Mast Cell Tumor in Dogs: An Introductory Guide

By Jennifer Dempsey / November 14, 2018

Finding lumps and bumps on your perfect furry friend can be quite troubling for a dog owner. It’s hard to train our minds to not automatically think the worst. Yet, if you jumped on Google and typed in “lumps on dogs,” you’re probably already in a downward spiral of panic. This is going to sound like an impossible thing to ask… but try to remain calm. Many lumps and bumps on dogs are nothing more than a mole or a minor allergic reaction. However, some lumps, of course, are much more serious than that.

In this article, we will cover everything you should know about mast cell tumors in dogs. It is imperative for pet owners to know how to recognize the symptoms of the disease in order to catch it early on and begin treatment straight away. While your dog’s lump may be something that is easily resolved, in some cases, such as mast cell tumors, immediate treatment is imperative. 

mast cell tumor dog

Mast Cells: The Basics

Before we dive straight into mast cell tumors, there is some basic terminology that is important for dog owners to recognize in order to fully understand their dog’s disease. Our dogs trust us to take the best care of them.  By understanding what is actually wrong with Fido, pet owners can ensure that they are making the best decisions for their wellbeing.

What Are Mast Cells 

Mast cells are a type of white blood cell that originates in the bone marrow but mature and settle in other tissues. In fact, mast cells are found within all connective tissues of the body. With that said, mast cells exist in much larger volumes in the skin, respiratory tract, and digestive tract. They are also found in large quantities around the eyes, nose, and mouth. Mast cells contain histamine and heparin and are involved in a number of important functions.

What Do Mast Cells Do 

Mast cells are part of our immune system and they help to provide the body with defense against parasitic infestations. Additionally, mast cells support the dog’s body in repairing tissues and therefore help promote the healing process. They also play a crucial role in a process known as angiogenesis or the formation of new blood vessels.

Also as a type of white blood cell, mast cells produce antibodies in part of an allergic reaction defense. Mast cells release chemicals and compounds (such as histamine) when exposed to allergens. Histamines are known for causing symptoms of allergies such as sneezing, itching, and runny eyes. However, when these substances are released in high amounts, they can also cause anaphylaxis and life-threatening allergic reactions.

mast cell tumor

Tumors in Dogs… What Exactly Are They

In simple terms, a tumor is the swelling of a specific body part. Tumors typically occur without inflammation and are caused by an abnormal growth of tissue. Tumors are either identified as malignant (cancerous and dangerous) or benign (noncancerous and usually harmless).

What is Mast Cell Tumor (also known as Mastocytoma in Dogs) 

A mast cell tumor or MCT is cancer that develops from mast cells. Mast cell tumors (also known as mastocytomas) are the most commonly diagnosed skin tumors in dogs. In fact, approximately one-third of all tumors in dogs are skin tumors. Of that one third, roughly twenty percent are mast cell tumors. Some mast cell tumors have a low level of malignancy and are relatively harmless to the dog. However, other MCTs have a high level of malignancy. These tumors can be life-threatening if appropriate treatment isn’t initiated in a timely manner.

Mast Cell Tumor in Dogs: Where Are They Found

Mast cell tumors can be found anywhere in the dog’s skin, but are most frequently seen around the trunk and limbs. However, MCTs can also be found in other places, including the intestines and lungs. Furthermore, even in cases where mast cell tumors originated in the skin, more aggressive MCTs are known to spread quickly to other organs, typically the spleen, liver, and bone marrow. This ultimately dictates the stage and grade of the mast cell tumor (more on that in a moment).

What Causes Mast Cell Tumors to Develop

Even with constant advancements in science that are being made every day, there is still so much about cancer and tumors that remain unknown. When it comes to mast cell tumors, in the majority of cases, the exact underlying cause is undetermined. With that said, scientists do know that MCTs are the result of a cell mutation (in a protein called KIT) within the mast cells. The mutation causes the uncontrollable reproduction and growth of the cancer cells, thus resulting in mast cell tumors.

