Mast Cell Tumors in Canines and Felines – What Your Need to Know

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What are Mast Cells?

Mast cells (AKA mastocytes) are white blood cells involved in the process of regulating a variety of bodily functions. White blood cells are immune system cells having responsibility to control inflammation, fight infection, clean up the breakdown products of day-to-day cellular function, and more. The liver, spleen, bone marrow, and lymph nodes are organs where white blood cells are produced and stored. Besides being in tissues, most white blood cells circulate throughout the body in the lymphatic system and blood vessels.

Mast cells are some of the body’s first-line defenders and are mainly found imbedded in tissues and organs. Unlike other white blood cells, mast cells don’t circulate throughout the body in the blood. Mast pacells primarily help protect the body from parasites, but in actuality the world in which our domesticated pets live is relatively parasite-free.

Mast cells are filled with granules, which are sack-like pockets of biochemicals that help protect the body from invading pathogens (parasites, etc.) and antigens (immune system stimulating substances, like pollen, etc.). Instead of their primary role protecting the body from the effects of parasites, mast cells are stimulated to release their granules after encountering common substances in our environment, like pollen, which leads to an allergic response.

When mast cells release their granules, a variety of physiologic changes take place. This process is called degranulation and can be a normal response to environmental allergens or as an allergic response to a chemical stimulant (bee/insect venom, etc.). This can lead to mild to severe clinical signs of lethargy, nasal cavity secretions, appetite changes, decreased mobility, and more.

The biochemicals released from mast cell granules include:

  • Bradykinin —A peptide (protein) that causes blood vessels to dilate, which in turn can cause runny eyes and nose, reduced blood pressure, and smooth muscle constriction.
  • Heparin—A complex of peptide (protein) bound to a complex sugar (polysaccharide), which delays blood clotting times by preventing platelets from aggregating to form a clot.
  • Histamine —A nitrogen-based biochemical which makes blood vessels more permeable to the movements of white blood cells in order for the white blood cells to do their job in defending against invading bacteria, parasites, and viruses. Histamine release also promotes stomach acid production. When histamine is released it can cause the sensation of itching, which is why antihistamines are commonly used when controlling allergies in pets and people.
  • Other

What are Mast Cell Tumors?

Mast cell tumors (MCT, or mastocytoma) are collections of cancerous mast cells that form a mass-like lesion. I use the term mass-like lesion to refer to any lump or bump that can be seen, felt, or detected via diagnostic testing that may or may not be concerning for cancer. Cancer occurs when cells that have lost their ability to stop dividing, grow in an out-of-control manner and form tumors.

Over months to years of life the body’s cells are exposed to a variety of noxious agents that cause cellular DNA changes ultimately manifesting as cancer. Carcinogens are substances, both manmade and natural, that are linked to cancer development. Ultraviolet radiation, chemical exposures (pesticide, second hand smoke, etc.), infectious diseases (bacteria, viruses, etc.), and other factors can alter DNA manner that prevents cellular division from shutting off. Hormonal changes and lifestyle factors like obesity, lack of exercise, and poor diet can also contribute to cancer.

MCT are malignant cancers, which means they can be locally invasive and have a high likelihood of spreading to other body parts/systems. As a result, malignant cancer has a poor prognosis. For clarification of terminology, benign cancer are less-likely to metastasize and thereby generally has a better prognosis (“benign = better”), yet they still can be locally invasive as they grow and require extensive treatment to manage.

Individual cancerous mast cells are considered microscopic disease as they are invisible to the naked eye and are only seen under a microscope. MCT are termed macroscopic disease as they have a mass-like appearance (i.e. a tumor).

MCT are considered the most-common skin tumor affecting dogs and make up “seven and 21 percent of all canine skin tumors” according to The exact reason or reasons why pets get mast cell tumors is unknown.

Some pets have a genetic predisposition as a result of their breeding. Pure-breed canines like the Boxer, Bulldog (American and French), Labrador and Golden Retriever, Pit Bull Terrier, Pug, Sharpei, Weimaraner, and others may be more-prone to developing MCT. Dogs are more affected by MCT than cats and there’s not a known correlation among particular feline breeds in the development of MCT.

Adult and senior life stages are more-commonly affected by MCT, but there’s no exact defining age where a pet could be considered too young. Diagnosing a MCT in a puppy or kitten having lived only a few weeks to months of life is unlikely but not 100% impossible (nothing in nature ever is).

What Symptoms Do Cats & Dogs Show When Having Mast Cell Tumors?

If your pet has a MCT you may or may not immediately see symptoms. When a MCT develops in the layers of your pet’s skin (cutaneous MCT) you’ll feel or see a mass-like lesion once it becomes large enough. Some owners are very astute and readily feel or see changes in their pet’s skin or coat, so a MCT may only need to be a few millimeters to be discovered. MCT are generally movable, non-painful, soft to the touch and pink to red and rapidly growing (seemingly overnight or over days).

