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Mast Cell Tumors (MCT): Symptoms, Diagnosis, Treatment

Mast Cell Tumors in dogs & cats

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What is a mast cell tumor?

Mast cell tumors (MCTs) are among the most common tumors found in dogs, and the third most common intestinal tumor found in cats. The tumor itself (aka: mastocytoma) is named after the cells that cause it. You guessed it, mast cells. 

Roughly 20% of all skin tumors found in dogs are diagnosed as MCTs. 

Not all mast cell tumors are malignant (cancerous). Fortunately, though, malignant MCTs are among the easiest types of cancer to treat.

Mast cell tumors most commonly appear on the skin as nodules, but can also form inside the body, affecting bone marrow, intestines, liver and spleen. They’re also commonly found on the chest, lower abdomen and on the limbs (typically the hindquarters).

Usually, mast cell tumors appear as a single growth. However, in roughly 10% of cases, MCTs have appeared in multiple locations on the body.

Large tumors, those that grow quickly, and those that manifest on the lower parts of the body, or near mucous membranes, are more likely to spread to other areas of the body. 

What are mast cells?

Mast cells are a type of white blood cell. When exposed to allergens, mast cells spring into action, releasing a chemical called histamine. The release of histamine (this process is called degranulation) is what causes itching, sneezing, watery eyes, runny nose … all those things that happen during an allergic reaction.

Sometimes, when too much histamine is released into the body, it can cause anaphylaxis--a life-threatening allergic reaction. 

RELATED POST: Pet Allergies: Types, Symptoms & Treatment

What causes mast cell tumors?

Like most cancers, the cause of mast cell tumors is not entirely known. However, MCTs do tend to affect older dogs (over the age of 8). 

While any dog is capable of developing mast cell tumors, some breeds are more susceptible, like our flat-faced friends: boxers, pugs, Boston terriers and bulldogs. Other breeds known to be affected by MCTs are Labrador retrievers, golden retrievers, Rhodesian ridgebacks and schnauzers.

Cancers may be caused by several factors, sometimes a mix of external elements and good old fashioned genetics. Scientists and veterinarians do know of several genetic mutations that cause MCTs. One in particular is KIT -- a protein that replicates and divides cells.

MCT symptoms

The dreaded C word can strike fear in the heart of even the strongest pet parent. As with many cancers, early detection can be crucial to a positive outcome

Finding lumps and bumps on your pet is always cause for concern, but try not to jump to worst case scenario assumptions. Not all bumps are tumors. And not all tumors are cancerous. 

Mast cell tumors can crop up anywhere on or in your pet’s body, and the shape and size of them can vary. So let’s look at mast cell tumors more closely so you can spot the signs and symptoms.

Some mast cell tumors might have the following appearance:

  • Raised lump or bump on or just beneath the skin
  • They may be red
  • They can be ulcerated (breaking through the skin)
  • They can be swollen
  • Some are small (and stay that way for months)
  • Others are large (and grow quickly)
  • Some MCTs change in size and appearance (sometimes slowly and other times daily)
  • MCTs can look like fatty tumors (lipomas)
  • They can look like a bug bite
  • Most MCTs are raised masses under the skin that feel soft yet solid

Nearly 25% of dogs with mast cell tumors also present with stomach ulcers caused by the over-production and release of histamine.

Clearly, mast cell tumors are not strict when it comes to their appearance and behaviour. That makes it difficult to diagnose mast cell tumors at first glance.

If you see a lump or bump, no matter it’s size, location or behaviour, book an appointment to see your veterinarian. 

How are mast cell tumors diagnosed?

Mast cell tumors are tricky little things. Although they often appear as a single mass, they’re usually surrounded by microscopic clusters of mast cells, invisible to the naked eye. 

After examining your pet’s lump, your vet will perform a fine needle aspiration (FNA). With a super thin needle, they’ll collect a sample of the cells (cytology). A pathologist will view those cells under a microscope in search of mast cells. 

Unfortunately, FNAs aren’t always conclusive. If the lump or bump is inflamed or infected, mast cells can be tough to see. 

If the lump is ulcerated or oozing, your vet might try an impression smear. For this, he’ll literally wipe a clean glass slike across the lump to collect a sample. 

If all else fails to return a definitive diagnosis, you may need to biopsy the lump (histology). 

WARNING: When you poke or prod a mast cell, degranulation may occur. If this happens, excessive amounts of histamine can be released into the bloodstream, causing issues elsewhere in the body. 