What Do Mast Cell Tumors Look Like

Due to the fact that MCTs vary so greatly in appearance, they can often be difficult to detect. Mast cell tumors can develop in differing shapes, sizes, textures, and in a number of different locations. MCTs can also be soft or firm, raised or flat, as well as covered in hair or ulcerated. They tend to start small but can rapidly increase in size. They have also been known to fluctuate in the size of the tumor.

Symptoms of Mast Cell Tumors in Dogs 

It is imperative for dog owners to be aware of the symptoms of mast cell tumors. The earlier the MCT is diagnosed, the better the prognosis and life expectancy times.

A Bump or Lump 

It probably goes without saying that the most prominent physical symptom of mast cell tumors is the presence of a lump or bump (a tumor) under or on the skin. The tumor may have existed for days, weeks, or even months before the owner noticed it. This is largely due to the fact that the tumor may have fluctuated in size, making it more difficult to detect. Furthermore, as we previously mentioned, the tumor may have only existed for a short period of time, but has rapidly grown, seemingly overnight.

It is important for dog owners to know that mast cell tumors (especially in early stages of their development) may resemble warts or insect bites. It is imperative that pet parents closely monitor any new lumps or bumps that they find on Fido. A wart or insect bite will likely resolve itself whereas an MCT will not.

mast cell tumor symptoms

Redness or Fluid Buildup

Another symptom of mast cell tumors is the presence of redness and/or fluid build up around the tumor. Skin irritation and inflammation are also commonly due to the release of higher histamine levels within the tumor.

Enlarged Lymph Nodes

Pet parents may also find lymph node enlargement around the area of the tumor. In many cases, the surrounding lymph nodes will be surgically removed as part of MCT treatment. 

Spleen and Liver Enlargement

High-grade mast cell tumors can spread to the liver and spleen, which may cause organ enlargement or fluid buildup in the abdomen, causing the belly to appear rounded or distended. 

Gastrointestinal Issues

Finally, gastrointestinal symptoms including vomiting, diarrhea, and loss of appetite are often signs that the cancer cells are spreading. Gastric ulceration can also occur when mast cells degranulate and release compounds into the bloodstream. Melena, or black tarry stools, is often a sign of a bleeding ulcer in the GI tract. 

Dog Breeds at Higher Risk of Mast Cell Tumors 

Although mast cell tumors are typically diagnosed in older dogs, they have also been found in puppies as young as three months old. With that said, It is possible for dogs of any breed or age to develop mast cell tumors.

However, based on extensive research, experts have also determined that certain breeds are at an increased risk of developing canine mast cell tumors. 

These breeds include:

  • Boston Terriers
  • Boxers
  • Weimaraners
  • Bulldogs
  • Pit Bull Terriers
  • Rhodesian Ridgebacks

Mast Cell Tumor Dog Diagnosis

Your veterinarian will likely perform a fine-needle aspiration and cytology in order to accurately diagnose whether your dog has a mast cell tumor.

Fine-Needle Aspiration

A fine-needle aspirate is a rather quick procedure that is done without having to sedate the dog. This involves taking a small needle and suctioning a sample of cells directly from the tumor. The sample is placed onto a microscope slide and examined by your veterinarian. The cytology can also be sent to a pathologist to confirm the diagnosis. The grade of the tumor cannot be diagnosed on cytology alone.  

Surgical Biopsy

If cancer cells are present on the cytology, a surgical tissue biopsy will be necessary in order to classify the grade of the tumor. The biopsy will also determine if the margins are clean or free of cancer cells. Generally, the veterinarian will take large margins around the tumor during surgery as long as the location permits. If the biopsy confirms a higher-grade tumor, further diagnostics will be needed to determine the stage of the tumor.

