Mast Cell Tumors in Dogs

Mast cell tumors are one of the most commonly diagnosed tumors of the skin in dogs. Mast cells are immune cells dispersed throughout the skin. The cells 'react' when an insult occurs (e.g. insect bite) by releasing local substances which cause reddening and swelling, part of the inflammatory reaction that combats the insult. Any skin lump on your pet should be examined by your veterinarian as mast cell tumors can have multiple different appearances in and under the skin. A needle sample taken of a lump is diagnosed as a mast cell tumor by using stains and the microscope.

Mast cell tumors are given a grade: I, II, or III. Grade I tumors are quiet, minimally aggressive tumors which rarely spread or recur (20% rate). A wide removal of the tumor with surgery will usually provide a cure. Chemotherapy is not recommended and radiation therapy is used only if the tumor is located in an area where a surgery cannot successfully remove the tumor completely.

Grade III tumors are aggressive with often large masses which have spread internally (lymph nodes, spleen, blood, bone marrow). Grade III tumors have a high rate of recurrence, incomplete removal, and spread (80% rate). Chemotherapy drugs are successful in many dogs diagnosed with aggressive mast cell tumors. Prednisone, Vinblastine, and Lomustine are used in combination to treat aggressive mast cell disease. A nine week protocol with 'rest' periods is recommended. The protocol includes pills and injections. Over 60% of dogs respond to treatment with control of the mast cell tumors for 1-1.5 years. Side effects are rare and short lived when noted.

Dogs diagnosed with Grade II tumors have a variable and unpredictable prognosis, somewhat between dogs with grades I and III tumors. Surgery, radiation, or chemotherapy may be recommended alone or in combination to provide the longest control and highest quality of life for your pet.

Two common medical problems can be seen in dogs that are diagnosed with mast cell tumors. The first is gastric ulcers. Antiulcer therapy is recommended to protect your dog's stomach. A second problem is flushing-reddening due to histamine release from the mast cells. This histamine release may be why owners commonly report that the mast cell tumors seem to change in size and appearance quickly from one day to the next. Antihistamine therapy is also recommended.