It is found in humans and many animal species; it also can refer to an accumulation or nodule of mast cells that resembles a tumor.

[4] Mast cell tumors are known among veterinary oncologists as 'the great pretenders' because their appearance can be varied, from a wart-like nodule to a soft subcutaneous lump (similar on palpation to a benign lipoma) to an ulcerated skin mass.

[6] Manipulation of the tumor may result in redness and swelling from release of mast cell granules, also known as Darier's sign, and prolonged local hemorrhage.

In rare cases, a highly malignant tumor is present, and signs may include loss of appetite, vomiting, diarrhea, and anemia.

Release of a large amount of histamine at one time can result in ulceration of the stomach and duodenum (present in up to 25 percent of cases)[6] or disseminated intravascular coagulation.

A needle aspiration biopsy of the tumor will typically show a large number of mast cells.

The grade depends on how well the mast cells are differentiated, mitotic activity, location within the skin, invasiveness, and the presence of inflammation or necrosis.

[8] However, there is a significant amount of discordance between veterinary pathologists in assigning grades to mast cell tumors due to imprecise criteria.

[5] Toceranib and masitinib, examples of receptor tyrosine kinase inhibitors, are used in the treatment[11][12] of canine mast cell tumors.

One study showed about 23 percent of incompletely removed grade II tumors recurred locally.

[16] Any mast cell tumor found in the gastrointestinal tract, paw, or on the muzzle has a guarded prognosis.

Dogs of the Boxer breed have a better than average prognosis because of the relatively benign behavior of their mast cell tumors.

Cases involving difficult to remove or multiple tumors have responded well to strontium-90 radiotherapy as an alternative to surgery.