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December 10, 2020

Mast Cell Tumors in Dogs: Here’s What You Should Know

Mast Cell Tumors in Dogs: Here’s What You Should Know


Key Points

  • Dog mast cell tumors are one of the most common forms of dog skin cancer.
  • Like many forms of cancer in dogs, we are unsure of the underlying cause of mast cell tumors, but certain breeds seem to be more at risk for developing them.
  • Mast cell tumor dogs often will have a lump on the skin that can look or feel like almost anything.
  • Many low-grade mast cell tumors can be cured with prompt surgical removal, but high grade mast cell tumors have a guarded prognosis potentially requiring radiation or chemotherapy.

What is a mast cell tumor in dogs?

Mast cell tumors are a common type of dog skin cancer. This invasive tumor is made up of mast cells which are a type of cell that is involved in the allergic response. Most dog mast cell tumors appear as a lump on the skin, but this cancer can also affect the bone marrow, spleen, or liver. They are one of the most commonly diagnosed types of dog skin cancer.

What causes dog mast cell tumors?

We don’t know for sure what causes mast cell tumors. Like other forms of cancer in dogs, there are many different risk factors including genetics or environmental risk factors. Certain dog breeds seem to be more predisposed to developing mast cell tumors. According to University of Illinois College of Veterinary Medicine, the breeds most prone to developing mast cell tumors include:

  • Boston Terriers
  • Boxers
  • Golden Retrievers
  • Labrador Retrievers
  • Bull Terriers
  • Pugs

What does a mast cell tumor look like on a dog?

Washington State University Veterinary Teaching Hospital states that mast cell tumors most commonly appear as a lump or mass on or under the skin. Dog mast cell tumors can have varying appearances. They can be mistaken for lipomas which are benign, soft-fatty lumps. They may also look like ulcerated skin lesions. Sometimes a mast cell tumor dog will have a lump that fluctuates in size especially when it is touched or manipulated. One day the lump could be bigger then suddenly it gets smaller over 24 hours.

How are dog mast cell tumors diagnosed?

If you have found a lump on your dog, it is a good idea to get it checked out by a veterinarian. Your veterinarian will likely want to do a fine needle aspirate (FNA) of the lump. This test can easily detect if the lump you are feeling is a mast cell tumor. 

How are mast cell tumors treated in dogs?

The treatment for mast cell tumor dogs is the surgical removal of the lump. Your dog will need general anesthesia so your vet can remove the lump. Often, the incision will look much bigger than the lump because your vet wants to be sure they get all of the cancer. They will often remove wide margins around the lump to be safe. Your dog will be sent home on pain medication to make sure your dog’s pain is controlled post-op. When your pet gets home from surgery, make sure you have a quiet recovery area set up with a nice soft cozy calming bed.


After removing the lump, your veterinarian will recommend sending the tumor to a diagnostic lab for grading. For lower grade mast cell tumors that have been completely removed, your vet will probably just recommend that you monitor for more lumps. For high grade mast cell tumors, your vet may recommend chemotherapy or radiation therapy. 

Mast Cell Tumor Dog Grades

Your veterinarian will order a biopsy of the tumor after removal. This is an important step because it will tell the vet if the lump has been completely removed and will provide a grade. There are 3 grades of mast cell tumors.

Grade I Mast Cell Tumor

This is the lowest grade and also the least aggressive form of mast cell tumor in dogs. This type of mast cell tumor typically does not spread (metastasize) to other locations, so generally, if your vet removes the entire tumor, it should not recur.

Grade II Mast Cell Tumor

Most mast cell tumors that I diagnose as a veterinarian are the grade II mast cell tumor dogs. This type of mast cell tumor is more aggressive than the grade I mast cell tumors. Fortunately, many times, if your vet removes the entire tumor with clear margins, it will not spread to other locations. That being said, there is a small subset of the grade II mast cell tumor dogs that will have a recurrence or invasive spread of their cancer.

Grade III Mast Cell Tumor

This is the worst grade of dog mast cell tumor. Luckily, it is also a less common form. Dr. Wendy Brooks, a veterinary dermatologist, stated that this type of mast cell tumor only accounts for 25% of mast cell tumors. Grade III mast cell tumors are very aggressive and malignant. They frequently move to other areas of the body including the liver, bone marrow, and spleen. Often, dogs with this form of mast cell tumor will need chemotherapy or radiation to control the spread of the cancer.

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

The mast cell tumor dog life expectancy is variable depending on the grade of the mast cell tumor and the location. For low grade mast cell tumors of the skin that are removed promptly and completely, the prognosis is generally excellent. Many of these low grade tumors never recur or spread once removed. If your dog has been diagnosed with a high grade mast cell tumor, the prognosis is guarded with average survival times of 4 to 6 months.

Can dogs live with mast cell tumors?

If your dog has a mast cell tumor, it should be removed by a veterinarian. Unfortunately, mast cell tumors can be invasive dog skin cancers that have the potential to spread to other areas of the body if left untreated. This form of cancer in dogs will often continue to grow and get worse if not treated. As with most cancers in dogs, early detection and aggressive removal of dog mast cell tumors may lead to better outcomes.

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