Cancer in Cats: Mast Cell Tumors

Cancer in Cats: Mast Cell Tumors

The series on cancer in cats for Pet Cancer Awareness Month continues with an in-depth look at mast cell tumors, also known as mastocytomas or MCTs. These fairly common connective tissue tumors represent about 20% of all feline skin cancers (the second most common skin cancer in cats) and 33% of all feline gastrointestinal tumors. However, they are less common in cats than they are in dogs.

What Are Mast Cell Tumors?

In a cat's normal immune system, mast cells are created by the bone marrow and migrate to the body's peripheral tissues to mature. They are found throughout the body in connective tissue as a part of the immune system, but mainly in the skin, the linings of the respiratory and digestive tracts, and the nose and mouth. Mast cells don't circulate in the blood, like the lymphocytes we discussed in a previous post, but remain in these connective tissues. They are also specifically designed to combat parasites.

Mast cells help regulate the nerves in the skin, blood circulation, the body's fibrous tissue, and immune cells. They are closely related to allergy, as they release histamine granules to combat allergens introduced to the body, which the mast cells react to just like they would to parasites. This helps the body fight off the irritating allergen. But histamine is not the only substance they release in response to parasites they encounter in the body.

When tissue is injured, the mast cells respond and assist with keeping blood flowing to cleanse a wound by activating the blood platelets to function properly. They release heparin, an anti-clotting agent that may also have additional unknown functions. They also release other granules that tell the phagocytes to "eat" foreign cells or dying cells. These substances they release cause inflammation (swelling) at the site of the wound. Within hair follicles, mast cells regulate their activity.

An MCT is an abnormal mass of unstable mast cells that form a small nodule. As the cells destabilize, they begin releasing large amounts of granules into the cat's system.

When released in normal quantities, these substances help the body's immune system to function properly. But when too much of them is released, they become irritating to the body's tissues and have unintended effects. They can cause clotting problems with the blood (90% of cats with a tumor on the spleen experience this) and ruin the collagen that supports the skin structure around the tumor.

If untreated, the tumor may clear on its own, or it may start spreading. It first moves to the cat's lymph nodes, and from there to the liver, spleen, and bone marrow. About half the time these tumors in cats will involve the spleen or intestines. They are the third most common type of intestinal tumor in cats.

What causes the cells to become abnormal and gather into a tumor is unknown. It's also unknown what causes some tumors to be benign and others malignant. Because mast cells vary with each individual's genetic makeup, it has been difficult to identify any particular factor or factors that cause the cells to become cancerous or to spread.

What Symptoms Would My Cat Have?

Because of the many different functions of mast cells, these tumors may cause a variety of symptoms in your cat that can be somewhat difficult to identify at home. Ulcers in the digestive tract and itchy lesions on the skin are two of the most common. The tumor may appear as a little hard, white, flattened area or nodule. Sometimes they are pink. Since these tumors have a lot of histamines in them, they tend to itch and get inflamed. Some describe them as looking like the eraser on a wooden pencil.

The growth may be on top of the skin or a lump just underneath it. These tumors under the skin are unique to cats and sometimes go away spontaneously; they're the ones to which Siamese and Sphynx cats are more susceptible. The tumor may appear to get bigger, and then smaller. It may be hairless...or not. At first, it may resemble a flea or mosquito bite, a wart, or some type of allergic reaction.

You might notice such a growth that appears to be fairly stable, but then after several months it starts to grow rapidly. It may redden and have an accumulation of fluid in it. There may be multiple MCTs, or only the one.

Typical locations for these growths are on the cat's head and neck or the trunk of the body. Common places to look for them are the base of the ear, cheek area, near the eyes, or the top of the head. Less commonly, they are found on the cat's legs or paws. Internally, they could be anywhere in the digestive tract, respiratory system, or spleen.

If you manipulate the little growth, it may get red and swollen. This is from the histamine being released by the mast cells in the tumor. Try not to do this much before taking the cat to the vet, as it can increase the chance of it spreading.