MCT can also form inside the body (visceral MCT) and may be undetectable unless your pet undergoes diagnostic testing (x-rays, ultrasound, tissue sampling, etc.) to look underneath the skin’s surface. Visceral MCT can be discovered if your veterinarian is going on a “tumor hunt” or incidentally found as part of routine screening (“senior screens”). Cutaneous MCT may lead to visceral MCT as a result of metastasis, but visceral MCT are especially common among cats and can be primary tumors not resulting from metastasis.

MCT are great imitators, so that movable, non-painful, soft mass-like lesion that you and your veterinarian suspect may just be a benign fatty tumor (lipoma) could actually be something more severe. Such is why I’m a big advocate of Dr. Susan Ettinger’s See Something, Do Something (Why Wait? Aspirate) campaign and apply the principles to my patients. “See something” means “when a skin mass is the size of a pea or larger or has been present for 1 month” we then must “do something” by taking the steps to “aspirate or biopsy, and treat.”

You may be unaware your pet has a MCT until a cutaneous mass-like lesion is seen or felt or clinical signs of illness are seen resulting from a visceral MCT. Clinical signs are often fairly obvious as a result of mast cell degranulation and include:

  • Itching/licking/chewing/rubbing, redness, and pain to the touch at or around a cutaneous MCT
  • Lethargy
  • Hyporexia (reduced appetite) or anorexia (no appetite)
  • Emesis—vomiting
  • Stool changes—diarrhea, soft stools, mucus, blood, melena (dark tarry stools from digested blood)
  • Decreased mobility
  • Petechia (pin-point bruises into the skin) and ecchymosis (larger areas of bruising caused by coalescence of petechia)
  • Altered respiratory pattern—panting, increased respiratory effort, etc.
  • Other

Severity of signs depends on the degree to which mast cells degranulate. Trauma such as handling, scratching at, and fine needle aspiration (FNA) of cutaneous MCT can cause rapid degranulation and severe and sudden growth of a MCT and worsening of clinical signs. Such signs should prompt you to get your pet to your general practice or emergency veterinarian for examination, diagnostics, and treatment.

How Are Mast Cell Tumors in Canines & Felines Diagnosed?

If your veterinarian suspects your pet has a MCT it’s crucial to take steps in establishing an official diagnosis. This process is called staging and is crucial in assessing whole body health so that the most-appropriate treatments can be established.

Many diagnostic tests are used in MCT diagnosis, including:

  • Blood and Urine Testing—Blood chemistry (organ system function, protein, electrolyte, calcium, thyroid, etc.), Complete Blood Count (CBC, for assessing red and white blood cell and platelet levels), and other blood and urine tests help determine overall health status and a patient’s ability to tolerate treatments like chemotherapy, surgery, radiation, etc. Severe cutaneous or visceral MCT can produce enough mast cells to be seen in the blood, which is not a finding characteristic of normal mast cells.
  • Radiographs (x-rays)—X-ray imaging creates a static (non-moving) image useful for assessing the inside of body cavities like the chest and abdomen and other body parts (bones, etc.). As the spleen, liver, and heart are common sites where MCTs can have a primary site or metastasize, radiographs can detect organ enlargement or other abnormalities.
  • Ultrasound —Ultrasound is an imaging modality commonly used to access information about body parts that can’t be fully discerned with radiographs. Although x-rays are a good first step in determining concern for the health of abdominal structures, ultrasound permits finer details about the potential invasiveness of MCT into organs of concern. Ultrasound waves don’t readily penetrate air-filled lung tissue, but they are useful in assessing the heart (echocardiogram).
  • CT scan—Computed tomography (CT) is an advanced imaging technique that permits deeper evaluation into the chest or abdomen when radiographs and ultrasound don’t provide sufficient assessment. CT scans require a pet to be fully anesthetized under gas anesthesia, while for radiographs and ultrasound a pet is usually kept awake or mildly sedated.
  • Fine Needle Aspirate (FNA) for cytology and biopsy—Insertion of a needle into a mass-like lesion to remove cells or fluid for cytology (microscopic evaluation of the cells). Although only a small portion of cells potentially present in a tumor may be collected via FNA, doing so is often the first step taken in diagnosing cutaneous MCT and can establish a definitive MCT diagnosis.

Biopsy requires attainment of a larger tissue sample so that cellular architecture of a mass-like lesion can be microscopically assessed and a more definitive diagnosis. Biopsy may be an evaluation of an entire mass-like lesion (excisional) or it can be performed on a chunk of tissue (incisional).

There are also two types of grading systems used when characterizing MCT, the Patnaik System and the Kiupel System, both of which can lend perspective as to the severity of your pet’s condition and expected outcome with treatment.

What Kind of Treatments Are Available to Treat Mast Cell Tumors in Canines & Felines?

With early diagnosis and appropriately thorough treatment, MCT cure is possible. Even if your pet’s MCT can’t be cured, there’s likelihood the disease can be controlled and quality of life can be improved or maintained; a very-realistic treatment outcome for many patients.

Cutaneous MCT generally have a better prognosis than visceral MCT. Yet, cutaneous MCT arising from the genital area, muzzle, nail bed, and oral cavity tend to have a worse prognosis. Because visceral MCT are already inside the body and commonly affect important organs like the spleen and liver they hold a grave prognosis.

