Mast Cell Tumors in Dogs: Is It Always Cancer?

Mast cell tumors are common in dogs. Early detection and staging is critical to diagnoses and successful treatment.


Mast cell tumors (MCTs) are one of the most frequent skin cancers seen in dogs. Mast cell tumors are the reason why careful monitoring of any skin growths is essential for maintaining a healthy canine. Any new masses on the skin should be evaluated by your veterinarian. In regards to MCTs, there are several predisposed breeds including Boxers, American Staffordshire terriers, and pit bulls.

Mast cells are important in the immune system, particularly in allergic reactions. They are found predominantly in the skin, but they are also found in lower numbers throughout the internal organs. Rarely are they found in the bloodstream. These cells are filled with substances such as histamine and heparin. During an allergic reaction, they degranulate – meaning they empty their contents onto or in the area of the offending allergen. The effect of mast cells can be seen when a patient develops hives and welts, as well as itching and redness.

As with any cell in the body, mast cells can develop cancer. The word cancer merely means the uncontrolled proliferation of cells. It can be divided into 2 broad categories – malignant and benign. Malignant cancers can be locally invasive and damaging, spread to other organ systems like the lungs, or both. Benign tumors do not spread to other organs and are cured by removal.

tumor on a dog
iStock/ Getty Images Plus/ Elen11

The symptoms of a MCT on dogs begin with a skin mass, most of the time (in rare circumstances, they can start in the internal organs, but this is more common in cats). They can be as small as a pea or as large as a softball. One important aspect is that they tend to wax and wane. They can start as small and suddenly become large, red, and irritated or weepy. This is a sign of degranulation, meaning the tumor has become irritated and released the nasty substances within it. The hallmark of a mast cell tumor is a tumor that grows and shrinks periodically.

If you note a skin mass on your dog, it should be checked by your veterinarian. As with any veterinary visit, your dog should have a nose-to-tail examination including weight and vitals, followed by a detailed history. Your veterinarian will ask how the mass has behaved, how long it has been present, and if it has changed significantly. They may also measure it with calipers.

After a history and physical exam, your veterinarian will focus on the mass with gentle palpation. It is likely they will recommend a fine needle aspirate. This involves taking a very small needle and obtaining a sample of cells from the tumor.  Another option is just having the entire mass removed and submitted for testing (excisional biopsy).

If your veterinarian is suspicious of a mast cell tumor and wants to sample it, they may recommend pre-medicating with Benadryl, an antihistamine. As we discussed above, one of the substances found in mast cells is histamine. Giving Benadryl may help prevent the tumor from degranulating during sampling. Sudden degranulation can cause a systemic reaction (anaphylaxis) and can be very serious or even life-threatening. Your veterinarian will handle any suspected MCTs gently, as a result.

Diagnosis is generally by a veterinary pathologist. MCTs are graded on two different scales – the older Patnaik scale (giving a number I through III with I being the least malignant), and the newer Kiupel system that simply evaluates high grade versus low grade.

dog tumor
iStock/ Getty Images Plus/ cynoclub

If your dog is diagnosed with a mast cell tumor, your veterinarian will recommend staging as the next step. This means determining if the cancer has spread by conducting bloodwork, urinalysis, chest xrays, and abdominal ultrasound. Once staging is completed, a clearer picture of prognosis can be seen.

Treatment of mast cell tumors in dogs involves initial surgical removal followed by evaluating whether the whole tumor was removed. If it wasn’t (called “dirty margins”), a second surgery may be needed. Radiation is also an option when the entire tumor wasn’t removed. It may seem “simple” to remove a whole tumor, but sometimes the cancer cells have infiltrated the surrounding tissue on a microscopic level. For low grade (Kiupel system) or grade I-II (Patnaik system), usually removal is sufficient if the margins are clear. Even with removal, a dog will be at higher risk for developing MCTs again.

With high grade/grade III tumors, following surgery, oncologists recommend chemotherapy. This is usually administered by a veterinary oncologist. If chemotherapy is not pursued, a dog with high grade MCT will likely stay on Benadryl and steroids to suppress the MCT until symptoms become too severe. As the disease progresses, the mast cell tumor will spread to distant sites like the liver, spleen, and lungs. Symptoms will correlate with the system that is affected.

A mast cell tumor is not the end of the world, but they can be very serious. It is important to notify your veterinarian when you find any skin masses so that they can be promptly evaluated. Early detection and treatment are critical to a successful outcome.


  1. Surgical removal is not always best! I have seen horrible consequences with surgical removal in recurrence with a vengeance even leading to the euthanization of a patient. Modulation of the immune system with natural remedies – herbs and such should be the mainstay of treatment. Whenever you remove the “primary tumor” you risk causing tumors in the same or other places. It seems to me that this article was not written from a holistic perspective, but from a conventional one.