Additional Diagnostic Tools

Additional diagnostics are used to see if the tumor has spread to lymph nodes or other organs. Radiographs of the chest will be performed to rule out small nodules or metastasis in the lungs. Radiographs can also be used to look at the size of the liver and spleen. Abdominal ultrasound will look internally at these organs to see if there is any evidence of nodules. The vet may also aspirate any enlarged lymph nodes to rule out tumor metastasis.

Grades of Dog Mast Cell Tumors

The tumor grade of the MCT refers to its level of malignancy. Once again, the grade cannot be determined with the fine needle aspirate, only with the biopsy of the tumor after surgical removal. The grade will help your veterinarian make predictions as to how the tumor will develop and if it will affect other internal organs.

Grade I

Mast cell tumors that are classified as Grade I are typically benign and usually always occur on the surface of the skin. However, just because they are benign doesn’t mean they should be ignored. Some Grade I MCTs can grow to be quite large and prove to be difficult to remove. Even so, the good news is that Grade I MCTs have not spread to the surrounding organs. Surgery alone should be curative as long as there are good margins on the biopsy report and no cancerous cells were seen on the margins of the sample. If cancerous cells were left behind, a follow up surgery for wider margins or radiation therapy may be recommended. 

Grade II

Mast cell tumors that are classified as Grade II are those that extend below the surface of the skin and into the subcutaneous tissues of the body. Treating Grade II MCTs can prove to be difficult to predict in terms of outcome. This is largely due to the unpredictable biological behavior of Grade II MCTs. The majority of tumors fall in this grade. A newer classification system has been implemented to determine if it will act more as a low-grade or high-grade tumor. An estimated 65% of Grade II mast cell tumors are cured with surgery. With that said, it is still possible for the tumor to return or for the spreading of cancer cells to occur.

Grade III

Grade III MCTs are those which prove to be the most difficult to treat. Mast cell tumors that are diagnosed as Grade III tend to spread quickly throughout the body. Even with aggressive treatment, the disease often has already developed deep below the skin’s surface. High-grade MCTs will often require surgery and cancer treatments including chemotherapy and radiation. Sadly, in most cases, Grade III mast cell tumors are not a disease that the dog can overcome.

Stages of Mast Cell Tumor Dog

The tumor stage of the MCT refers to its metastasis. The stage will help your vet determine to what degree the tumor has developed and spread. Different stages also refer to the degree to which the surrounding lymph nodes are affected. Staging is determined using further diagnostics such as chest radiographs (to look for metastasis and lymph node enlargement), abdominal ultrasound (to evaluate the spleen and liver), and lymph node aspirates. 

Stage I

Mast cell tumors that are classified as Stage I are those that involve only one tumor in the skin. They also do not have any lymph node involvement.

Stage II

Stage II MCTs are also a single tumor confined to the skin but there is evidence of lymph node involvement. 

Stage III

Stage III mast cell tumors are characterized by either multiple tumors or by a large tumor that has spread into the subcutaneous tissues. Mast cell tumors that are Stage III may or may not have lymph node involvement.

Stage IV

Finally, Stage IV MCTs are characterized by one or more tumors with metastasis in the skin. It is also common for the tumors to have spread to other internal organs. Additionally, Stage IV mast cell tumors involve the lymph nodes.

Treatment for Mast Cell Tumor 

The grade and stage of the mast cell tumor will ultimately dictate the appropriate and necessary treatment.

As we previously mentioned, mast cell tumors have high levels of histamine. Therefore, any manipulation of the tumor may cause a sudden and extreme release of histamine to enter the bloodstream, causing a severe reaction. For this reason, your veterinarian may prescribe antihistamines (most commonly Benadryl) to combat any adverse effect. An antihistamine medication will also help to protect the internal organs that could be harmed from the sudden histamine release.

mast cell tumor treatment for dogs

Surgical Removal

Surgery to excise the tumor is typically the preferred method of treatment, as long as it can be removed easily and there is no evidence of spread. The Grade of the MCT and margins on the surgical biopsy will help your veterinarian to predict the success rate of the surgery. A more aggressive surgery, as well as additional treatment such as chemotherapy or radiation, will often be necessary in cases where the cancer cells have spread to close to the surgical margins.

