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.

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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.

mast cell tumor on a dog
If a mast cell tumor is benign, it will not spread to other organs and can just be removed. © 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.

mast cell tumor in dog eye
Boxers, American Staffordshire terriers, and pit bulls are prediposed to getting mast cell tumors as some point in their lives. © 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.

dog after mast cell tumor removal surgery
This dog is recovering from having two mast cell tumors removed. ©Greg King

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.

For more information on mast cell tumors in dogs, read our other articles on the topic:
About Mast Cell Tumors in Dogs
What Are The Treatment Options for Dogs With Mast Cell Tumors?

79 COMMENTS

  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.

    • I WOULD ALSO LIKE MORE INFO ON HOLISTIC TREATMENTS – I AM FEEDING CBD, MUSHROOM BLEND, BENADRYL, TURMERIC, GINGER, FLAX SEED OIL, PROBIOTIC, ETC…. THE LOW GRADE MCT HAS RECURRED FOR A THIRD TIME AFTER SURGICAL REMOVAL BUT IS ON THE BACK PAW WHERE IS NOT MUCH TISSUE.

          • Hi! Just recently discovered my dog has one on its leg and it’s also spread.. just curious what you choose to do?

          • Hi Ashley I didn’t know if you were commenting on my post but I didn’t choose to do chemo I brought her to tufts in Walpole mass they really couldn’t do much they did offer chemo and did a liver aspiration and when they did that it made everything worse ,, they should have gave her Benadryl before aspiration so if u do aspirate make sure they give Benadryl!!!!!she is on prednisone and Benadryl and it was helping at first but today her leg is swollen were the cancer has spread into her lymph node so now we have to come to terms on what to do it’s been a night mare ! And it all started w a little cyst on her toe and my vet should have done biopsy and they didn’t ,, and I think to my self maybe they could have bought it early .. best of luck to you ,, does your dog cave mast tumor?

          • Hi Linda, Have you heard of the Tagamet/Benadryl protocol? My 7 yo female Labrador has a large MCT on her lip. Vet said inoperable. I found the protocol and began it 2/9/21. We’ve seen the mass shrink about 50%. If you’re on FB, look up the protocol by Vera. She’s very responsive and the page has been very helpful!

    • 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.

    • My sweet 9 yr Boson Terrier, Molly, had a small, pea size (maybe tad bigger) wart looking bump on lower back area. First noticed something sev months ago, reminded me of pencil eraser or smaller- no growth. In early summer months it chgd some, so 5 weeks ago took him to vet, she thought it was nothing but would biopsy if I wanted ease of mind. I did. 9/19/19 & a clean cut, 3 days later found it was grade 3 MCT 😢 in past week noticed a very small bump on forehead, had it I think 2 yrs ago but went away on own. And 4 days notice another small pin head size bump on thigh.
      I’ve been giving her Benadryl for a few years, had skin allergies. Going to try & get her in vets, need to know if it’s MCT or maybe something else. & if or when she should take Pepsi’s/prednisone. Prior to removing first bump I started giving her turmeric, only because I thought it was good for her. Anyway I’ve been very sad past few weeks. Grade 3 is suppose to be most aggressive. 💔

    • I write my current story w/my sweet loving BosonTerrier below. Molly had a small wart looking thing removed- & biopsied. Clean edges so got all of it. However it came back grade 3. If u look below there is more Hx I will say in past week I see 2 more bumps, very tiny. Maybe size of small seeds. On head larger of 2 sunflower size bump. Seeing vet next week.
      Plz read what I wrote below and if you can give me even a close prognosis- & what I can do mean while, I’d truly appreciate it.
      Thank you! 💕

      I recently lost my dad to a very aggressive/deadly skin cancer- Merkel Cell. I got him into a trial, extended his life 2.5 years but hardship to get to treatments. This reminds me of that a bit. Aggressive & malignant.

    • I would live to know more on your methods, please. Our almost 17 (human years) year old Jack Russell x Shih-tzu has developed one near his ear that had flared up, and is weepy and gross. The vet said he would not recommend doing anything given our dog’s age.

