12 Pictures of Mast Cell Tumors in Dogs [With Vet Comments]

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This article was updated on October 10th, 2023

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Mast cell tumors are the second most common cancerous tumor seen in dogs; they account for 16-20% of all tumors. And the effects of even the tiniest mast cell tumors can be a matter of life and death.

These tumors have the potential to spread throughout the body incredibly quickly. Once they spread, they have the potential to destroy the body. So, it’s particularly important to recognize and diagnose these tumors early.

In this article, our veterinarians have put together a collection of 12 pictures of mast cell tumors in dogs to help you understand what these tumors can look like.

dachshund getting a physical exam with owner

Pictures of Mast Cell Tumors in Dogs

There is no way to tell if your dog has a mast cell tumor by its appearance alone: in fact, in veterinary medicine, these tumors are known for their unpredictable appearance. They can look like something harmless, even in their most life-threatening forms. Let’s look at a few different types of mast cell tumors:

1. Small mast cell tumor

The tumor below is a small mast cell tumor. As you can see, it looks harmless and could easily be mistaken for a sebaceous cyst or lipoma. But don’t let its small size fool you: small tumors can be high-grade with the potential to spread quickly.

mast cell tumor grade 1
This is a picture of a small mast cell tumor. The tumor is slightly raised and flesh-colored and could be mistaken as a cyst.
mast cell tumor on a dog
This is a picture of a small mast cell tumor that is small and flesh-colored.

Small mast cell tumors can appear:

  • Red or flesh colored
  • Raised or flat
  • Ulcerated

Small tumors can feel:

  • Hard or soft
  • Fixed or mobile

2. Medium/large mast cell tumors

Below is a Pit Bull mix with a large mast cell tumor. The tumor appears damaged from the dog chewing and licking it. Tumors of this size are usually cancerous, high-grade, and at risk for metastasis (spreading). High-grade tumors need protection. A great way to prevent further damage is to put an e-collar on your dog. E-collars block the dog’s ability to reach the area and cause damage.

Large mast cell tumor that the dog chewed on, exposing the underlying tissue.

Large mast cell tumors look like:

  • Large lumps
  • Ulcerated skin on or around lumps
  • Bleeding
  • Itching
  • Oozing skin

Large high-grade tumors can cause:

  • Nausea
  • Vomiting
  • Weakness
  • Diarrhea

Pictures by location

On a dog’s paw

closeup image of a mast cell tumor on a dog's paw

On a dog’s face or eyelid

Mast cell tumors near the eye are usually treated locally. However, they can spread to other parts of the body. Pictured below is a tumor on the eyelid of a senior Golden Retriever.

mast cell tumor on a dog's eyelid

On a dog’s nose

Mast cell tumors on the muzzle (nose) are usually aggressive. These tumors have an increased risk of spreading.

red mast cell tumor growing near a dog's nose

Is it a mast cell tumor or something else?

Keep in mind that it is usually not possible to diagnose a tumor or growth just by looking at it: your veterinarian will likely want to do an FNA or biopsy to confirm diagnosis. For example, the picture below shows a possible cancerous growth that has ulcerated and is very inflamed.

The skin appears to be necrotic (dead) and considerations for tumor type would include a melanoma, mast cell tumor or squamous cell carcinoma.

Possible squamous cell carcinoma, melanoma, or mast cell tumor.
Possible squamous cell carcinoma, melanoma, or mast cell tumor.

The picture below shows a pink and raised bump that is scabbed over and ulcerated in some areas. There also appears to be some flaked skin or keratin.

It could be that this is a benign wart or skin tag, but given how the surface is ulcerated, we should be concerned that there could be something more sinister going on here (such as a tumor like a mast cell tumor). The only way to confirm diagnosis for sure is to sample it.

benign wart or skin tag, or cancerous mast cell tumor
Possible skin tag, wart or mast cell tumor

To learn more, view our article featuring the most common types of lumps and bumps in dogs (with pictures).

Pictures of the surgery

While no two dogs will have the same surgery and recovery, knowing what to expect when your dog comes home from surgery is helpful. Here are some photos of what it may look like before, during, and after surgery.

Pre-surgical marking

In the image below, a veterinarian marked the skin around a mast cell tumor using a black surgical marker.

This tumor is circled to guide the veterinarian’s incision during surgery.

