free 2 to 5 years after being treated with a combination of surgery and radiation therapy. Please see additional information on radiation therapy.

Medications commonly used for mast cell tumors:

  • Benadryl—this is an H1 blocker that is given to block the effect of histamine release. Mast cell tumors have histamine in their granules.
  • Ranitidine (Zantac®)—This is an H2 blocker that is used to help block the effect of histamine release, as well as to help with acidity in the stomach.
  • Prednisone—This is a steroid that is used to decrease inflammation associated with the mast cell tumor, and some mast cell tumors will respond to prednisone alone. This medication will cause your pet to urinate more, drink more, pant more, and may increase his/her appetite. Please be sure that he/she has free choice water at all times and goes outside frequently to urinate. Your pet may NOT receive any aspirin or aspirin like products, such as Carprofen (Rimadyl®), Deracoxib (Deramaxx®), Piroxicam (Feldane®), Tepoxalin (Zubrin®), or Meloxicam (Metacam®), while receiving prednisone, for the combination could cause a stomach ulcer.
  • Omeprazole (Prilosec OTC®) – This is a proton pump inhibitor that is helpful in controlling signs of gastrointestinal ulceration.


Feline Mast Cell Tumors

Mast cell tumors in cats are most commonly found as incidental findings on the head and neck. Cats may have only one mass, but 13 to 43% will present with multiple mast cell tumors. The masses are often raised, white or pink in color, and may be haired or nonhaired. They may be mistaken for a “bug bite.” Mast cell tumors in cats are usually benign (do not spread to other areas of the body), but some do spread to local lymph nodes and can spread to other areas of the body (spleen, liver, or bone marrow). Sometimes cats will present for mast cell disease isolated to their spleen. The Siamese cat has been cited in multiple studies as being predisposed to developing mast cell tumors.

The treatment of choice for feline mast cell tumors of the skin is surgery, and usually local recurrence is low. It is very important for you to monitor for any new masses, and bring these to your veterinarian’s attention.

If your cat has a primary mast cell tumor of the spleen, removal of the spleen with surgery may be recommended, and chance of long-term survival is typically high.


Veterinary Professionals