Mast Cell Tumor in Dogs: Cause, Symptoms & Diagnosis

Mast Cell Tumor in Dogs: Cause, Symptoms & Diagnosis

Author WDF Staff


Getting the news that your dog has the dreaded “C” word - cancer, can be pretty devastating. About a million thoughts run through our heads, and we start thinking of possible treatments. Most of us will probably become desperate when we hear the news. Yes, cancer is potentially deadly, and learning that your dog has a mast cell tumor is pretty bad.

For those of us who are not familiar with this type of dog canine cancer, there are some things we should know before we start jumping to conclusions. Not all mast cell tumors are the same, and there are different treatment options available. Unfortunately, there are many dogs across the world diagnosed with mast cell tumors. If that happens to be the case with your dog, take a few deep breaths, calm down, and ask your vet for available options. Here is what you need to know about it.

What are mast cells?

To thoroughly understand what mast cell tumor is, we need to know what mast cells are. Mast cells are a type of white blood cells that can be found in different body tissues. These cells are responsible for allergy response, and when they get in contact with common allergens, they start releasing chemicals.

What is a mast cell tumor?

A mast cell tumor is a tumor that consists of mast cells. They are the most common skin tumor in the canine kingdom, and 60 - 70 percent of dogs develop only one mast cell tumor. This type of tumor usually forms masses or nodules in the skin. Unfortunately, the skin is not the only part of the dog that can be affected by this tumor; bone marrow, spleen, liver, and intestine contain mast cells, so they can be affected as well.


It is still not entirely clear what causes this disease; it is most likely a mix of different risk factors. Some factors can be hereditary, and others can be environmental. Scientists managed to confirm that one of these risk factors is the KIT protein, which plays a vital role in mast cell division.

All dog breeds can be affected by this tumor, but like with any other disease bugging our canine friends, some breeds are simply more prone to it than others. It is not entirely clear why that is; we just know that the largest numbers of mast cell tumor cases come from these breeds:

What are the symptoms?

Spotting a mast cell tumor can be tricky. They can vary in appearance and can form all over the dog’s body. Most of them appear on the skin or just under it. Unfortunately, they can also form on the lining of the lips, salivary glands, conjunctiva, and in some rare cases, on the spine.

You must understand your dog’s behavior, and if you notice something strange going on with them, you should get them for a check-up as soon as possible. The most common symptom is a lump on or under the skin. Some nodes can stay the same size and not bother your dog, which means that some owners postpone their dogs’ check-ups. Other owners can notice the lump’s aggressive forming, and it can grow in a matter of days. The worst part is; all skin lumps can be mast cell tumors, so as soon as you notice anything, you should take your dog to the vet.

If you notice a lump in your dog’s skin changing sizes, you should check it out. Since mast cells secrete chemicals, the tumor may pressure it, and it can literally grow during the day and drop in size at night. Also, ulcers on the surface of the skin can be a sign of mast cell tumors. Other symptoms include:

  • Dark stool
  • Stomach ulcers
  • Lethargy
  • Loss of appetite
  • Enlarged lymph nodes, spleen, and liver
  • Swollen belly
  • Vomiting


Your vet can suspect your dog has a mast cell tumor based on all the symptoms, but they cannot be absolutely sure until they observe cell samples. They can harvest the cells by needle biopsy, and that will most likely confirm or deny it. If the tests confirm a mast cell tumor, your vet will grade it on this scale;

  • Grade I - Grade I tumors are benign and not that dangerous. They can usually be entirely removed, and the prognosis is great.
  • Grade II - Grade II mast cell tumors are the most common, and some of them can be surgically removed. Choosing the right treatment can be tricky, and 43% of all these tumors are Grade II.
  • Grade III - Grade III mast cell tumors are aggressive and spreading over the dog’s body. The only treatment option is chemotherapy.


The most crucial thing all dog owners whose dogs are diagnosed with mast cell tumors want to know is - prognosis. However, different grade tumors have different prognoses. Here is the prognosis for each grade;

  • Grade I prognosis - Grade I prognosis is usually pretty good. Benign tumors mean they are not as aggressive or developed as a malign tumor, and most dogs will completely recover after surgery.
  • Grade II prognosis - This is where it gets tricky, even for vets. They cannot accurately guess how the tumor will act and how it will progress. Some will act benign, but a small percentage of them will grow and spread. It is impossible to know which will act in what way. Some might be surgically removed, but recurrence and spread can also happen.
  • Grade III prognosis - There is no way to sugarcoat Grade III mast cell tumors. The prognosis is pretty bad. This type of tumor will most likely reappear after surgery, and there is a strong possibility it already started spreading. Most dogs are expected to live up to a year after the diagnosis of Grade III mast cell tumor. 

World Dog Finder team

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