The mark is a guide to tell the surgeon where to cut during surgery. During surgery, the goal is to remove wide margins of skin around the tumor, decreasing the chance of tumor re-growth.

However, sometimes there isn’t enough room to remove extra skin. In this case, the veterinarian injects a steroid into the tumor. The steroid will shrink it, making more room to cut around the tumor.

Your vet will also recommend a pre-surgical screening. These include a physical exam, blood work, and a surgical consultation. During the consultation, your veterinarian will explain the plan for surgery and answer any questions you have.

Recovering from surgery

The dog in the photo below is getting his sutures checked. Sutures are typically removed 10-14 days after surgery, but large incision sites and ones in odd places may take longer to heal.

This dog is being restrained to check his sutures

Sutures need to be clean and dry to prevent infection. Your dog may benefit from an e-collar or wrap to prevent them from licking the incision during the healing process.

Licking and chewing are the most common reason for suture failure, so watch your dog closely during this time. If you have an e-collar you can remove it when your dog eats and drinks, but make sure you watch them. All it takes is seconds for a dog to chew its incision open.


After surgery, your dog will have a recovery period before they wake up from anesthesia. The veterinarian will monitor their vital signs and ensure the incision is clean and protected. The dog in the photo below is in the process of waking up from a tumor removal surgery. After removal, the entire tumor, including the skin around it, goes out for biopsy.

At the lab, a pathologist will examine the edges of the sample for cancerous tissue. If the tissue sample shows cancer cells around the edges, your vet will recommend another surgery. Most tumors that return are high-grade and at risk of spreading. In these cases, veterinarians recommend a combination of medications, chemotherapy, and radiation. If your dog has symptoms related to the tumor, your vet may prescribe medications.

The most common medications used for dogs with mast cell tumors are:

  • Steroids
  • Anti-nausea medication
  • Allergy medication

Your vet may also send your dog home with devices or aids to help during healing like:

  • An E-collar (Elizabethan Collar)
  • Wrap
  • Bandage

Throughout the healing process, your dog will need periodic exams. During the exam, the veterinarian will check for the following:

  • Swelling
  • Infection
  • Bleeding
  • Failed or ripped sutures
  • Any sign that the tumor may be coming back.

Picture of a dog with a wrap on his leg after mast cell tumor surgery

Each dog will have a different recovery. The length of recovery depends on the severity of the tumor. Dogs with higher-grade tumors may have a tough time recovering, while dogs with low-grade tumors may recover quickly. During recovery from surgery, wraps should stay clean, dry, and intact. Dirty, wet, or worn bandages can cause secondary infections. In the picture below, a senior Labrador Retriever is recovering from mast cell tumor surgery. His back leg has a wrap to prevent licking and protect it from damage and bacteria. Your dog may not have a wrap, depending on where the tumor is. 

lab with mast cell tumors
Senior dog recovering from mast cell tumor surgery.

Below is a picture showing a senior Labrador drying off after a swim. The dog lost her ear to a mast cell tumor and is a courageous survivor!

A senior Yellow Labrador Retriever lost her ear to a mast cell tumor.

Life expectancy:

The life expectancy of a dog with a mast cell tumor varies and depends on the grade, stage, and location. Dogs with tumors in areas that are hard to treat (mouth, eyelid, and groin) will have the worst prognosis. For these dogs, surgery is not an option. They will benefit from chemotherapy, radiation, and medication. Out of the dogs who have low-grade tumors, 90-100% of them never come back. Dogs with high-grade tumors have a median survival rate of only six months after surgery. Learn more.

Mast Cell Tumor Grades: What Do They Mean?

There is no reliable way to know what grade a tumor is by looking at it (small mast cell tumors may be high grade, and larger tumors may be low grade). However, grading helps us tell how a tumor will act, grow, and respond to treatment. It will also help your vet determine the best treatment for your dog and whether or not to treat the tumor locally (one spot) or systemically (full-body, through chemo, radiation, or medication).

There are three grades of mast cell tumors. The grade is determined when a pathologist looks at a tissue sample under a microscope.

Tumors are graded using two systems:

The Patnaik system:

This system determines the grade by the number of normal mast cells to abnormal mast cells. There are three grades in this system:

Grade 1:

Grade 1 tumors have mostly normal mast cells with a low chance of spreading throughout the body. Surgery is the best option for these tumors. While many dogs recover with surgery alone, there is always a chance tumors could return. These dogs should have regular exams to check for any new tumor growth.