Watch for signs of stomach or intestinal ulcers:

  • Vomiting
  • Diarrhea
  • loss of appetite
  • Lethargy
  • Melena (black, tarry stools)

Although it may not happen often, mast cell tumors on the skin can spread to the organs, causing:

  • Enlarged lymph nodes
  • Hepatitis (inflammation of the liver)
  • Fluid build-up (peritoneal effusion) in the abdomen, causing a bloated or swollen belly

Some additional ways to diagnose MCTs may include:

  • Blood and urine samples (to assess organ function)
  • Abdominal ultrasound (to assess organs for evidence of MCT spread)
  • CT Scan (to identify the location and size of MCTs)

Once your pet has been diagnosed with a mast cell tumor, your vet may recommend a prognostic panel. Closer review of the tissue will give your vet the added information he needs about the genetic makeup of the tumor, so he can give you a more accurate prognosis.

Mast cell tumor grades, stages & prognosis 

When your pet is diagnosed with a mast cell tumor, he will likely need surgery to remove the mass and prevent it from metastasizing (spreading to other areas of the body). 

Tumors larger than 1.25 in (3 cm) are considered to decrease survival time. 

As with most other cancers, when a tumor is diagnosed, it’s graded from I-III. Lower-grade tumors have a better prognosis, while higher-grade tumors are more likely to metastasize.

Grade I tumors are typically benign (not cancerous), small lumps confined to one area on your pet. They tend to grow slowly just beneath the dermis.

Grade II tumors can also be found just beneath the dermis, but tend to also affect deeper tissues. Grade II MCTs often affect the lymph nodes. They can start out as benign and then change to become malignant.

Grade III MCTs are malignant (cancerous), often spreading quickly and affecting the lymph nodes and organs. 

Prognoses tend to be less favorable when:

  • The pet is a susceptible breed
  • The tumor is located where skin meets mucous membranes
  • The number of cells actively replicating is high

Single, low-grade mast cell tumors that have not metastasized are frequently cured when they’re completely removed surgically, or partially removed surgically along with radiation therapy. 

MCTs that have spread to local lymph nodes often have a good prognosis when treated with surgery, radiation and chemotherapy. In fact, 5-year survival times have been reported

Expected survival time for pets with high-grade MCTs may only be a few months, even with treatment.

Stages are a little different than grades. There are five stages 0-IV. According to the National Canine Cancer Foundation, staging depends on the number of tumors and lymph node involvement:

Stage 0: A single tumor just beneath the dermis, which is easy to remove. 

Stage I: One tumor in the skin, with no lymph node involvement.

Stage II: One tumor in the skin with lymph node involvement.

Stage III: Multiple large, deep skin tumors, with or without lymph node involvement.

Stage IV: One or more tumors with metastasis in the skin with lymph node involvement. This stage is divided further into tumors with no other signs (substage a) and tumors with other clinical symptoms (substage b). 

Mast cell tumor treatment options

MCT treatment options depend on the grade and stage of the tumor. For example, stage I tumors are frequently handled by complete surgical removal. For smaller tumors, vets will generally remove 3cm of healthy tissue from all sides of the tumor. This ensures removal of all those tiny clusters of mast cells circling the mass. 

Sadly, there is no one agreed-upon treatment for tumors that fall into stages II through IV. Still, the most common treatment options include surgery, radiation and chemotherapy -- sometimes a combination of all.

When pets are diagnosed with a higher grade of cancer, parents may opt for palliative care, which can include:

  • antihistamines (diphenhydramine: also known as Benadryl)
  • pain killers 
  • steroids (prednisone or prednisolone)
  • supplements to boost immune system function

Veterinary cancer specialists (oncologists) may recommend new types of treatments to help with difficult cases. Because scientists know there is a genetic component to mast cell tumors, they’re actively creating drugs designed to specifically target the proteins that cause this type of cancer

For pets with mast cell tumors that can’t be surgically removed, or for those with recurrent MCTs that did not respond to chemotherapies, targeted therapy may be the best option.

Talk to your vet about all your options. There’s no such thing as a stupid question, so don’t be shy. Ask all the questions you want answers to before making a decision on treatment. This is your fur-baby after all!

Prevention and care

When you find a lump or bump on your pet, call your vet immediately. Early detection of MCTs is critical. Better safe than sorry!

Degranulation (itchiness, swelling and discomfort) is easily triggered by pressure, so try to avoid feeling or pressing on the tumor.

Do not let your pet chew, lick, or scratch at the tumor. Not only can this trigger degranulation, but it may cause the area to ulcerate and become infected.

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