Mast Cell Tumors

One of the more common types of cancer we seen in dogs (about 20% of all skin tumors)(less common in cats) are mast cell tumors. Mast cells are a part of the normal immune system. They are responsible for the release of histamines durine an allergic reaction but they also play a major role in wound healing and defense against pathogens. They can be found in almost all tissues and in very small numbers in the blood stream.

A mast cell tumor (MCT) is the result of uncontrolled growth of neoplastic mast cells. The most common MCT sites in dogs and cats are skin, the spleen can be a common primary location in the cat and a common site of metastasis in the dog. Mast cell tumors can also be found in the GI tract, though this is rare.

Signs of mast cell cancer can include identification of a skin lesion(mass), an allergic type reaction including redness and hives, swelling, shock, hypotension, vomiting, diarrhea, itching, defective coagulation, and delayed wound healing. GI and colonic ulcers are also a common side effect from MCT.

MCT are known as the "big imitators" because they can look like any other skin tumor. They can be hairy or hairless, pink or black, raised or under the skin, etc. so any skin mass should be examined closely. A raised, pink, hairless mass that seems to itch the pet should be aspirated for cytology as soon as possible.

Diagnosis of MCT can be made by fine needle aspiration(FNA) and cytology, but it is only about 70% accurate due to the delicacy of the cells and chance of getting an adequate sample size. It may be necessary to take a full biopsy and send it out for histopathology. Special stains may be required to make the diagnosis.

MCT's have been reported in dogs less than one year of age but are much more common in older animals. Boston terriers, Boxers, Bull terriers, Dachshunds, English bulldogs, Fox terriers, Labrador retrievers, Pit bull terriers, Staffordshire terriers, and Weimaraners are breeds that are at increased risk for developing MCT.

Grading of a MCT is important to help develop a treatment plan and a prognosis. The most commonly used grading system in the dog is the three-grade Patnaik system. This grading system has excellent correlation with 1500-day survival rates (Grade I = 83%, Grade II = 44%, and Grade III = 6%).

Grade I tumors are much less aggressive and can be surgically excised without further treatment. They rarely metastasize.

Grade II tumors provide the greatest difficulty in determining staging for the dog and on deciding an appropriate treatment course. For Grade II tumors, more involved staging (further testing)may be necessary to determine how aggressive the tumor is likely to be.

Grade III tumors are are invariably aggressive and need to be treated aggressively.

The next important step is to stage the MCT. Staging allows one to find out where else the MCT may be.  MCTs typically metastasize to lymph nodes, liver, spleen, and bone marrow.  Common staging procedures include FNA  of local lymph nodes, abdominal ultrasonography, and possibly splenic and liver aspiration cytology. Sometimes bone marrow aspiration may be done. 

Prognosis for a dog or cat with MCT can be difficult. In addition to the tumor Grade, location of tumor can play a significant role in prognosis.   MCTs of the oral cavity and nail bed have a higher metastatic rate to regional lymph nodes. Masses located on the muzzle and ear tend to be more aggressive. Gastrointestinal tract MCTs have a grave prognosis.

Specific treatment for MCT includes surgical resection with wide margins into healthy tissue.  Margins should be at least 2 cm wide and a 1 cm deep.   For unresectable or Grade III masses, additional treatments such as radiation and chemotherapy are necessary.  Mast cells tend to be very sensitive to radiation and can be used for long-term local control (up to a 90% control for several years).  Chemotherapy should be administered in higher grade tumors or when there is indication of metastasis.  It can be given in conjunction with radiation and surgery.

So the take home lesson is that every mass found on your dog is potentially a MCT especially in those breed's prone to it.   The appearance of a new mass should be monitored for changes and for it's affect on the pet.  Any mass that suddenly appears should be aspirated and evaluated.  Chances are that it probably is a benign growth such as a sebaceous adenoma, but better safe than sorry.