Radiation Therapy

Radiation is recommended for tumors that were not completely excised and a second surgery is not possible or for tumors that cannot be surgically removed due to their location on the body. In these cases, your veterinary oncologist (a veterinarian who specializes in cancer patients) will likely recommend radiation therapy. Radiation therapy is more appropriate for tumors that are localized and have not yet spread. If metastasis is evident, chemotherapy may be the more appropriate option.  


Chemotherapy is recommended for high-grade tumors where there is evidence of spread to local lymph nodes or other organs. It can also be used when the tumor is too large for surgery or radiation to control. There are several different drugs and protocols including prednisone, injectable chemotherapy (such as vinblastine, Lomustine) or oral medications (Palladia and Kinavet). These treatment protocols can vary in expense and side effects. 

Palliative Therapy

Palliative care is focused on relief from symptoms and improving the quality of life for patients in which chemotherapy or radiation is too expensive or not an option. These medications do not slow the progression of the disease. Corticosteroids, such as prednisone, have been shown to be directly toxic to mast cells and reduce inflammation. Antihistamines, such as diphenhydramine (Benadryl), are used to reduce the effects of histamine that is leaked from mast cells throughout the body. Histamine also signals production of stomach acid, which can cause stomach ulcers, and other gastrointestinal symptoms.  Famotidine, an over-the-counter histamine blocking antacid, is also used to prevent gastric ulcers. 

Mast Cell Tumor Dog Life Expectancy

Survival times and overall prognosis for dogs diagnosed with mast cell tumors ultimately depends on several factors. First, prognosis depends greatly on the grade and stage of the MCT. Complete surgical excision on low-grade tumors generally has a good prognosis for a cure.  Survival times also depend on whether or not the dog receives the appropriate treatment. Surgeries and radiation treatment prove to be extremely pricey. Some pet owners will, unfortunately (and understandably), not be able to afford such treatment.

Sadly, dogs suffering from Stage III MCTs typically have a life expectancy of less than one year, even with aggressive treatment. 

As you can see, while we urge pet owners to try to remain calm when it comes to new lumps and bumps you may find on your dog, it is imperative to not ignore them either. You should always have any new lumps checked by your regular veterinarian. Your dog’s life may very well depend on whether or not they were diagnosed early on.

mast cell tumor dog

Mast Cell Tumor in Dogs: A Final Thought

Mast cell disease is a diagnosis that can devastate a dog owner. We understand the absolute heartache that a cancer diagnosis can cause and we are so sorry for our readers who are going through this currently. We want to encourage our readers to do their research when it comes to mast cell tumors and the available treatment options. Even something as simple as changing your dog’s diet to the highest quality possible can significantly help your dog through their recovery process. 

Due to the fact that the underlying cause of dog mast cell tumors remains unknown, prevention can prove to be nearly impossible. However, by making sure that you stay aware of any changes you find in your dog is an effective way to catch any disease in its early stages. By being implicitly aware of your dog’s “normal” you can make sure you’re able to recognize when something is off and act accordingly.

From all of us here at Honest Paws, we sincerely hope your beloved four-legged friend feels better soon.


About the author

Jennifer Dempsey

Dr. Jennifer Dempsey is a small animal veterinarian and freelance medical writer. She received her Bachelor of Science from the University of Central Florida and her Doctorate of Veterinary Medicine from the University of Florida (Go Gators!) She has resided in the Orlando area since graduation and has gained years of experience helping cats and dogs live happier and longer lives. As a general practitioner, she has found client education to be one of the most important aspects of day to day life in veterinary medicine. Medical writing has helped her to connect with a larger audience and make sure that pet owners are fully aware of their loved one’s medical condition. She currently shares her home with two rescued mixed breed dogs named Primo and Morgan.



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