Mast Cell Tumors in Cats

By Debbie Stoewen DVM, MSW, RSW, PhD; Christopher Pinard, DVM

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 (substances that stimulate allergies), mast cells release chemicals and compounds, a process called degranulation. One of these compounds is histamine. Histamine is most commonly known for causing itchiness, sneezing, and runny eyes and nose – the common symptoms of allergies. But when histamine (and the other compounds) are released in excessive amounts (with mass degranulation), they can cause full-body effects, including anaphylaxis, a serious, life-threatening allergic reaction.

What is a mast cell tumor?

A mast cell tumor (MCT) is a type of tumor consisting of mast cells. Mast cell tumors can form nodules or masses in the skin (and other organs), and cause enlargement of the spleen and intestine. Mast cell tumors are the most common splenic tumor (tumor of the spleen), second most common skin tumor, and third most common intestinal tumor in cats.

What causes this cancer?

Why a particular cat may develop this, or any cancer, is generally 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. A genetic mutation in a protein involved in the replication and division of cells (called KIT) has been well-described in the development of MCTs in dogs. In cats, about 67% of MCTs also have this mutation.

What are the signs that my cat may have a mast cell tumor?

Most mast cell tumors are seen as firm plaques (hard, flattened areas) or nodules (small lumps) in the skin. The head and neck regions are the most commonly affected areas, especially the top of the head and either or both ears. There may be itching because the tumors produce substances that cause inflammation.

If your cat has the splenic form of the disease, the most commonly observed signs are weight loss, vomiting, and loss of appetite. This is a result of the compounds released by the cancer that make your cat feel sick.

"If your cat has the splenic form of the disease, the most commonly observed signs are weight loss, vomiting, and loss of appetite."

The intestinal form, depending on how severe the disease is, may cause vomiting, diarrhea, fresh red blood in the stool, or black/tar-colored stool (the discoloring evidence of digested blood). In some cases, your veterinarian may be able to feel a mass within the abdomen during a physical examination.

How is this cancer diagnosed?

Typically, this cancer is relatively easily diagnosed by cytology. Cytology is the examination of cells under a microscope. These cells are retrieved by placing a needle into the mass, suctioning out some cells, and placing them on a microscope slide. This procedure is called a fine needle aspiration (FNA). A veterinary pathologist then examines the slide under a microscope.

In cases that are questionable, surgical excision of a piece of the tumor (a biopsy) or the entire tumor can be performed. Pieces of the tumor will then be examined under the microscope. This is called histopathology. Histopathology gives the pathologist much more information about the type of tumor, how aggressive it is, and the margins (the border between healthy and cancerous tissue). If your veterinarian submits the entire tumor to the pathologist, the likelihood that the cancer has been fully removed can be determined.

How does this cancer typically progress?

There are three distinct forms or syndromes of MCT in the cat:

Cutaneous MCT
Siamese cats appear to be predisposed to this form of MCT. Based on how the cells appear under the microscope – well-differentiated (i.e., what a more normal mast cell looks like) or poorly-differentiated (i.e., what a very abnormal mast cell looks like) – the disease progression and prognosis may vary. The well-differentiated tumors tend to act less aggressively. Staging (searching for potential spread to other locations in the body) should also be pursued. This may include bloodwork, urinalysis, X-rays of the lungs, and possibly an abdominal ultrasound. If any lymph nodes are enlarged or feel abnormal, further sampling may be pursued to determine if spread is present. Some cats with cutaneous MCT will have abnormal mast cells in their spleen, but can have them elsewhere as well. Staging, therefore, is always recommended.

Splenic/visceral (associated with internal organs) MCT
The spleen is a filtering organ that contains red blood cells and white blood cells (including mast cells). Approximately 15% of cats with abnormal or diseased spleens are diagnosed with splenic MCT. This cancer has the potential to spread to other organs as well (e.g., the liver, lymph nodes, bone marrow, lungs, and less commonly, the intestines).

Intestinal MCT
Intestinal MCT typically involves the small intestine, but there are some reports citing MCT of the colon as well. Unfortunately, intestinal MCT commonly spreads to neighboring organs and lymph nodes. When this happens, some cats will develop fluid in their abdomen (called an effusion).

What are the treatments for this type of tumor?

Surgical removal of the mass(es) is the treatment of choice whenever possible. Depending on the findings with histopathology and staging, chemotherapy may be suggested. In some cases, if the mass is not completely removed (meaning some cancerous cells are left behind) or in a location that makes surgery too difficult or risky for your cat, radiation therapy may be suggested.

"Depending on the findings with histopathology and staging, chemotherapy may be suggested."

In cases of intestinal MCT and MCT involving the spleen, surgery is also the treatment of choice.

All three forms of mast cell tumors can release compounds that increase acid production in the stomach, causing stomach upset and heartburn-like symptoms. Your veterinarian may prescribe anti-nausea and antacid medications to help your cat feel better.

Is there anything else I should know?

Preventing your cat from rubbing, scratching, licking, or biting the skin tumor(s) will reduce itching, inflammation, ulceration, infection, and bleeding. Any ulcerated areas need to be kept clean.

After surgery, the surgical site needs to be kept clean and your cat should not be allowed to lick or chew at the site. Your veterinarian may recommend the use of an Elizabethan collar (E-collar or cone). Be sure to report any loss of sutures or significant swelling or bleeding to your veterinarian. If you require additional advice on post-surgical care, please ask your veterinary healthcare team.