A mast cell originates from the bone marrow and is normally found throughout the connective tissue of the body. It is a normal component of the immune system and as it releases histamine it is associated with allergic reactions. Mast cells also respond to tissue trauma. Mast cell granules contain histamine, heparin, platelet-activating factor, and other substances. Disseminated mastocytosis is rarely seen in young dogs and cats, while mast cell tumors are a common malignant tumor of the skin in older dogs and cats. Up to 20 to 25 percent of skin tumors in dogs are mast cell tumors, with a similar number in cats.
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. The presence of these signs usually indicates mastocytosis, which is the spread of mast cells throughout the body. 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), or disseminated intravascular coagulation. When metastasis does occur, it is usually to the liver, spleen, lymph nodes and bone marrow.
A needle aspiration biopsy of the tumor will typically show a large number of mast cells. This is sufficient to make the diagnosis of a mast cell tumor, although poorly differentiated mast cells may have few granules and thus difficult to identify. The granules of the mast cell stain blue to dark purple with a Romanowsky stain, and the cells are medium sized. However, a surgical biopsy is required to find the grade of the tumor. 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.
However, there is a significant amount of discordance between veterinary pathologists in assigning grades to mast cell tumors due to imprecise criteria.
The disease is also staged.
Mast cell tumors that are grade I or II that can be completely removed have a good prognosis. One study showed that about 23 percent of incompletely removed grade II tumors recurred locally. Any mast cell tumor found in the gastrointestinal tract, paw, or on the muzzle has a guarded prognosis. Previous beliefs that tumors in the groin or perineum carried a worse prognosis have been discounted. Tumors that have spread to the lymph nodes or other parts of the body have a poor prognosis. Any dog showing symptoms of mastocytosis or with a grade III tumor has a poor prognosis. Boxers have a better than average prognosis because of the relatively benign behavior of their mast cell tumors. Multiple tumors that are treated similarly to solitary tumors do not seem to have a worse prognosis.
Mast cell tumors do not necessarily follow the histological prognosis. Further prognostic information can be provided by AgNOR stain of histological or cytological specimen. Even then there is a risk of unpredictable behaviour.
Mast cell tumors of the skin are usually located on the head or trunk. Gastrointestinal and splenic involvement is more common in cats than in dogs; 50 percent of cases in cats primarily involved the spleen or intestines. Gastrointestinal mast cell tumors are most commonly found in the muscularis layer of the small intestine but can also be found in the large intestine. It is the third most common intestinal tumor in cats, after lymphoma and adenocarcinoma.
Diagnosis and treatment are similar to the dog. Cases involving difficult to remove or multiple tumors have responded well to strontium-90 radiotherapy as an alternative to surgery. The prognosis for solitary skin tumors is good, but guarded for tumors in other organs. Histological grading of tumors has little bearing on prognosis.