Grade 2:

These tumors have some normal and some abnormal mast cells. The abnormal cells may eventually spread into deeper layers of the skin. Surgery, radiation, and chemotherapy are the best option for grade 2 tumors. Dogs with grade 2 tumors are more likely to need repeat treatment.

Grade 3:

These tumors have mostly abnormal mast cells and are the most dangerous type. They have the highest chance of spreading throughout the body. These dogs will need surgery, radiation, chemotherapy, and supportive medications. Many dogs with grade 3 tumors will also need palliative care at some point in their life. Palliative (comfort care) is supportive care given to terminal cancer patients.

The Kiupel 2-tier grading system:

This system is similar to the Patanaik system, but it breaks the tumor types into two categories:

  • High grade- highest chance of spreading; life expectancy is four months after surgery.
  • Low grade- lowest chance of spreading; dogs often live a normal life after surgery.

Picture of the rupture of a Mast Cell Tumor

Below is a photo of a mast cell tumor chewed open by a dog. Mast cell tumors release a substance called histamine that causes itchy skin. Some dogs itch so badly that they end up chewing their skin raw and creating a wound. The best way to prevent tumor rupture is by covering it with a clean wrap or putting an e-collar on your dog. To learn more about what to do when a mast cell tumor ruptures, read our article: Dog’s Mast Cell Tumor Itches, Bleeds or Bursts.

open mast cell tumor on a dog
This mast cell tumor has ruptured, exposing underlying tissue.

Early signs of Mast cell tumors in dogs

Because mast cell tumors look like many other skin conditions, it is hard to narrow the symptoms down. These tumors can look and act like ANY other skin condition! However, mast cells have one characteristic symptom that sets them apart from other skin conditions; they tend to change in size and appearance and grow larger or smaller from one day to another.

The earliest symptoms include:

  • Lump or lumps that are itchy, or not
  • Enlarged lymph nodes
  • Irritated skin

Symptoms of more progressed mast cell tumors are:

  • Welts
  • Vomiting
  • Diarrhea
  • Weight loss
  • Bloody or black, tar-like feces
  • Abdominal pain
  • Collapse

When should I call my vet?

You should make an appointment with your vet when you notice a new lump on your dog. While many lumps are benign, it is always possible that your dog has a malignant tumor. The quicker you act, the better the chance your dog will live a long life.

Mast cell tumor or histiocytoma?

A mast cell tumor is a cancerous tumor that forms when mast cells (immune cells) over-replicate. They come in all shapes and forms, but can sometimes look like histiocytomas (read our article: histiocytoma or mast cell tumor? or view our pictures of histiocytomas). Veterinarians are usually not able to diagnose a lump or bump just by looking at it: diagnostic tests are often required to confirm diagnosis and determine the right treatment.

Learn more about mast cell tumors:

veterinarian examining dog's skin with a light Mast Cell Tumors vs Histiocytomas in Dogs [10 pictures] - According to research, histiocytomas make up 15.9% of tumors found in dogs, while mast cell… [...]
dog on operating table to remove mast cell tumor The Stages of Mast Cell Tumors and Life Expectancy [With Pictures] - When you find a new lump on your dog, it’s easy to dismiss it as… [...]
labrador fighting mast cell tumors Mast Cell Tumors In Dogs: A Veterinarian’s Guide for Owners - Mast cell tumors are one of the most common forms of cancer in dogs. They… [...]
labrador fighting mast cell tumors What is My Dog’s Life Expectancy With Mast Cell Tumor? A Vet Weighs In - Just this past week, I cared for a dog who was recovering from a large… [...]
labrador fighting mast cell tumors What to Do When a Dog’s Mast Cell Tumor Itches, Bleeds, or Bursts - Mast cell tumors are a common skin cancer in dogs, accounting for 20% of all… [...]
labrador with mast cell tumors When to Euthanize Your Dog with Mast Cell Tumors [Veterinarian Advice] - The worst part about owning a dog is that their lives are far too short.… [...]


  • Amber Cheney

    Amber has worked as a Veterinary Technician in the United States for over ten years. She is also a certified dog trainer and specializes in fear-free patient care. Her goal is to share her veterinary knowledge with pet owners, so they can make the most informed decisions about their pet's health care.

Disclaimer: This website's content is not a substitute for veterinary care. Always consult with your veterinarian for healthcare decisions. Read More.

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