    • Elisa, I would be interested in knowing more about what I can do for my dog in terms of diet and herbal remedies if you are willing to provide your insight. Harley is a 4 year old frenchie that had her first mast cell cancer removed (left ear) in 11/2018. She had a second (back leg) removed just 3 weeks ago and today I found another suspicious lump above her right back foot heel pad. Her staging has been negative (ultrasounds, aspirates, chest X-ray) and both tumors have been a grade 2/low. I want to do more for her if I can! Thank you! Amy

    • Many of us are looking for holistic treatment available for dogs. Can you please give recommendations?

      My 20 pound Schnoodle was just diagnosed with a mast cell tumor today and will be undergoing removal this Thursday.
      If I can get some holistic treatment into her after the surgery I will feel much better. Can you recommend what to give her for her weight and for how long?
      Thank you.

  2. My little pug has 2 mct’s half way down on his back leg. They have been there almost all of his life. They are small, about the size of the end of my little finger. They are light in color, almost white and are flat. At one time they were a little swollen, but since I put him on antioxidants and both probiotic and prebiotic’s plus immune defense herbs plus a change in diet, then adding fish oil, acv, and curcumin they have remained the same size and flat. Does this mean that his mct’s are most likely benign?

    • Could you tell me more about the treatments you used . Just today I had a biopsy done on a small pinhead size tumor that’s growing on my dogs ear. His brother had multiple mast cell tumors that spread to his liver and I lost his over a year ago. They were removed once and then came back with a vengeance a year later. Then a year after that he was gone. Brody is my service dog. He has terrible skin allergies so he is producing lots of histamines. Today I got him on an allergy med and antibiotics to get the skin irritations under control. I won’t know about the tumor for a week. But I have been giving him probiotics, fish oil and quercetin for years.

    • Hi our dog was diagnosed with a mast cell yesterday. The doctor aspirated the bump and determined it’s supposedly mild. We scheduled surgery for Thursday. No I’m scared that it might making it worse reading these comments. How long has your dog been diagnosed with mast cells and did they also determine it with extracting tissue with a needle?

      • My dog had 3 MCT’s removed 4 years ago, they were low grade, today she had another one removed, she is an 8 year old AMSTAFF . As I find them I will keep getting them removed.

  3. Lost my lovely Holly 3 yrs ago
    Nasal cancer
    She started with nose bleeds
    Sadly we had to make the most heart breaking decision and have her put to sleep

  4. I noticed a lump on my dachshund and have read the material on mast cells. I sched. an appt. with the vet on 7/24/2019 and hoping for the best. Reading the articles on cancer and the lump that is on her has enlightened me and praying that she will be fine. She comforted my mother when she had colon cancer and me during my mother’s death and I must do the same for my friend, my dog.

  5. My sweet Paxton a Puggle was diagnosed a week ago with mass cell. He is almost 16 so they would not even consider surgery. This lump appeared out of nowhere. He is one of the sweetest most loving dogs we have ever had. The cancer seems to be taking a real toll on him. They gave him cortisone and he has been on antihistamine. It went down a little but I noticed him limping today. There are now two more lumps very close to the original. I can’t see a good out come at this point. Bless his heart it’s so hard to watch and not be able to do anything.

    • You are doing what you can, Leslie. We just found out our sweet pitbull has a mass that is cancer. We are walking right beside you.

  6. My 13 terrier mix is having surgery in a week for a mct on her belly area. I hope this is the right decision. I’m so worried about the anesthesia/surgery etc. she’s been on prednisone, Benadryl and Pepcid for 10 days and not seeing much change. She seems to be acting absolutely normal, eating, drinking, playing etc
    Would she be seeming “sick” is it were a high grade?

  7. Rita my staffordshire had a large lump behind her front leg. We had it removed and now there is about 10 new lumps growing in the same spot, 6 weeks after removing. Took her to the vet today and basically was told that treating it with strong antihistamines is the only option. The first mass removed was very large and the vet said he does not recommend recutting a larger area and more grow back.

  8. Please also try the combination of CBD capsules around 45 mg a day ( My dogs weights 65 lbs ) and Graviola capsules ( 1400 mg ). My dog had autoimmune disease and VET recommended chemotherapy. Within two months ALL symptons were gone ( she was very weak, pale gums, tongue, couldn’t walk much ). Now 2 years after I just noticed a skin bump…I will start with the Graviola and CBD again. Please research it.

    • Can you give me more detail information for CBD oil and Graviola Capsules?
      I have a mixed breed male dog has a lump now.
      I had taken him to vets several times, all vets said it is just a fat pocket.
      I have scheduled an appointment with oncologist next week.
      I already met a surgeon for removal surgery but not recommended due to size in large.


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