The lymph nodes may swell near the tumor as the cat's immune system tries to fight it. This can also be a symptom that the cancer has spread to the lymph nodes. Once it becomes widespread in the cat's system, the liver and spleen will become enlarged.

If the tumor is internal or if it has spread to the internal organs, your cat may have a loss of appetite and experience vomiting and/or diarrhea. There may be some uncontrollable bleeding, especially if the tumor is in the intestines (90% of tumors located here cause blood clotting difficulties). If the tumor is in the lungs, it may be difficult for the cat to breathe. About 1/3 of cats will also become anemic and lose weight as body fat and muscle are depleted (also known as "wasting" or "cachexia"). Anemia is more common in cats with a tumor on the spleen.

Cats average around 10 years old when these tumors appear, but they have been seen in cats under a year old, and in cats as old as 18 years. The more benign form of the tumors are more likely on cats averaging around 2.5 years. Intestinal MCTs are more common in older cats.

Siamese cats seem more susceptible to mast cell tumors than other breeds of cats. Young ones are especially prone to getting the ones under the skin known as "histiocytic" MCTs. About 50%-90% of the time, these are benign and will go away on their own.

Young Sphynx cats may also develop small lumps of mast cells of this type that resolve on their own. The older the cat when the tumors appear, the less likely they are to disappear spontaneously.

How Would I Know For Sure If My Cat Has An MCT?

Your veterinarian will ask you about the symptoms you have noticed, so make careful notes on these at home when you find them. A physical exam will include checking the lymph nodes for swelling.

Some cells will be needle-biopsied to check for mast cells in the cat's blood (fine-needle aspiration). A surgical biopsy will help identify the grade and stage of the disease. Special precautions will need to be taken for cats that have

Depending on the preliminary examination, cell samples may also be taken from a nearby lymph node, the bone marrow, or from the cat's kidneys or spleen. These will be sent to a specialized lab for analysis.

The vet may also take an X-ray and ultrasound imaging and/or a CT scan. This will help locate the exact placement of the tumor and see how far it may have invaded the adjacent tissues.

How Are Mast Cell Tumors Treated?

It's important to seek treatment by a veterinarian for a mast cell tumor quickly, as they can be malignant and spread to other sites in your cat's body. You will likely be referred to a veterinary oncologist, who specializes in treating cancer in pets. Which treatment protocol is used will depend on the nature of the tumor and the stage of cancer when diagnosed.

Since these tumors typically contain a lot of histamine, an antihistamine may be prescribed for your cat. This will help with the itchiness of the tumor. It also benefits the organs so they are not damaged by so much histamine being released into the system as the tumor is manipulated and removed. This type of drug is often used prior to surgery for this purpose. A great amount of histamine released into the system can cause ulcers in the digestive tract or small blood clots.

Surgery to remove the tumor itself is typically the optimal, and often the only, course of treatment. It's recommended that the surgeon remove a "wide margin" of surrounding tissue to make sure and get all of the cancerous cells. If the tumor has affected the spleen, the spleen will be removed. The nearby lymph nodes may also be removed if the mast cells have begun invading them, but not always.

Radiation is not normally used on this type of cancer unless the tumor cannot be completely removed by surgery. This could be due to its location or the inability to remove adjacent tissue. Chemotherapy is often used in conjunction with spleen removal for tumors that have affected the spleen when mast cells are detected in the cat's blood. H2 or calcium channel blocker drugs may be given to protect the stomach from histamine.

After surgery, your cat will need to have blood work done at your vet from time to time, especially when chemo is used for treatment. You'll need to protect your cat from infectious diseases during treatment and recovery, as the immune system will be compromised.

A healthy diet with good nutrients and supplements to boost immunity is also recommended. This will strengthen your cat's body and speed recovery. Eliminating carbohydrates, which encourage inflammation, is recommended. Your vet can advise you on the types of supplements that would most benefit your cat. Omega-3 fatty acids are one type of supplement that seems to help.

