Mast Cell Tumor

Mast cell tumors are the most common type of skin cancer in dogs. A mast cell tumor is made of malignant, or cancerous, immune cells called mast cells that are found in high concentrations in tissues such as the skin, gastrointestinal tract, and lungs. Mast cells are important participants in allergic responses. They contain inflammatory substances that are released during inflammation, allergic reactions, and tissue healing.

While Portuguese Water Dogs are not considered to have a breed disposition for developing mast cell tumors, and this is generally thought to be a disease of older dogs (the median age at diagnosis is about 9 years old), dogs of any breed and any age can develop mast cell tumors.


A mast cell tumor is typically a lone skin mass, although sometimes the tumors may present as a series of bumps (miliary appearance). While mast cell tumors of dogs most commonly originate in the skin, they can also originate from other organ sites, as these cells normally have access to every organ and tissue of the body.

Not all mast cell tumors look alike. Mast cell tumors may be itchy or non-itchy, variable in color (most commonly white, pink, or red), ulcerated or non-ulcerated, and may grow in size or may fluctuate in size.

Mast cell tumors can release inflammatory substances during a process called degranulation, leading to clinical signs in dogs which may include swelling, itchiness, redness, vomiting, diarrhea, fever, and collapse.

Mast Cell Tumor, created with
Images created with

Systemic symptoms of mast cell tumors may include:

  • Hives
  • Lack of appetite
  • Widespread bruising/swelling
  • Wheezing
  • Vomiting
  • Dark feces
  • Bloody vomit

If you think your dog has any of these symptoms, you should have him or her evaluated by your veterinarian.


Your veterinarian might use a needle to obtain a sample of cells from a suspicious lump to make a diagnosis. A biopsy, where part or all of the mass is removed, can also be used to make a diagnosis.

In addition to a diagnosis of a mast cell tumor, a series of tests to determine whether the tumor is confined to one location or has spread to other locations are used to determine if there is any cancer progression. This process is called staging, and it may include bloodwork, urinalysis, chest x-rays, abdominal ultrasound, and lymph node aspirates. In addition to revealing cancer progression, staging can help determine if there are other disease processed, called comorbidities, that may affect the dog’s treatment and/or prognosis. Finally, there are specialized tests that look for specific genetic mutations or other abnormalities that contribute to the behavior of the tumor and can help guide therapy.


Dogs with mast cell tumors are commonly prescribed medications such as antihistamines and antacids to reduce the risk of degranulation. The outcome for a dog is largely dependent on the features of the mast cell tumor, the number of mast cell tumors, and if there is any spread to other areas of the body. Based on these features, your veterinarian can determine the optimal treatment options.

  • Surgery

If your dog is diagnosed with a mast cell tumor, surgical removal is the treatment of choice. Mast cell tumors vary in their grades, or level of aggressiveness. For less aggressive tumors, complete surgical removal is considered curative.

  • Radiation therapy

Radiation therapy may be pursued if surgery is not feasible or in addition to surgery.

  • Chemotherapy

Mast cell tumors may be treated with chemotherapy/systemic therapy in addition to surgery/radiation therapy.

If you haven’t done so already, you are encouraged to read through our cancer overview article. Other articles on this website address some other forms of cancer that you may find informative.

This article has been written by Courtney Labé and Jaime Modiano, VMD, PhD, Director, Animal Cancer Care and Research Program, University of Minnesota, St. Paul, MN