Mast Cell Tumors

It is normal for your dog to get a few lumps and bumps on its body as he or she gets older. Most of these lumps are harmless – or benign – but they could also be the sign of something more serious, like a mast cell tumor.

What is a mast cell?

Mast cells are a type of white blood cell that play an important role in the immune system. Most are found in the skin and areas that come in contact with the outside environment. They contain chemicals like histamine, which is released during allergic reactions and certain immune responses. 

What is a mast cell tumor?

A mast cell tumor is a tumor consisting of mast cells. They are common in dogs, accounting for approximately 20% of all skin tumors. In addition to forming in nodules or masses in or on the skin, they can also affect regional lymph nodes, the spleen, liver, intestine, bone marrow, and other areas of the body. For most dogs, the underlying cause promoting the development of the tumor is not known

What are the signs of a mast cell tumor?

These tumors can form on any skin site on the body and with varying appearances. Most develop as solitary lumps or masses in or underneath the skin, but occasionally dogs will have multiple masses. Some are slow growing and cause little irritation, while others can be ulcerated, angry lesions that spread rapidly. Some dogs may have signs of systemic disease – disease affecting other parts of the body – which can be caused by some of the biologically active compounds found within mast cells.

How are mast cell tumors diagnosed?

Veterinary oncologists recommend collecting cells from skin lumps before removal to rule out the growth as a mast cell (or other malignant) tumor. Veterinarians can easily identify mast cells by examination of a fine-needle aspirate of the suspect mass.

To determine the extent of the tumor, your veterinarian may collect cells from regional lymph nodes and bone marrow for microscopic examination, in addition to performing imaging of the thorax and abdomen. A complete blood count, biochemical profile, and urinalysis can help assess overall health and in forming treatment recommendations.

A grading scale is used to describe the aggressiveness of mast cell tumors, with grade I the least aggressive and least likely to spread to other organs (metastasize). Most grade II tumors do not to metastasize, although they can. Grade III tumors are highly aggressive and likely to metastasize.

How are mast cell tumors treated?

While most mast cell tumors are considered locally invasive and can be difficult to completely remove, surgical removal is still the most recommended treatment. Wide areas of surrounding tissue that appear to be normal also need to be removed to increase the likelihood that the tumor is completely removed. For cases in which the tumor was not able to be completely removed, radiation therapy is often recommended, although a more aggressive second surgery is possible for some dogs. Chemotherapy is used as a treatment, but it is usually reserved for dogs with grade III tumors. Some dogs will also be treated with medications to fight the secondary effects of the tumor.

What is the prognosis for dogs with mast cell tumors?

If tumors are caught when small, surgical removal is usually adequate. Mast cell tumors, though, can be unpredictable, and even grade I and II tumors can be difficult to control and aggressive in terms of metastasizing.

The prognosis for completely removed grade I and grade II tumors, however, is excellent. The prognosis for incompletely removed grade I and II tumors treated with radiation therapy after surgery is also excellent, with approximately 90-95% of dogs having no recurrence of tumor within 3 years of radiation therapy. The prognosis for dogs with grade III tumors is considered guarded, as local recurrence is likely.

What should I do if my dog has a lump?

If you are concerned about a growth on your dog, schedule an appointment with your veterinarian or our community practice veterinarians by calling 509-335-0711.

This information is not meant to be a substitute for veterinary care. Always follow the instructions provided by your veterinarian. Washington State University assumes no liability for injury to you or your pet incurred by following these descriptions or procedures.