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.
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.
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