The Dreaded Mast Cell Tumor

More on PawNation: Dogs, Health, Tumors

Of all the tumors I treat, probably the most unpredictable would be the dreaded canine mast cell tumor. An oncologist I worked with during my internship described his take on this particular form of cancer in dogs by telling owners, "If ever there was going to be a tumor to fool me and do what it wants, it's a mast cell tumor."

The more cases I see, the more I find myself repeating those humbling words over and over again when talking about this challenging disease.

Most dogs develop mast cell tumors in their skin or subcutaneous tissue. They may also develop tumors internally, but this is less common. The tricky part comes when the skin tumors spread internally, or an internal tumor spreads to the skin. It can be nearly impossible to determine the "chicken or egg" in those cases.

Some dogs will be diagnosed with a mast cell tumor when a lump that's been present for many years is finally tested one day. Other dogs will develop a rapidly growing tumor that changes dramatically in a few short days to weeks. Some will have only one tumor over their entire life, while others will have a dozen or more develop in a short period of time.

I've also seen dogs that develop a new tumor every year like clockwork. I would also venture to guess it's probably the most common "second cancer" I diagnose in dogs I'm treating for a completely different tumor type.

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Mast cells are immune cells that normally play a role in allergic reactions and inflammatory responses. They reside within many tissues of the body, and dogs have a great deal of these cells located within their skin. Mature mast cells contain granules, which are basically packets of chemicals. When signaled by an allergen or the immune system, mast cells will release the chemicals by a process called degranulation. The chemicals can cause changes locally, right at the area where they are released, and can also travel through the blood stream to affect distant organs and tissues, and even the entire body, in what is known as an anaphylactic reaction.

We don't really understand completely what causes mast cell tumors to develop, but we do know they are more likely to occur in certain breeds of dogs, including Boxers, Boston Terriers, Beagles, Pugs, Labrador retrievers, and Golden retrievers (to name a few). This suggests a likely genetic component to their origin. Chronic skin inflammation and chronic topical application of irritants may predispose dogs to developing tumors.

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We also know that between 20-30% of mast cell tumors will have a mutation in a specific gene called c-kit. This will come up again in a future article discussing treatment options for mast cell tumors, and is the target for a new class of chemotherapy drugs called tyrosine kinase inhibitors (see article on Palladia).

For cutaneous mast cell tumors, one of the biggest predictors of how "good" or "bad" it will behave is something called the grade of the tumor. The grade can only be determined via biopsy, which means either a small portion of the tumor, or the entire tumor, needs to be removed and evaluated by a pathologist.

The most common grading scheme for mast cell tumors in dogs is something called the Patnaik scale, where tumors will be classified as either a grade 1, grade 2, or grade 3. The vast majority of grade 1 tumors will behave completely benign, and surgical excision is considered curative.

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On the other side of the spectrum are the grade 3 tumors. These are invariably malignant, with a high chance of regrowth after surgical removal, and a high propensity to spread to lymph nodes, internal organs, and even the bone marrow.

Perhaps the toughest of all to know how to treat are the grade 2 tumors. Most grade 2 tumors behave very much like grade 1 tumors, but a small subset will act very aggressively, and it's difficult to predict which ones will do so. Some information can be garnered from the biopsy report itself, but often we are making our best "guesses" as to what to do.

Because of the confusion surrounding the grade 2 tumors, a new grading scheme was proposed about two years designed to place all tumors into one of two categories. Using this new scheme, a mast cell tumor is designated as being high-grade or low-grade. Finally, it seemed the muddy waters would be cleared and tumors could simply be designated as "bad or good."

As is true for so many things, new isn't always better to some people, and not every pathologist has readily adopted the two-tier scheme. I do find it actually very helpful for a pathologist to include both designations on a biopsy report, and more and more pathologists are doing so as this newer system seems to be slowly catching on.

Although more than 80% of skin lumps and bumps on dogs are completely benign, and although most canine cutaneous mast cell tumors behave in a non-aggressive fashion, it's still very important to have any new or old lump or bump evaluated by your veterinarian (see Evaluating Lumps and Bumps).

Never assume a skin tumor is benign, or just a "fatty tumor" by feel. At minimum, a fine needle aspirate should be performed to determine the cause of the lump. Take it from someone who's been fooled one too many times by this cancer.

Next week I'll discuss treatment options for mast cell tumors in dogs, including surgery, radiation therapy, and chemotherapy.

Dr. Joanne Intile

"The Dreaded Mast Cell Tumor" originally appeared on PetMD.com.

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Julie Tower

We just go back from our vet, my pug has a lump on his leg. They did an FNA and there was a lot of blood in the slide but she said she did see quite a few mast cells. So tomorrow I am calling a specialist that she referred me to to get a consultation for removal.She said it's in a tough place for removal too. Mean while we are trying to shrink it the best we can with Benadryl. I'm so scared for him. He is 8yrs old and has only been with me for 2yrs and we are very attached to each other. I hope they can get it all and he won't have any more come back. I hate that he and anyone else has to go thru this :(

Tuesday at 10:28 PM Report abuse rate up rate down Reply
longsunjohn

I have had great results treating a mast cell mass/tumor with the herb skullcap. Skullcap works by suppressing/inhibiting the production of histamine in mast cells. My dog developed a mass on her foot after being bitten; it never healed properly and a round hairless lump developed with growing open sores. After some experimentation I determined that for my 80lb dog, 5 scullcap pills (425 mg) , 3 times a day (for a total of 15 pills) worked very well at returning the mast cells to their normal benign state (I have used both American and Chinese skullcap and they both work). The open sores dried up and eventually healed. Hair grew back and and the lump began to slowly shrink in size. There were a couple times that I ran out of skullcap and within a few days the open sores redeveloped and the lump began to grow again, this despite the fact that I continued to administer the OTC anti-histamine loratadine. When I restarted the skullcap treatment the effect was obvious within 12-24 hours; the open sores stopped oozing and started to heal again. My dogs mass/tumor was in a fairly bad spot, on the foot, so I think lower doses of skullcap may be effective on tumors located where they are subjected to less irritation. High doses of skullcap may cause liver damge so liver tests are necessary. Vets please take note!

May 13 2014 at 11:59 AM Report abuse rate up rate down Reply
llylyk

We found a growth on my German shepherd at her visit and tried antibiotics and it grew larger in a month tried to get a sample to biopsy but the mass was so solid they did not get anything anyone have any more helpful info on mast cell tumors I am very worried! What are symptoms and what is the chances it could be cancer it is about the size of a quarter dollar round and looks like a extra pad under her paw

November 23 2013 at 8:44 PM Report abuse rate up rate down Reply
llylyk

My dog has a growth on her paw she is a purebred German shepherd but they think it may be a mast cell tumor they tried to aspirate with a needle but it is so solid they were unable to get anything! I am very worried anyone with more info

November 23 2013 at 8:41 PM Report abuse rate up rate down Reply
Dawn Klauder Catramb

Paw nation GREAT & informative site for whatever is on ur mind concerning your dog or cat, or just about any domestic pet.
I just read an article about the dreaded mast cell tumor occurring in felines & canines, someone sd it came from the form of Purina dog food I feed him, Wrong. lol
Common in Golden's & Labs, abt 5 other breeds as well. Just in case ur interested,.I'll attach the url.
Thanks guys

September 30 2013 at 3:56 PM Report abuse rate up rate down Reply

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