Mast Cell Tumors in Dogs

By Malcolm Weir, DVM, MSc, MPH; Krista Williams, BSc, DVM, CCRP; Christopher Pinard, DVM; Robin Downing, DVM, DAAPM, DACVSMR, CVPPDVM

What is a mast cell?

A mast cell is a type of white blood cell that is found in many tissues of the body. Mast cells are allergy cells and play a role in the allergic response. When exposed to allergens, mast cells release chemicals and compounds (called degranulation). One of these compounds is histamine. Histamine is commonly known for causing itchiness, sneezing, and runny eyes and nose – the common symptoms of allergies.

When histamine and the other compounds are released in excessive amounts (with mass degranulation), it can cause full-body effects, including anaphylaxis, a serious, life-threatening, allergic reaction. Other complications include delayed wound healing, bleeding disorders, and gastrointestinal (GI) ulceration.

What is a mast cell tumor?

A mast cell tumor (MCT) is a type of malignant (cancerous) tumor consisting of mast cells. Mast cell tumors typically form nodules or masses in the skin, but they can also affect other areas of the body, including the spleen, liver, intestine, and bone marrow. MCTs are the most common skin tumor in dogs (7%–21%). Most dogs with MCT (approximately 85%) only develop one tumor.

What causes this cancer?

Why a particular dog develops MCTs or any cancer is not straightforward. Very few cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary. Several genetic mutations are known to be involved in the development of MCTs. One well-known mutation affects a protein, called KIT, that is involved in the replication and division of cells.

While any breed of dog can get MCT, certain breeds are more susceptible. MCTs are particularly common in boxers, English bulldogs, Boston terriers, pugs, golden retrievers, cocker spaniels, schnauzers, Staffordshire terriers, beagles, Rhodesian ridgebacks, Weimaraners, Chinese shar peis, and Labrador retrievers. Affected dogs are usually older (average of 8–9 years); males and females are equally affected.

What are the clinical signs of a mast cell tumor?

Mast cell tumors of the skin can occur anywhere on the body and vary in appearance. They can be a raised lump or bump on or just under the skin, and may be red, ulcerated, or swollen. While some may be present for many months without growing much, others can appear suddenly and grow very quickly. Sometimes they can suddenly grow quickly after months of no change. They may appear to fluctuate in size, getting larger or smaller, even daily. These size changes can occur spontaneously or when the tumor is agitated, which causes degranulation and subsequent swelling of the surrounding tissue due to the histamine release.

"While some may be present for many months without growing much, others can appear suddenly and grow very quickly."

When mast cell degranulation occurs, some chemicals and compounds can go into the bloodstream and cause problems elsewhere. Ulcers may form in the stomach or intestines, causing vomiting, loss of appetite, lethargy, and melena (black, tarry stools that are associated with bleeding). Less commonly, these chemicals and compounds can cause anaphylaxis, a serious, life-threatening allergic reaction.

Although very uncommon, MCTs of the skin can spread to the internal organs, causing enlarged lymph nodes, spleen, and liver, sometimes with peritoneal effusion (fluid build-up) in the abdomen, causing the belly to appear rounded or swollen.

How is this cancer diagnosed?

This cancer is typically diagnosed using fine-needle aspiration (FNA). FNA involves taking a small needle with a syringe and suctioning a sample of cells directly from the tumor and placing them on a microscope slide. A veterinary pathologist then examines the slide under a microscope. A tissue biopsy (surgically removed sample) can indicate how aggressive the tumor is, allowing your veterinarian to determine the best course of action.

MCTs have been classically called “the great pretenders” because they may resemble an insect bite, wart, allergic reaction, or other less serious skin tumors. Therefore, any abnormalities of the skin that you notice should be evaluated by your veterinarian.

Once a diagnosis of MCT has been made, your veterinarian or veterinary oncologist (cancer specialist) may recommend a prognostic panel on a tissue sample. This panel provides information on the genetic makeup and abnormalities of the tumor and provides valuable information that your veterinarian will use to determine the prognosis (the likely course of the disease) for your dog.

How does this cancer typically progress?

This tumor's behavior is complex and depends on many factors. Typically, when a biopsy sample is examined under a microscope, the pathologist can assess how aggressive the cancer is based on several criteria. The whole tumor is graded from I-III, with Grade I as much less aggressive than Grade III. Higher-grade tumors have a higher tendency to metastasize (spread to other parts of the body).

Another classification system is also used to classify MCTs as either high-grade or low-grade. The average survival time with high-grade tumors is less than four months, and with low-grade tumors it is more than two years.

Typically, the prognosis is less favorable if:

  • the patient is one of the susceptible breeds,
  • the MCT is located at a junction where the skin meets mucous membranes (e.g., the gums), or
  • the number of cells actively replicating is high, when viewed under the microscope.

How are mast cell tumors treated?

In lower-grade tumors with no evidence of spread, surgery is likely the best option. Surgery for lower-grade tumors provides the best long-term control, with chemotherapy rarely required. However, in higher-grade tumors, even without evidence of spread, a combination of surgery and chemotherapy is often recommended. Radiation therapy is another option if the mass is not in a suitable location for surgical removal or if the surgical removal is incomplete (with cancerous cells left behind).

"A veterinary oncologist is the best resource for determining what is best for your dog."

It is now known there is an underlying genetic basis for MCT, so drugs such as toceranib phosphate (Palladia®) are being designed to target the proteins associated with the development of cancer. Other drugs, such as tigilanol tiglate (Stelfonta®) are designed to cut off the tumor’s blood supply. In patients with non-surgical MCT, or recurrent MCT that has failed to respond to other chemotherapies, targeted therapy becomes a much more appealing option. A veterinary oncologist is the best resource for determining what is best for your dog.

Is there anything else I should know?

You should avoid palpating (feeling) or manipulating the tumor(s), given how reactive MCT is, with degranulation easily triggered with pressure. Do not allow your dog to chew, lick, or scratch it, as this may also trigger degranulation. Degranulation may lead to further itchiness, swelling, discomfort, or even bleeding. Your veterinarian may recommend using an Elizabethan collar (E-collar or cone). Supportive treatments such as diphenhydramine (Benadryl®) and omeprazole (Gastrogard®, Prilosec®) or famotidine (Pepcid®) are recommended for dogs with visible tumors to reduce the side effects of degranulation.

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