Mast Cell Tumor
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Mast Cell Tumor

The following information is simply informational. Its intent is not to replace the advice of a veterinarian nor to assist you in making a diagnosis of your pet. Please consult with your own veterinary physician for confirmation of any diagnosis. Your pet’s life may depend on it.


These tumors (also called mastocytomas, mast cell sarcomas) are the most frequently recognized malignant or potentially malignant neoplasms of dogs. In addition, leukemic and visceral forms can occur. A viral etiology has been speculated but remains controversial. These tumors may occur in dogs of any age (average 8-10 yr). They may occur anywhere on the body surface as well as in internal organs, but the limbs (especially the posterior upper thigh), ventral abdomen, and thorax are the most common sites; ~10% are multicentric. Many breeds appear to be predisposed, especially Boxers and Pugs (in which tumors are often multiple), Rhodesian Ridgebacks, and Boston Terriers. The tumors vary markedly in size, and clinical appearance alone cannot establish a diagnosis.


Most commonly, they appear as raised, nodular masses that on palpation may be soft to solid. Although they often seem encapsulated, mast cell tumors in dogs are seldom discrete. Rather, they consist of a highly cellular center surrounded peripherally by a “halo” of smaller numbers of mast cells that palpate as normal skin. Dogs can also develop clinical signs associated with the release of vasoactive products from the malignant mast cells. Most common is gastroduodenal ulceration that may be present in up to 25% of cases.

peanut back1.bmp

This is Peanut, a Boston Terrier presenting with what his vets have called, “the worse case” of mast cell tumors they have ever seen.


This is Cucumber, a beagle mix, with Mast Cell in it’s advanced stages. Hers began as a small lump on the left side of her muzzle. As you can see in the photos above, the cancer has caused deformity of her face, listing her nose to the right and pushing her left eye from proper seating in it’s socket. The open area by her mouth was caused by scratching most likely due to the discomfort it caused her.


The behavior of mast cell tumors is variable in that some are rapidly fatal and others are benign. One in eleven cases will appear as multiple nodules involving all the skin. I like to refer to mast cell tumors as cancer and “tricksters” because they can’t be trusted to behave according to their classification. Most pathologists will report them as Grade II, which means they don’t know how they’ll behave. The Grade III cases are almost always fatal. Some will appear rapidly on the face feet or axilla and resemble insect bites.

One can distinguish mast cell tumors from benign fatty tumors with cytology, the examination of cells from a fine-needle aspirate. It is excellent practice to perform cytology before surgery. I like to use New Methylene Blue stain on all my cytology specimens. The dark blue storage granules of mast cells are easy to see under microscopic examination of the stained aspirate. Early diagnosis and aggressive treatment are most effective against this common cancer.

Cytologic evaluation of Wright’s-stained, fine-needle aspirates or impression smears can be used to establish the diagnosis of mast cell tumors in dogs. However, cytology is not a substitute for histopathology—only the latter has been correlated with prognosis. Two systems of histopathologic grading have been defined, and to avoid confusion, it is essential to know which of the two systems is being used.

Although there is believed to be a benign variant of canine mast cell tumor, there is no clinical or microscopical means of identifying it. In addition, small mast cell tumors may remain quiescent for long periods before becoming aggressive. Thus, all should be treated as at least potential malignancies.

Treatment depends on the clinical stage of the disease. For Stage I tumors (a solitary tumor confined to the dermis without nodal involvement), the preferred treatment is complete excision with a wide margin; at least 3 cm of healthy tissue surrounding all palpable borders should be removed in an attempt to excise both the nodule and its surrounding “halo” of neoplastic cells. If histologic evaluation suggests that the tumor extends beyond the surgical margins, reexcision should be attempted. Alternatively, because mast cells are sensitive to radiation, radiation therapy may be curative if the remaining tumor is small or can only be seen microscopically. Combined radiation and hyperthermia may be more effective than radiation alone.At present, there is no agreed upon mode of therapy for Stage II-IV mast cell tumors. For Stage II tumors (a solitary tumor with regional lymph node involvement), options include excision of the mass and the affected regional node (if feasible), prednisolone, and radiotherapy, used either singly or in combination. Treatment of Stage III (multiple dermal tumors with or without lymph node involvement) or Stage IV (any tumor with distant metastasis or recurrence with metastasis) tumors is generally palliative. One recommended therapy is prednisolone (2 mg/kg body wt, PO, for the first 5 days, followed by a maintenance dose of 0.5 mg/kg, daily) or intralesional injections of triamcinolone (1 mg/cm diameter of tumor, every 2 wk).Treatment with H-receptor antagonists for the peripheral and gastric effects of histamine, respectively may be indicated for animals with systemic disease or clinical signs referable to histamine release. Chemotherapy with vinca alkaloids (vincristine, vinblastine), L-asparaginase, and cyclophosphamide has also been used with some effectiveness.Since intraoperative radiation therapy is the most aggressive approach being used on sarcomas in the U.S.A. today, we treat dirty tumor beds and recurrent mast cell cancer in this fashion. Our data suggests that tumor bed implants with steroids and delivering 1,000 centiGray of radiation during surgery into the tumor bed increases long term remissions and survival in animals with sarcomas of any histologic type and especially mast cell.We also deliver intratumor injections on a weekly basis and evaluate the reduction in size of non operated mast cell tumors. We also use cryotherapy to freeze small mast cell tumors in patients who have multiple small nodule disease.These techniques are attractive to clients who have old pets for which they decline anesthesia and surgery.




Mast Cell Tumor



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Last modified: 06/22/10