Grade I – well differentiated and mature cells with a low potential for metastasis
Grade II – intermediately differentiated cells with potential for local invasion and moderate metastatic behavior
Grade III – undifferentiated, immature cells with a high potential for metastasisHowever, 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 according to the WHO system:

Stage I - a single skin tumor with no spread to lymph nodes
Stage II - a single skin tumor with spread to lymph nodes in the surrounding area
Stage III - multiple skin tumors or a large tumor invading deep to the skin with or without lymph node involvement
Stage IV – a tumor with metastasis to the spleen, liver, or bone marrow, or with the presence of mast cells in the bloodX-rays, ultrasound, or lymph node, bone marrow, or organ biopsies may be necessary to stage the disease. Treatment and prognosis
Removal of the mast cell tumor through surgery is the treatment of choice. Antihistamines, such as diphenhydramine, are given prior to surgery to protect against the effects of histamine released from the tumor. Wide margins (two to three centimeters) are required because of the tendency for the tumor cells to be spread out around the tumor. If complete removal is not possible due to the size or location, additional treatment, such as radiation therapy or chemotherapy, may be necessary. Prednisone is often used to shrink the remaining tumor portion.
H2 blockers, such as cimetidine, protect against stomach damage from histamine. Vinblastine and lomustine are common chemotherapy agents used to treat mast cell tumors.Toceranib and masitinib, examples of receptor tyrosine kinase inhibitors, are used in the treatment of canine mast cell tumors. Both were recently approved by the U.S. Food and Drug Administration (FDA) as dog-specific anticancer drugs.Grade I or II mast cell tumors that can be completely removed have a good prognosis. One study showed 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. Dogs of the Boxer breed 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 behavior. Other animals
Mast cell tumors are an uncommon occurrence in horses. They usually occur as benign, solitary masses on the skin of the head, neck, trunk, and legs.