Some vets also recommend that cats who have had a mast cell tumor never again be vaccinated. However, depending on your local laws regarding rabies vaccinations, this may not be possible.

What is My Cat's Prognosis If Diagnosed With a Mast Cell Tumor?

This depends on the type of tumor, as well as its location. Obviously, catching it sooner is always better. You can detect some of these tumors within days of their appearance. Many of them are benign.

Once examined under a microscope, mast cell tumors get a "grade" from 1 to 3 that is influenced by several factors, including the tumor's location, how inflamed it is, and its appearance compared to a normal (non-cancerous) cell, or "differentiation". However, this system is more useful for analyzing tumors in dogs than in cats. And because there are no clearly defined standards, grading varies greatly from one pathologist to the next.

Within cats, mast cell tumors on the skin (cutaneous MCTs) are generally classified as "compact" or "diffuse". The former type is generally more benign, while the latter is more likely to spread. Internally (visceral MCTs), there are three forms of tumors: "smooth", "diffuse" and "nodular". 18% of cats who have a cutaneous MCT also have one on the spleen.

The cancer is also given a "stage," depending on its progress at the time it is diagnosed and its likelihood of spreading.

A Stage 1 tumor will likely be a single tumor that has not spread any further. This is the typical type of mast cell tumor that will be found in cats.

Stage 2 would be a single tumor that has spread into nearby lymph nodes.

Stage 3 involves multiple tumors, or by one single large tumor that has invaded adjacent tissue under the skin. There may or may not be involvement of the lymph nodes.

Stage 4 involves a tumor that has spread to the organs, or where the mast cells are present in the cat's blood.

If the tumor can be completely removed, your cat has a good chance of surviving. Some tumors will return (about 23% of the time). Some may simply disappear on their own, even if there are some neoplastic (cancerous) cells in the nearby lymph nodes.

If the tumor is on your cat's paw, muzzle, or in the digestive tract, the prognosis is more guarded. But even cats with internal tumors can enjoy long-term survival if treatment is sought before it has spread.

Once the cancer has spread widely, the prognosis is not good. This is more common with tumors in the spleen or intestines, as they are more difficult to identify early. Most of the cats with these types of tumors either die or are euthanized soon after diagnosis.

Cats with poorer outcomes are those that have lost a lot of weight or who are unable to eat. If your cat is eating well at the time of treatment, the prognosis is much more optimistic. Male cats also have a slightly less likely chance of recovering well from mast cell tumors in the spleen.

The key is to get your cat to the vet as soon as you suspect a mast cell tumor. Keep the cat from scratching, rubbing, or licking the area, as this will only irritate the tumor and increase the amount of histamine being released by the mast cells. A specially designed collar to prevent the cat from grooming will be helpful.

Keep any ulcerated growths clean. This also holds true for the surgical incision area once the tumor is removed. If you notice severe swelling or bleeding at the incision site, call your veterinarian.

If the tumor can be removed and the cancer has not spread, once it shows no sign of returning after about six months, the prognosis is good. Check your cat carefully for tumors that may appear elsewhere. Malignant tumors will usually reappear in another site within two or three months after removal of the initial one.

Watch for more upcoming posts on other types of cancer in cats throughout May for Pet Cancer Awareness Month. While it's a lot more fun to look at cute and funny pictures of cats, it's also important that we pay attention to the health of our feline companions.

Sources: "Types of Cancer in Cats", PetWave; "Mast Cell Tumor (Mastocytoma) in Cats", Pet M.D.; "Mast Cell Tumors in Cats", Pet Cancer Center; "Mast Cell Tumors in Cats", VCA Animal Hospitals; "Intestinal Mast Cell Tumors in Cats", "Mast Cell Tumor", and "Splenic Mast Cell Tumor", Veterinary Society of Surgical Oncology; "This is One Lump You Can't Ignore", Healthy Pets with Dr. Karen Becker; "Mast Cell Tumors", Marvista Vet; "Mastocytoma", Wikipedia.