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Mast Cell Tumor (Mastocytoma) in Dogs

What are Mast Cell Tumors?

Mast cell tumors, also known as mastocytomas, are the predominant form of skin cancer observed in dogs. These tumors originate in the connective tissue throughout the dog’s body, where mast cells, a type of white blood cell crucial for the immune system’s allergic response, are located. When mast cells undergo abnormal division and form tumors, they become cancerous. Mast cell tumors can often be misinterpreted as other skin abnormalities such as warts or benign lumps due to their diverse appearance in terms of shape, firmness, size, and location. Typically, they manifest as firm, solitary masses on the skin that grow slowly. Additionally, mast cell tumors in dogs have the potential to trigger severe allergic reactions, known as anaphylaxis. Despite their prevalence, identifying mast cell tumors solely by visual examination, even for veterinarians, is challenging. These tumors are more commonly found in middle-aged dogs, with certain breeds like Boxers and Boston Terriers exhibiting higher susceptibility. Consequently, it’s advisable to promptly seek veterinary evaluation for any unusual skin masses observed on your dog.


Symptoms associated with mast cell tumors may not always be apparent. To help detect potential issues early, veterinarians advise periodically examining your dog’s skin for any masses. If you notice any of the following, it’s recommended to schedule an appointment with your vet:

  • Discovery of a new skin mass
  • Changes in the size or color of a known mass
  • Unexplained allergic reactions or hives

Some masses may appear small and easily movable under the skin with minimal swelling, while more aggressive tumors may present as larger, hairless sores.

In certain instances, mast cell tumors can manifest symptoms, particularly strong allergic reactions triggered by an activated immune system. Furthermore, if a tumor is disturbed or bumped, it may undergo “degranulation,” releasing its inflammatory contents all at once. Degranulation can lead to symptoms such as swelling and redness in the affected area, or in severe cases, anaphylactic reactions characterized by:

  • Weakness
  • Nausea
  • Vomiting
  • Diarrhea
  • Swelling of the face and limbs
  • Rarely, collapse and death


The exact causes behind mast cell tumors in dogs remain unknown. Similar to other forms of cancer, it is likely that a combination of genetic and environmental factors contributes to their development. However, insufficient information is available to suggest specific changes to your dog’s environment for preventing mast cell tumors.

Certain breeds exhibit a higher likelihood of developing mast cell tumors. These breeds include:

  • Boxers
  • Pugs
  • Pit Bull / Bull Terrier
  • Boston Terrier
  • Bulldogs
  • Retriever breeds (such as Golden Retrievers)
  • Rhodesian Ridgeback
  • Weimaraner
  • Schnauzers


When diagnosing mast cell tumors, your veterinarian typically begins by creating a skin chart documenting all known masses. Subsequently, they may proceed with a tissue sampling procedure such as fine needle aspiration (FNA) or biopsy for more concerning masses. Many veterinarians can initially identify mast cell tumors through fine needle aspiration.

Fine needle aspiration is a straightforward procedure usually performed without sedation. It involves inserting a needle into the mass to extract tissue samples. Once a few cells are collected, they are examined under a microscope by a specially trained veterinarian pathologist to make an initial diagnosis.

Upon diagnosis, the mast cell tumor is graded based on its level of aggressiveness. Low-grade tumors exhibit less aggression, whereas high-grade tumors are more aggressive. More aggressive tumors are also more prone to spreading (metastasizing) to other locations beyond the original tumor site.


Upon diagnosis, mast cell tumors are typically treated through surgical removal, followed by sending the excised tissue to a pathologist for assessment to determine the tumor’s grade—whether it’s low grade (less aggressive) or high grade (more aggressive). This differentiation is crucial as treatment approaches vary significantly between low- and high-grade tumors.

Approximately 80% of mast cell tumors fall within the low- to intermediate-grade category and are unlikely to recur following surgery. Therefore, surgical removal is often curative for the majority of mast cell tumors.

High-grade tumors have the potential to aggressively metastasize to lymph nodes and internal organs. In such cases, additional diagnostic tests such as lymph node aspirates and chest x-rays may be recommended by your veterinarian. They might also advise consulting with an oncologist regarding potential chemotherapy or radiation therapy.

In situations where surgical intervention isn’t feasible, veterinarians may explore alternative treatments such as needle radiation therapy. However, not all dogs are suitable candidates for non-surgical interventions.

Living and Management

Following surgical removal of a low-grade mast cell tumor, the recovery process typically involves two weeks of rest, administration of pain medications and antihistamines like Benadryl, and the use of an e-collar (commonly referred to as the “cone of shame”). After this period, sutures are removed by the veterinarian, and the dog can gradually resume normal activities.

For several months post-surgery, it’s essential to monitor the area where the mass was excised for any signs of recurrence, although recurrence is rare for most low-grade tumors. In most cases, surgery is curative, allowing the dog to live out its natural lifespan.

In the case of high-grade tumors, recovery after surgery remains similar, but additional treatments such as radiation or chemotherapy are typically recommended by the veterinarian to prevent potential spread of tumors or cancer cells.

Survival rates with radiation or chemotherapy usually range from 10 months to 2 years, depending on the tumor and the specific therapy administered. Without any treatment, survival averages about 4 months.

Mast Cell Tumors FAQs

What is the survival rate for dogs with mast cell tumors?

Survival rates hinge upon the grade of the mast cell tumor. Various grading systems exist, generally categorized into less aggressive tumors referred to as “low grade” and more aggressive tumors termed “high grade.”

For low-grade mast cell tumors, surgical removal often proves curative. While there’s a slight chance of recurrence, most dogs typically enjoy their natural lifespan post-removal. Conversely, with high-grade mast cell tumors, survival time following surgery alone averages around 4 months.

When additional treatments like radiation therapy or chemotherapy are employed, the average survival time extends to 1 year or longer. Survival rates for high-grade tumors can significantly vary based on the chosen treatment option and the unique characteristics of the tumor.

Are mast cell tumors in dogs always cancerous?

Yes, all mast cell tumors are deemed cancerous, thus any identified mast cell tumor should undergo surgical removal if feasible. However, it’s important to note that not all mast cell tumors will metastasize, nor will they all necessitate chemotherapy. These outcomes are contingent upon the grade of the tumor at the time of removal.

How serious are mast cell tumors in dogs?

Mast cell tumors in dogs are considered quite serious. Without treatment, they can potentially induce anaphylactic shock. Moreover, if they advance into a more aggressive form, they may metastasize and ultimately result in death.

Does a mast cell tumor cause my dog pain?

Mast cell tumors typically do not cause pain in dogs unless the pet exhibits symptoms resulting from a tumor-induced allergic reaction.

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