Mast Cell Tumor Dog: An Introductory Guide

By Chelsea Hunt-Rivera / May 28, 2018
Mast cell tumor in dog

Before we begin, let’s cover a couple of terms and concepts that you’ll need to know in order to better understand the following article on mast cell tumors in dogs.

dog with massive tumor

What Are Mast Cells

Mast cells are cells that originate in the bone marrow but mature and settle in the connective tissues in the body. Mast cells particularly reside in nerves and vessels that are closest to external areas such as the skin, nose, mouth, and lungs but are found in all tissues of the body.

What Do Mast Cells Do

mast tumor

Mast cells serve several important functions in the body including providing a defense against parasitic infestation, aiding the body in repairing tissues, and a primary role in the formation of new blood vessels (known as angiogenesis).

Additionally, mast cells communicate with cells of the immune system to produce antibodies as part of an allergic reaction defense. Mast cells act as a cellular barrier to protect the body against potentially harmful, external agents.

What is a Tumor

A tumor is a swelling of a specific part of the body, typically without inflammation. The swelling is caused by a growth of tissue that is labeled either malignant (dangerous) or benign (noncancerous and often harmless).

What is Mast Cell Tumor (Mastocytoma)

Mast cell tumors (MCTs) are also called Mastocytoma. MCTs are the most common skin tumor found on dogs. Approximately 1/3 of all tumors are skin tumors, and an estimated 20% are mast cell tumors. Mast cell tumors are cancerous proliferation or reproduction of the mast cells.

When the cancerous tumors invade, they inhibit the roles that mast cells are responsible for, such as protecting against parasitic infestation, tissue repair, and aiding to produce new blood vessels. Mast cell tumors also affect the dog’s heart rate, blood pressure, and overall homeostasis in the body.

Because Mastocytoma in dogs is so common, it is important for pet owners to know as much about them as they can. Canine mast cell tumors can have a low level of malignancy and be relatively harmless, but they can also have a high level of malignancy and can be life-threatening.

Where are Mast Cell Tumors in Dogs Found

Mast cell tumours are typically found in the skin’s surface, but can also exist in large numbers in other places such as the dog’s intestines and lungs.

Also, while MCTs are most commonly found on the dog’s skin, they can also quickly spread, typically to the spleen, liver, and bone marrow.

MCTs on the skin surface are often on the dog’s limbs and trunk. However, mast cell tumors can be difficult to recognize as they vary greatly in shape, appearance, size, texture, and location. They can be raised or flat, firm or soft, covered in hair or ulcerated. MCTs can also quickly fluctuate in size. They can become smaller and then rapidly larger.

The danger of canine mast cell tumors does not actually come from the MCTs themselves, but rather the severe amount of chemicals that they release into the dog’s body. If you notice a new bump on your dog, be sure to get them to a veterinarian as soon as possible.

Symptoms of Mast Cell Tumor in Dogs

symptoms of mast tumors in dogs

The symptoms of canine mast cell tumors vary depending on the location, grade, and stage of the tumor. However, the following are the typical symptoms associated with MCTs.

  • A bump or lump (tumor) on or under the skin.
  • The tumor has been present for days, weeks, or even months.
  • The tumor may fluctuate in size.
  • Inactivity or rapid growth of the tumor is also possible.
  • Recent onset of redness or fluid buildup.
  • The lump or bump may resemble an insect bite, a wart, or an allergic reaction.
  • An estimated 50% of mast cell tumors develop on the trunk of the dog.
  • Enlarged lymph nodes around the area of the tumor.
  • Inflammation or skin irritation around the tumor due to higher histamine levels in the tumor.
  • An enlarged spleen and enlarged liver characterize wide-spread mast cell cancer.
  • Vomiting, diarrhea, and loss of appetite may also be signs of the stage of MCTs progressing.

What Causes Dog Mast Cell Tumor to Form

The underlying cause of mast cell tumors is unknown. However, we do know that the tumors are a result of cell mutation in the mast cells that lead to an uncontrollable reproduction and growth of the mutated (cancerous) cells.

Breeds at a Predisposition for Mast Cell Tumor

Experts have also identified that MCTs are prevalent in certain breeds of dogs.

These breeds include:

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

MCTs commonly occur in older dogs but have been diagnosed in dogs as young as three months. The sex of the dog does not affect the likelihood of the dog developing a mast cell tumor.

