Understanding Mast Cell Tumors in Dogs

Finding a lump on your dog can be alarming, and receiving a diagnosis of a mast cell tumor (MCT) often raises even more questions. At Cascade Heights Veterinary Center, one of the most common questions we hear is:

“Is this cancer, and what happens next?”

While mast cell tumors are considered malignant (cancerous), the diagnosis is not always as frightening as it first sounds. Many dogs go on to live long, happy lives after treatment. Understanding what a mast cell tumor is and what to expect can help you make informed decisions for your pet.

What Is a Mast Cell Tumor?

Mast cells are a normal part of your dog’s immune system. They play an important role in allergic reactions and inflammation by releasing substances such as histamine and other inflammatory chemicals.

When these cells begin to grow uncontrollably, they form a mast cell tumor. Mast cell tumors are among the most common skin cancers seen in dogs and can develop almost anywhere on the body.

One of the unique challenges with mast cell tumors is that they can look like almost anything. They may appear as:

  • A small, hairless bump
  • A soft lump under the skin
  • A raised pink or red mass
  • A lesion that changes size from day to day
  • A lump that becomes swollen, itchy, or irritated after being touched

Because they are so variable in appearance, every new skin mass should be evaluated by your veterinarian.

How Is a Mast Cell Tumor Diagnosed?

The first step is usually a fine needle aspirate (cytology). During this simple procedure, a small needle is used to collect cells from the mass, which are then examined under a microscope.

In many cases, cytology can identify that the mass is a mast cell tumor. However, cytology cannot determine how aggressive the tumor is.

That information comes from histopathology, which requires surgical removal of the entire mass and evaluation by a veterinary pathologist.

Why Does the Tumor Grade Matter?

After surgery, the pathologist assigns the tumor a grade, which helps predict its behavior.

Low-Grade (Grade I / Low Grade)

These tumors are the most common and generally have an excellent prognosis if complete surgical excision is obtained.  Complete excision is often curative.

Intermediate to High-Grade (Grade II–III / High Grade)

These tumors are more aggressive and have a greater chance of recurring or spreading to other parts of the body. Additional treatments, such as chemotherapy or consultation with a veterinary oncologist, may be recommended.

Fortunately, most mast cell tumors diagnosed in dogs are low grade, which is encouraging news for many families.

Why Is Surgery So Important?

The cornerstone of treatment for most mast cell tumors is surgical removal.

Unlike many benign skin masses, mast cell tumors often send microscopic extensions into the surrounding tissue. For this reason, veterinarians aim to remove not only the visible tumor but also a margin of healthy tissue around it whenever possible.

Obtaining clean surgical margins significantly reduces the risk of the tumor returning.

Depending on the tumor’s size and location, achieving wide margins may not always be possible, but surgery remains the most important first step in treatment.

What Is Recovery Like?

Most dogs recover very well after surgery.

Your veterinarian will typically recommend:

  • Wearing an Elizabethan collar (cone) to prevent licking
  • Pain medication to keep your dog comfortable
  • Restricting activity for approximately 10–14 days
  • Monitoring the incision closely for swelling, redness, or discharge

Because mast cell tumors release inflammatory chemicals, some surgical sites may experience increased swelling or slower healing than other routine skin surgeries. Your veterinary team will provide detailed instructions for monitoring recovery.

Should My Dog Have Cancer Staging?

Depending on the characteristics of the tumor and your dog’s overall health, your veterinarian may recommend cancer staging.

Staging helps determine whether the cancer has spread elsewhere in the body and guides treatment recommendations.

Common staging tests include:

  • Complete blood work
  • Chest radiographs (X-rays)
  • Abdominal ultrasound
  • Occasionally sampling nearby lymph nodes

Staging is particularly important when:

  • A high-grade tumor is suspected
  • Chemotherapy is being considered
  • There are concerns that the cancer may have spread

For dogs receiving comfort-focused or palliative care, staging may not always be necessary.

What Is the Prognosis?

Every mast cell tumor is unique, but many dogs have an excellent outcome.

Prognosis depends on several factors, including:

  • Tumor grade
  • Whether the tumor was completely removed
  • Tumor location
  • Whether the cancer has spread
  • Whether additional therapy is needed

Dogs with completely removed low-grade mast cell tumors often enjoy an excellent long-term prognosis.

When Should You Have a Lump Checked?

One of the most important messages we share with pet owners is this:

Don’t wait to have a lump evaluated.

Many cancerous masses—including mast cell tumors—can look harmless. The sooner a mass is diagnosed, the more treatment options are typically available, and the better the chances of achieving an excellent outcome.

At Cascade Heights Veterinary Center, we recommend that any new lump or bump be examined promptly. A quick needle aspirate is often all that’s needed to determine the next best step.

We’re Here to Help

A diagnosis of a mast cell tumor can feel overwhelming, but you don’t have to navigate it alone. Our team at Cascade Heights Veterinary Center is here to guide you through every step—from diagnosis and surgery to pathology results and, when necessary, advanced cancer care referrals.

If you’ve discovered a new lump on your dog or have questions about an existing mass, schedule an appointment with our team. Early evaluation can make all the difference.