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There are various types of tumors that affect dogs, but there is one in particular, the mast cell tumor, which can behave quite erratically and that deserves a dog owner's attention. 

Dog owners should keep their vigilant antennas up and pay attention when petting and grooming their dogs as these tumors are, not only quite common, but also quite unpredictable in nature. 

Mast cell tumors, also known as mastocytomas, are one reason why, even the most innocent looking bump or lump should be checked out by a veterinarian. 

This is why it's never really a good idea to take a wait-and-see approach with any lumps and bumps, unless a veterinarian has determined that it's safe to do so.

 So today, let's take a look at some surprising facts about mast cell tumors in dogs.


Picture of mast cell tumor on the side of a dog

Mast Cell Tumors are Copy Cats...

Mast cell tumors in dogs are often referred to as "the great imitators," why is that? 

Mast cell tumors gain this reputation from the fact that they can clinically resemble many other types of dog skin tumors. (See pictures for an idea)

Mast cell tumors may therefore look like an innocent bump, a fatty mass under the skin, an ugly ulcerated mass or a bug bite. You name it!

 Mast cell tumors in dogs can also be smooth, bumpy, solitary or in groups and they may be present on the skin or underlying tissues.

This is again why, one can't never say what a mass really is until it gets checked out.

 However, despite having a variety of clinical appearances, if one must describe how a mast cell in a dog looks like on average, a mast cell tumor can be described as a hair-less, pink, raised mass that prefers to show up on the dog's torso and legs, explains Steven Neihaus, a board-certified veterinary surgeon in an article on DVM360..

"Mast cell tumors can occur anywhere on the body. Approximately 50% occur on the trunk, 40% on limbs, and 10% on the head. " Source, DVM360

And They Are Good at Hiding Too


Dog mast cell tumors can be quite unpredictable tumors.

 For instance, some mast cells tumors may have a history of shrinking for some time and then swelling up again.

The shrinking may lead dog owners to assume that the growth is something that is getting smaller which may cause them to delay treatment.

 Then, after a while, they get a wake-up call once the growth starts swelling up again.

This history of shrinking and then swelling is due to the mast cell tumor's tendency to degranulate and release histamine, explains Tracy Geiger, a board-certified veterinarian specializing in internal medicine.


Mast Cell Tumors Release Substances...

Mast cell tumors don't just sit there all day long doing nothing. 

These tumors originate from the bone marrow but then finish up maturing in the tissues of a dog's body and this can includes skin, digestive tract and respiratory tract.

Once stimulated by the immune system, mast cells tumors release their granular contents along with several chemicals and these may include histamine, proteoglycans, neutral proteases (enzymes that break down protein) and chemotactic growth factors.

Histamine in particular, is basically the same stuff that causes an area of the skin affected by a bee stings to become red, inflamed and painful. 

When all these chemicals are released, they can, not surprisingly, wreck quite some have havoc on a dog's body.

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That Can Cause Complications

As mentioned, mast cell tumors are quite insidious in nature causing a variety of problems when they release chemicals.

Among dog mast cell tumor complications, at a skin level, recurrent swelling may occur due to degranulation and associated release of histamine, while local bruising, this time from the release of heparin, may be present as well.

When mast cells tumors release histamine into the dog's bloodstream, they may trigger the dog's stomach lining to produce too much acid and this may lead to a decrease in appetite, nausea, lip smacking, drooling and vomiting, explains veterinarian Dr. Dressler on the Dog Cancer Blog.

Because of these dog mast cell tumor complications, anti-acids are often prescribed to manage the excess acid production, while antihistamines are used to block the release of histamine, allowing the body a better chance of coping with the high histamine levels. Both cancerous and non-cancerous forms of mast cell tumors may release histamine.

" Dogs can also develop signs associated with the release of toxins from the malignant mast cells. For example, up to a quarter of dogs with mast cell tumors also have stomach ulcers due to histamine release." ~Merck Veterinary Manual

They Can be Quite Easily Diagnosed

In the case of a suspected lump or bump, it can be aspirated with a fine needle to determine whether the growth is cancerous or not. 

The needle aspiration is done with a small gauge needle and shouldn't be painful hence, the term "fine needle aspiration."

The fine- needle aspirated cells can then be evaluated under a microscope (cytology) in house or the sample can be sent out to be evaluated by a pathologist.

Under a microscope, the sample typically shows a large number of mast cells which is enough to make a diagnosis of mast cell tumor. 

Mast cells show up as purples granules that contain histamine. Once mast cell tumor is confirmed, a surgical biopsy is needed to discover the grade of the tumor.

"Diagnosis can often be made with a needle aspirate, which collects some cells of the tumor with a needle, and the cells are examined under the microscope. The granules have distinct staining characteristics leading to their recognition. An actual tissue biopsy, however, is needed to grade the tumor and grading of the tumor is crucial to determining prognosis."~Dr. Wendy C. Brooks, Veterinary Information Network

But Removal Requires Wide Margins

If you think that surgery to remove a mast cell tumor involves just simply removing only the the lump or bump, think again.

In order to get rid of this pesky tumor, wide margins are required. Because it's difficult to tell where the tumor begins and where it ends, a large area of about 3 inches of 'healthy' tissue in all directions must be removed. So this explains why dog mast cell tumor need wide margins.

Getting wide mast cell tumor margins though can sometimes be a problem depending on where the tumor is located. For instance, mast cell tumors on the neck or in the mouth may be problematic.

Should the mast cell tumors have metastasized to other areas, a combo of anti-cancer drugs may be used along with surgery and radiation. For the best treatment plan, it's often a good idea to consult with a veterinary oncologist.

More Mast Cell Tumor Facts 

Did you know? Mast cell tumors in dogs account for up to 20 percent of all tumors affecting the skin in dogs. While they may mostly affect older dogs, they can be found in dogs of any age dog. 

Mast cell tumors may also affect any breed, even though certain breeds such as boxers, pugs, Boston terriers, bulldogs, bull terriers, and retrievers are known for being predisposed.

 As with other types of tumors, it's important to promptly report to the vet any suspicious growth, lump or bump for the best possible outcome.

Disclaimer: this article is not meant to be used as a substitute for professional veterinary advice. If your dog has a growth on his skin, please consult with your vet promptly for proper diagnosis and treatment.


  • Pet Education: Canine Mast Cell Tumors, retrieved from the web on November 19th, 2016.
  • Dog Cancer Blog, Why use stomach medication for mast cell tumors? retrieved from the web on November 19th, 2016.
  • Dog Cancer Blog, Why Benadryl For Mast Cell Tumors? retrieved from the web on November 19th, 2016.
  • DVM360, Mast cell tumors in dogs and cats, retrived from the web on November 18th, 2016

Photo Credits:

  • Mast cell tumor on the side of a dog Joel Mills Own work, CCBY3.0 
  • Mast cell tumor on the inner thigh of a dog, by Joel Mills Own work CCBY3.0
  • Mast cell tumor of the paw, Joel Mills Own work, CCBY3.0 Mast cell tumor cytology Joel Mills - Own work CCBY3.0

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