Fortunately, they are many treatments available to pets having MCT. Unfortunately, your pet likely requires more than one type of treatment depending on disease severity, including:

  • Surgery—“A chance to cut is a chance to cure” is a common adage in veterinary medicine, as surgery has curative power and reduces potential for MCT to metastasize. Not all MCT are surgically resectable, but many cutaneous MCT can be completely removed leaving behind cancer-free tissue.

If margins of 2-3 centimeters around the MCT are not attained in all directions at the time of surgery then the need for further treatment with radiation, chemotherapy, and even additional surgery may be indicated.

  • Radiation—When MCT isn’t resectable or insufficient margins are attained then radiation treatment can be used to destroy cancer cells. Radiation requires a pet to go under anesthesia for each treatment’s administration.
  • Chemotherapy—Chemicals can be used to kill cancer cells or slow their growth. Even when a MCT is surgically removed and/or the patient underwent radiation there’s potential for microscopic disease to be present in the body. Such is where chemotherapy fits in.

Chemotherapeutic drugs may be oral or injectable and are generally well-tolerated by most canine and feline patients. Even common veterinary medications like steroidal anti-inflammatory drugs (Prednisone, Dexamethasone, etc.) can have a chemotherapeutic effect and help a patient by reducing inflammation and swelling, directly damaging mast cells, improving appetite, water consumption, energy, and more.

  • Adjunct medications—Antihistamines block the release of histamine and are generally safe with few to no side effects (sedation, etc.). Antacids (Famotidine, etc.) block the production of stomach acid caused by the release of histamine and can help with gastric irritation, nausea, appetite, and more.
  • Complementary and alternative medicine (CAM)—Many aspect of complementary and alternative medicine (CAM) can benefit patients having MCT.

Supplements and herbs like omega fatty acids, turmeric, ginger, and others can have a natural anti-inflammatory effect. Silymarin (milk thistle) has a liver-supporting effect so the organ can properly detoxify the blood. Pre—and probiotics help maintain normal digestive tract function, which reduces stress on the immune and makes for better tolerance of chemotherapy. Xue Fu Zhu Yu Tang, a Chinese herb also known as Drive Out Stasis in the Mansion of Blood Decoction, has led to a favorable response in some of my canine patients. Compounds found in Miatake, Reishi, and Shiitake mushrooms can inhibit the growth of or kill cancer cells.

My patients eating whole-food, human-grade meals (such as The Honest Kitchen’s low carb recipes like the Limited Ingredient Turkey Recipe, Grain Free Turkey Recipe and Grain Free Beef Recipe for dogs and the Grain Free Turkey Recipe and Grain Free Chicken Recipe for cats) and treats generally are healthier, have improved digestive and immune system function, and tend to show fewer side effects from chemotherapy, radiation, and anesthesia. Chinese medicine food energy can quell some of the Yang (heating) energy associated with MCT by having a cooling effect on the body.

Acupuncture and acupressure can have a calming effect on the nervous system to promote a better sense of well-being, minimizes pain, reduces nausea, and improves digestive tract function.

Selecting the most-appropriate single or combination treatments is a complicated undertaking, so I always recommend clients seek help from specialist veterinarians working exclusively in the realm of pet cancer. Mona Rosenberg DVM, Diplomate ACVIM (Oncology), owner, CEO, and Chief of Staff of Veterinary Cancer Group (VCG) states “when treating cancer, there is hope for your pet. Pursuing a consultation with a board certified veterinary oncologist will provide you with perspective on the best options available,” including clinical trials about which your general practice veterinarian may not be aware. A veterinary oncologist in your area can be found via the American College of Veterinary Internal Medicine (ACVIM).

Additionally, if you are seeking the CAM approach, seek consultation with a holistic veterinarian via the American Holistic Veterinary Medical Association (AHVMA).

Can Mast Cell Tumors Be Prevented in Dogs & Cats?

The multifactorial nature of cancer makes it crucial that owners take preventive measures to minimize a pet’s exposure to carcinogens. Such is why I recommend my patients eat diets made with human-grade, whole-food ingredients in attempt to minimize exposure to carcinogens like aflatoxins (from molds), heavy metals (arsenic, etc.), chemical preservatives (BHA, BHT, etc.), acrylamide (from cooking starches at 248F or greater), artificial flavors and colors, and others that are found in some commercially available pet foods and treats that are suspected or known to be carcinogenic.

A UC Davis study established an intriguing correlation between the development of certain cancers in Golden Retrievers. Early-spayed females had greater occurrence of hemangiosarcoma (malignant red blood vessel cancer) and MCT than females spayed at or after 12 months of age (late-spayed) as compared with unspayed females.

This does not mean that you should not spay or neuter your dog due to fears of cancer. Instead have a frank discussion with your veterinarian the health benefits and concerns of permanently altering your pet’s hormones.

Author: Dr. Patrick Mahaney VMD, CVA, CVJ


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