Mast cell tumors are one of the most common skin cancers observed in dogs.  Mast cells are normal immune cells found in areas of the body exposed to the environment (skin, lungs, gastrointestinal tract, ect) and contain histamine, heparin and vasoactive amines within granules within the cells. They play a role in allergic and immune responses.  With manipulation of the tumor, the granules (degranulation) are released from the the cells causing reddening of the skin and swelling.  In severe cases, degranulation can cause coagulation abnormalities, hypotension and anaphylaxis.

Diagnosis:

A presumptive diagnosis of a mast cell tumor is easily obtained with a fine-needle aspirate, which involves passing a small needle into the mass, aspirating the cancer cells onto a slide and documenting the characteristic cells.  Although this can diagnose mast cell tumors it provides no prognostic information.  Grade, which is obtained with a surgical biopsy, provides imperative information about the potential behavior of your pet’s mast cell tumor. Low-grade mast cell tumors are considered the least aggressive and high-grade mast cell tumors are often highly aggressive and metastasize (spread of cancer distantly) easily.

If we have a suspicion that your pet may have a high-grade form based on your pet’s history and our physical examination, we will recommend performing tests to determine if the cancer has already spread potentially before surgical biopsy.  These tests will often include x-rays, an abdominal ultrasound, fine-needle aspirates and cytology, bone marrow aspirate and urinalysis.

Treatment and Prognosis:

Assuming that no spread of the mast cell tumor is identified, treatment may involve several steps.  First we must remove the tumor in its entirety.  Then, if the tumor has characteristics of being more aggressive, we may recommend treatments to manage potential metastases.

Recommended treatments for mast cell tumors depend on their grade as well as whether or not they have spread to other locations.  Low-grade mast cell tumors are minimally invasive, unlikely to spread, and can be cured by surgery. High-grade mast cell tumors are locally invasive, more likely to spread, and may recur after excision.  If they spread, they tend to go to local lymph nodes as well as the liver and the spleen.

Local therapy involves surgical removal of the tumor.  Because microscopic cancer cells (nonvisible) are present in the tissue that surrounds the mass, we would ideally remove a margin of normal tissue around the tumor in the hopes that all the cancerous mast cells will be surgically removed.  Low-grade mast cell tumors can be removed with a smaller margin of normal tissue whereas high-grade mast cell tumors need wide surgical margins.  It is important to be appropriately aggressive with the surgery because if even a few cancer cells remain, the mass may regrow in the same spot.  If we take off more normal tissue than necessary, we may put your pet through a larger surgery than indicated, potentially increasing the potential for complications or lengthening healing time.

If based on the surgical biopsy results, your pet has a less aggressive tumor and it was completely removed, we may not recommend any more therapy.  However, if your pet has an aggressive tumor, or if the tumor was not completely removed, additional treatment will be recommended.  Depending on the case, this may include a scar revision surgery (ie, removing more tissue from the site of the surgery), radiation therapy (directing a beam of energy at the site of the surgery to kill more cancer cells at that spot), and/or chemotherapy.

If chemotherapy is indicated, there are numerous drugs that are affective against mast cell tumors including vinblastine, CCNU and palladia, a targeted chemotherapeutic. We administer chemotherapy with the goal of maintaining a very good quality of life and attempt to avoid toxicities; therefore, although we attempt to be aggressive against the cancer, we dose our drugs with these goals in mind.  Chemotherapy is typically very well tolerated in dogs, but potential rare side effects may include decreased appetite, vomiting, diarrhea, lethargy, and susceptibility to infection.  Please see the handout for additional details about chemotherapy.

Until the tumor is removed, your pet should receive Benedryl and Pepcid AC to prevent side effects that are associated with mast cell tumors.  These side effects may include stomach ulcers and allergic-type reactions.

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