Mast cell tumors or mastocytomas arise most commonly in the skin. They develop from a normal component of body tissues called the mast cell, which normally play a role in the process of tissue repair by releasing inflammatory mediators.

Mast cell tumors vary greatly in their biological behavior. Some mast cell tumors remain localized for extended periods of time, but others invade local areas causing much inflammation, and they eventually metastasize or spread to distant sites in the body. Malignant mast cell tumors can spread to the lymph nodes, spleen, liver and bone marrow.

Mast cell tumors rarely affect the skin of cats as they do in dogs. Malignant mast cell tumors in cats usually involve the intestine or spleen. The spleen of a cat with a splenic mast cell tumor can become dramatically enlarged.

Mast cell tumors occur as one of three types:

Well-differentiated

Moderately-differentiated

Poorly-differentiated

This classification refers to how closely the mast cells of the tumor resemble normal mast cells and ultimately to the biological behavior of the tumor or its tendency to remain localized or spread throughout the body.

Well-differentiated

The cells of well-differentiated mast cell tumors closely resemble normal mast cells.

Well-differentiated mast cell tumors tend to remain localized and have benign biological behavior.

Poorly-differentiated

Poorly-differentiated mast cell tumors can be difficult to identify as mast cells without special stains.

Poorly-differentiated mast cell tumors tend to spread through the body and have malignant biological behavior.

Moderately-differentiated

This type is between well and poorly differentiated.

The cause of mast cell tumors is unknown. Mast cell tumors have been transmitted experimentally using tumor extracts suggesting possible viral origin, but this hypothesis remains unproved. Among cats, the Siamese breed may be more commonly affected. Males and females are affected equally.

Mast cell tumors generally respond well to treatment. Well-differentiated mast cell tumors in the skin often can be removed successfully by wide surgical excision. The less common, poorly-differentiated mast cell tumors can cause severe swelling and inflammation locally and tend to spread throughout the body. These aggressive tumors are much more difficult to treat effectively and often result in death. Mast cell tumors that occur in areas around the mouth, anus and genitals tend to be more aggressive and have a worse overall prognosis than those occurring elsewhere in the skin.

Mast cell tumors in the skin of cats usually are benign and can be managed effectively by simple surgical excision. Some mast cell tumors in young Siamese cats may even spontaneously regress in time without any treatment.