    • I had to have my beautiful Yorkshire terrier Milly put to sleep 3 days ago, we rescued her from Spain didn’t know how old she really was maybe 3 years, we had Milly nearly 9 years, in 2018 I noticed a wart on her bottom right gum it was removed but never tested, in June 2019 I noticed it had grown back could also feel a lump in her neck,she had both removed then the worst call I got from the vet to say it was an aggressive mass cell tumour, the vet said it would come back over time, surely it did and was very aggressive in July 2020, they couldn’t operate this time so put her on steroids, with steroids it was still growing and fast, the size of the tumour in her neck was taking over her little body it was also rock hard, the one on her bottom gum and lip was the size of a golf ball, it also started to weep and smell, she was loosing all interest in walks 2 weeks ago she ate her food but I think that was down to the steroids she couldn’t chew she just gulped it down, she would cough and breathing up her nose was like she was honking it was an awful sound, Friday 4th December at 3pm my beautiful girl went to sleep at the vets, I couldn’t let her suffer anymore, she was messing more in the house and had terrible smelly runny poo, I could see she wasn’t happy anymore and was quickly going down hill, I feel so guilty that I let her go on this long as I didn’t want to let her go, I’m heartbroken and feel so lonely without her, 💖

      • Awww karen i feel your pain and sadness as im going through the same thing. I have a 10 year old Bichon Frise and he as been my world for a decade. He has lumps on his body and a big one on his neck. Ive never had them tested as didnt want to know and put him through chemo and pain. He has had them 2 years but he has started scratching the one on his neck so its bothering him. I know its just a matter of time and i wont let him suffer but i dont know how i will live without him. Its just him and I in the house so i dread that day.x

      • I’m so sorry for your loss:'( my best friend and beloved Yellow lab/Boxer mix Jackie Brown was diagnosed with a MCT last week she is 13 and on diphenhydramine and Prednisone and the tumor suddenly tripled in size and over the last 4 days appears to have disappeared. Not sure what to make of it but i changed her diet to strictly protein and no o carbs of sugars and also give her cbd and cannibis for pain. She seems fine but i am absolutely terrified.

    • I agree – when my AMSTAF first got an MCT, (about the size of the tip of my thumb) I asked for it to be removed (I am glad that there was no histamine release) – the cut of the margins left my pup with staples across his entire backside!! (a shock to see for a tumor so small). I was not really given any real explanation or advice or treatment options.

      Years went by with no return, then another popped up (close to his backside again), we opted for another surgery – as that one was healing, another popped up (on his flank). I couldn’t continue these surgeries so they offered Benadryl and prednisone – I wanted these to be the last resort as he was an older dog though very active and felt that these medicines were only masking the root cause and could make him fat and sleepy.

      I sought out holistic care.
      We stopped feeding him dry dog food (these foods over time can increase a histamine/inflammation issue), and instead, I started feeding him a blend of cancer-fighting vegetables and cooked human grade meats (blended for absorption, and frozen in ice cube trays for convenience) – eventually, he did get a lil bit of grain-free, high-quality kibble to help maintain his weight.

      I started seeing a holistic vet who prescribed vitamins (Vit A was a big one), as well as Chinese medicine and acupuncture.

      I know that sounds like a lot or questionable, but in less than 3 weeks, the tumor on his flank healed over and disappeared completely – he never had another one flare-up anywhere for the 3 years he lived after that. (he had a different tumor inside a joint rupture when he was 14.5yrs). In that time he remained a healthy and active dog – even jumping into the car and playing the sad day that I had to put him down because of the rupture.

      Knowing what I do now – I would make every effort to feed my dog a whole diet (not just kibble) and focus on getting at the inflammation issues that can lead to this histamine response from day 1.
      Hope this is a lil helpful for all those below who are interested in what holistic options are. I’m not a vet, just a pet owner who has seen the benefits of chinese and alternative medicine work wonders in humans as well as animals (and not anti-conventional medicine, but prefer an integrative approach)

    • In Februari 2011 our small poodle got a lump in her neck, our vet suggested to wait how it would develop. Eventually it grew to a 2 inch mass in a few weeks, but our vet not wanted to poke it with a needle and removing wasn’t his first choice either. For the same reason as Dr. Elisa Katz.
      Now, almost 12 years later, the 2 inch tumor is still there. Not growing, not hurting her and there never came another one. Our poodle girl sees her favorite vet twice a year. She is a very active and healthy lady with her upcoming 17th birthday next month. 🥰
      Greetings from the Netherlands.

  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?

    • Hoping so… our pets become our family. My kids are 30’s early 40’s and I’ve been by myself for many years. I got her 6 months after I had to put asleep my 11 yr old Boston.
      Plz keep us updated. Hoping & praying she lives 8+ more years, w/good health. 💕

  3. 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.

  4. 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.