Diagnosing Dog Mast Cell Tumor

In order to diagnose the abnormal growth on your dog, your veterinarian will likely perform a fine-needle aspiration and cytology.

This process involves inserting a fine needle into the growth and removing cells of the tumor. The fine needle aspirate will be able to identify the amount of abnormal mast cells (cancer cells).

Additionally, your veterinarian may take a sample from the bone marrow, a kidney, or spleen or by draining a lymph node for further testing as well as order an abdominal ultrasound or x-ray of the chest and abdominal area.

In order to identify the grade and stage of the MCTs, a surgical tissue biopsy will be necessary.

Grades of Dog Mast Cell Tumors

MCTs are graded from one to three. The tumor grade refers to its level of malignancy and helps the veterinarian predict how the tumors will behave in the body.

Grade I

MCTs that are characterized as Grade I are typically benign and occur on the skin. While they may be large and can be difficult to remove, a Grade I MCT will not move to other parts of the body.

MCT Grade II

Grade II MCTs extend below the skin’s surface and into the subcutaneous tissues. The biological behavior of Grade II MCTs can be unpredictable therefore making the treatment of the MCTs also difficult to predict. Surgical removal cures approximately 65% of Grade II mast cell tumors but it is possible for the tumors to return or metastasis (the tumor spreading to other parts of the body) to happen.

Grade III

Mast cell tumors that are categorized as Grade III are typically difficult to treat as they can rapidly spread throughout the body and occur deep below the skin’s surface. High grade MCTs will typically require surgery, aggressive radiation therapy, as well as chemotherapy. The prognosis for Grade III MCTs is not favorable for most dogs.

Stages of Mast Cell Tumor Dog

The stage of the MCTs measures the metastasis, in other words, the extent to which the tumor has spread causing secondary malignant growths, and to what degree the surrounding lymph nodes were affected.

MCT Stage I

Stage I involves one tumor in the skin that results in no lymph node involvement.

Stage II

MCTs with a Stage II diagnosis are categorized by one tumor that has spread into the surrounding lymph nodes causing secondary malignant growths.


Stage III mast cell tumors involve either multiple tumors or a large tumor that has invaded the subcutaneous tissues. MCTs that are Stage III may or may not have lymph node involvement.

Stage IV

Stage IV involves one or more tumors with metastasis in the skin. The metastasis to other internal organs is also common. MCTs that are Stage IV also have lymph node involvement.

Treatment of Mast Cell Dog Tumors

treatment for mast tumors in dogs

Treatment of mast cell tumors will depend on the stage and grade of the tumors.

Due to high histamine levels in the tumor, any manipulation of the tumor may cause the histamines to release into the blood stream. Therefore, your vet may prescribe antihistamines to prevent any symptoms from the release as well as protect the organs. A large release of histamines can cause adverse effects on the dog’s internal system.

In most cases, surgical removal of the mast cell tumor and surrounding lymph node areas is the treatment of choice. A thorough examination of the mast cell tumors can help predict the success rate of the surgery. Veterinarians will also be able to predict the future behavior of the tumor(s). If the cancer cells have spread too close to the surgical margins a more aggressive surgery will be necessary right away. Additionally, chemotherapy may be necessary to prevent further metastasis of tumor cells.

In cases where the mast cell tumor is not in a removable location, radiation therapy is typically the treatment that a veterinary oncologist will use.

Mast Cell Tumor Dog Life Expectancy

tumor free dog

Prognostic factors including the life expectancy of a dog with mast cell tumors are dependent on the grade and stage of the tumor. Additionally, whether the dog receives appropriate, aggressive treatment when necessary also determines the ultimate prognosis.

Dogs with Stage III MCTs, even with the necessary, appropriate treatment, usually survive less than one year.

For this reason, and the series of aforementioned concerns, it is paramount that your dog receives an accurate diagnosis for any new lumps or bumps that appear.


What is mast cell tumor dog life expectancy?

How many stages of Mast Cell Tumors in dogs are there?

Is my dog high risk for becoming a mast cell tumor dog?


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About the author

Chelsea Hunt-Rivera

Chelsea Rivera is a Dedicated Pet Parent who loves to create amazing content for pet owners and is helping change pet wellness as the Head of Content for