Mast cells are a type of normal white blood cell and are part of the immune system. Mast cell tumors (MCT) comprise about 20% of all cutaneous (skin) tumors in the cat. These tumors can be benign (not cancerous) or malignant (cancerous and able to spread to other parts of the body). Over 90% of MCTs that occur in the skin of cats are benign while visceral (occurring in internal organs) MCTs may behave more aggressively. This disease is seen mostly in middle-aged cats, and Siamese cats appear to be predisposed to getting this disease.
What causes this type of cancer in dogs and cats?:
- The actual cause of MCT is unknown. A genetic predisposition has been proposed due to the high incidence in Siamese cats. Mutations in certain genes (such as c-kit) have been studied extensively though a specific cause of this mutation has not been isolated.
What are the common signs of this cancer?:
- If your cat suffers from cutaneous (skin) MCT, you may notice small, firm, raised, hairless nodules on the head, neck, trunk, limbs, or other sites on your cat. They can intermittently be itchy or red and self-trauma can cause ulceration. If a mass is manipulated or “bothered”, a local allergic reaction canensue, and you may notice significant redness and wheal formation in the area of the tumor. This is due to the release of histamine and other inflammatory substances. Typically cats with skin MCT are otherwise healthy and show no signs of systemic illness.
- Cats with the visceral, or disseminated, form of MCT (involving an internal organ or multiple organs) may show signs of systemic illness. They can be depressed, anorexic (loss of appetite), show weight loss, and vomiting. This visceral type is much more common in cats in comparison to dogs.
How is it diagnosed?:
- A fine needle aspiration (inserting a small needle into a mass) and cytology can be performed on skin masses to obtain a diagnosis of MCT. This can typically be done without any sedation and only takes several minutes. If the spleen is suspected to contain MCT or an intestinal mass is seen on ultrasound, a fine needle aspiration can also be performed, but may require some sedation.
- The draining lymph nodes closest to the points of disease may be affected by some aggressive forms of this cancer. If a lymph node is enlarged, it may indicate that the disease has progressed and spread. A fine needle aspiration and cytology can be used to determine if a MCT has spread to the lymph node.
- Blood tests help determine the basic health of the body and organs. They can also identify mast cells in the blood (called a buffy coat test), which indicates that the tumor is spreading. Identification of low blood cell counts may indicate issues (such as bone marrow involvement or gastrointestinal ulceration) created by the growth of the mast cells.
- A bone marrow evaluation may be recommended to evaluate for disease in the bone marrow. This procedure does require some sedation but is typically not painful.
How is this cancer treated?:
- Surgery is the treatment of choice for cats with the cutaneous form of MCT (as long as there are not too many MCTs for the surgeon to remove), since most of these tumors are benign and surgical removal is likely to be curative.
- Surgery is typically not recommended for cats with disseminated MCT, unless the spleen is involved. It has been shown that cats with splenic MCT usually benefit from removal of their spleen, even when there is extensive disease such as bone marrow involvement.
- Relatively little is known about the effectiveness of nonsurgical therapy for cutaneous MCTs in the cat. Responses to some chemotherapy medications have been reported and are typically recommended for cats with the malignant (cancerous) form of MCT.
- Novel therapy with small molecular targeting drugs has shown great promise in fighting mast cell disease in cats.
- Drugs which block histamine receptors (such as cyproheptadine and famotidine) can reduce clinical signs of inflammation or stomach ulceration (by decreasing acid production induced by histamine).
- Steroids are also useful for decreasing inflammation associated with these tumors.
What is the prognosis for dogs and cats with this cancer?:
- Many cats with the skin form of MCT can do very well for years with appropriate therapy.
- Cats with the visceral form of MCT (spleen, intestine, liver) typically do worse than cats with the skin form, although the use of combination therapies (surgery, chemotherapy and TKIs) are allowing many cats to live well over a year.
What is on the horizon for this cancer?:
- The combination of small molecule inhibitors (Palladia and Kinavet for example) are being used more frequently in cats.
- The use of combinations of chemotherapy and small molecule inhibitors is an active area of research.