Mast Cell Tumors in Cats

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, a process 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. But when histamine (and the other compounds) is released in excessive amounts (with mass degranulation), it 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. MCTs can form nodules or masses in the skin (and other organs), and cause enlargement of the spleen and intestine. They are the most common splenic tumor, second most common skin tumor, and third most common intestinal tumor in cats.

What causes this cancer?

Why a particular cat may develop MCT 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. 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.

"Why a particular cat may develop MCT or any cancer is not straightforward."

What are the clinical signs of a mast cell tumor?

MCTs affecting the skin are seen as firm plaques (hard, flattened areas) or nodules in the skin. The head and neck regions are commonly affected, 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, 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?

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.

"A tissue biopsy can indicate how aggressive the tumor is..."

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 (what a more normal mast cell looks like) or poorly-differentiated (what a very abnormal mast cell looks like) – the disease progression and prognosis may vary. The well-differentiated tumors tend to behave less aggressively. Staging (searching for potential spread to other locations in the body) should also be pursued. This may include blood work, 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.

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 an effusion (fluid build-up in the abdomen).

How are mast cell tumors treated?

Surgical removal of the mass(es) (cutaneous, splenic, or intestinal) 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. 

"Surgical removal of the mass(es) is the treatment of choice whenever possible."

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?

Prevent your cat from rubbing, scratching, licking, or biting the skin tumor(s) to 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.