Mast cells are inflammatory cells that originate in the bone marrow of cats and dogs and circulate in many body tissues. They are important in allergic responses, tissue growth, wound healing, and nonallergic skin diseases. Mast cell tumors are more common in dogs than in cats. They are most common in boxers, bull terriers, Boston terriers, Labrador retrievers, shar-peis, golden retrievers, schnauzers, cocker spaniels, and Siamese cats. Although most mast cell tumors in dogs originate in the skin, they can be seen elsewhere, including the lungs, liver, and spleen.

Signs can be nonspecific and depend on the location of the tumor. These tumors are called the "great imitators" because they resemble many other tumor types. In dogs, mast cell tumors commonly appear as red, itchy lumps. Their size may change rapidly, and they may disappear and return. In cats, there are two types of mast cell tumors: cutaneous, which are found mostly on the skin around the head and neck, and visceral, which are found in the spleen, liver, and intestines. The tumor produces inflammatory proteins that can cause inflammation at the tumor site and elsewhere in the body. Nonspecific signs include weight loss, vomiting, diarrhea, and lack of appetite in your pet.

Treatment largely depends on the stage of the tumor(s), which refers to determining the extent and severity of disease. Taking a biopsy of the tumor is useful for staging, as is the number and location of tumors and the level of lymph node involvement. Many other factors are also considered in staging, including results of blood tests, x-rays and ultrasounds of the chest and abdomen, and analysis of bone marrow samples. Surgery to remove the tumor from your pet is always the first and most important treatment. Further treatment, such as chemotherapy or radiation therapy, may be recommended depending on the staging.

Q&A

What are mast cells?

Mast cells are inflammatory cells that originate in the bone marrow and circulate in many body tissues.  They are important in allergic responses, tissue growth, wound healing, parasitism, and nonallergic skin diseases.

What pets are most likely to get mast cell tumors?

Mast cell tumors are more common in dogs, especially boxers, bull terriers, Boston terriers, Labrador retrievers, shar-peis, golden retrievers, schnauzers, cocker spaniels.  They are sometimes seen on cats, especially Siamese cats. 

Where are mast cell tumors found in pets?

Most mast cell tumors in dogs originate in the skin, but can sometimes also be found in the lungs, liver, spleen, or other organs.

How are these tumors diagnosed in pets?

Mast cells have a characteristic appearance that can be identified during microscopic examination of a biopsy sample.  Other tests, including blood tests, medical imaging, and analysis of bone marrow samples, may be needed to gauge the stage and extent of the cancer. 

How are these tumors treated in pets?

Surgery to remove the tumor is always the first and most important treatment.  Further treatment, such as chemotherapy or radiation therapy, may be recommended depending on the severity of the cancer.

Written and reviewed by John A. Bukowski, DVM, MPH, PhD
and Susan E. Aiello, DVM, ELS