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Featuring Demian Dressler, DVM and Sue Ettinger, DVM, Dip. ACVIM (Oncology), authors of The Dog Cancer Survival Guide

Diagnosing mast cell tumors

Updated: November 27th, 2018

When should you see an oncologist for a mast cell tumor (MCT)? I recommend you get an oncologist involved early. But before you see me, you need to know what a MCT looks like, and how they are diagnosed.

Most dogs are not feeling or acting sick when they are diagnosed with MCT. Usually the first sign of illness is when you or your vet find lump, or mass, on or just under the skin. Most MCT tumors look like raised, hairless, pink bumps – but their appearance can vary widely. Because of this, MCT is called “the great impersonator.” Tumors can look like benign skin tags, or harmless lipomas. The tumor can be ulcerated (an open sore), swollen, and inflamed — or relatively benign looking.

It can also be confusing because MCT can get bigger and bigger, and then smaller — without any seeming rhyme or reason. We don’t typically think of tumors waxing and waning on their own. The reason these tumors do is because histamine is involved. Histamine causes swelling, and as it is released from the tumor cells, it causes inflammation and swelling. When it dissipates, the swelling and inflammation resolves. So it’s not the tumor changing in size, but the inflammation associated with the tumor.

Tumors can be found anywhere, but the trunk and the limbs are the most common locations. Some tumors are present for months or even years with little change in growth or appearance. Others appear suddenly and grow very rapidly.

Some MCT masses itch, so your dogs may scratch or lick them. If your dog chews, scratches, or bangs the tumor against the ground, it can release inflammatory chemicals from the granules. This is called degranulation, and the release of histamine can cause a localized swelling that looks like a hive.

Massive degranulation can also cause system-wide symptoms, like full-body swelling or in very severe cases, anaphylaxis (shock). This is uncommon with a single, smaller MCT. It’s more likely when there is a very large tumor, or if there is metastasis (spread) to internal organs.

Want to learn more on Mast Cell Tumors? Get a copy of this highly informative seminar!

Because of the wide variation in MCT appearance, I strongly recommend that every skin mass be aspirated for a microscopic evaluation. Without this evaluation, you might not catch MCT early.

MCT are typically diagnosed with a fine needle aspirate, which has proven very reliable in confirming the diagnosis. No one – not even experienced oncologists like me – can be sure that a suspicious lump is benign just by feeling it, which is why every skin and subcutaneous mass (those just below the skin) should be aspirated. If the mass cannot be aspirated, a surgical biopsy should be considered.

But for the majority of MCT, a simple and inexpensive fine needle aspirates will typically confirm MCT.  After the aspirate confirms MCT, the next step is typically surgery, especially if we are dealing with your dog’s first MCT and there is only one. If possible, the goal of surgery is to remove the entire visible tumor along with a wide (usually 2 to 3 cm on all sides and a layer of tissue below) margin of surrounding normal tissue. Then the removed tumor is biopsied. The surgical margin must be confirmed with the pathologist’s report. “Clean margins” on the report will mean that the tumor was removed completely. (Remember, there is always a chance that cancer cells remain that we can’t detect with our current technology — but in general clean margins are a good sign.)

Get a copy of the Dog Cancer Survival Guide to read more on Mast Cell Tumors in Chapter 30

The biopsy report will also provide the MCT grade, which we will discuss in the next blog. Grading MCT tumors has always been a challenge, because they are so changeable. There is the classic, 3-tiered system (grade 1, 2, and 3), but also a newer 2-tiered system that rates tumors are “low grade” or “high grade.” We’ll talk more about this in the next blog. We will also discuss who should do the surgery, when should staging like abdominal ultrasound be done, should you include a buffy coat in staging tests, and what the implications are if your dog has more than one MCT.

This and so much more about MCT is also discussed in the Guide.


Dr. Sue

Leave a Comment

  1. mary schroeder on September 17, 2015 at 6:56 am

    thanks for your encouraging words!! i have an appointment monday to have my 6 year old chiweenie biopsied i had gone to my regular vet he could not get any cells out of the aspirator and so he gave her antibiotics for 10 days and metacam and benedryl and never really explained why she had to be on benedryl well i finally got the right info and i made an appointment with another vet who is less expensive because the other place wanted to charge me $700 for the entire procedure this vet is at least 100 to 200 less and highly recommended so actually i really learned a lot by talking with the new vets tech she was the one who explained about histamines and why my baby more then likely needed to take this so i have 2 more days of meds and she goes in on monday morning.the area where this round raised lump is on her backside of hind leg or i think they call it her hock its scabby on top because she licks it occassionally but shehas been prety much not licking that area i have a cone for her neck but it is irritating so i will find a more comfy one. well all i can do is Pray and asking everyone who reads this to say a Prayer that its nothing terrible, it could be a callous!! thanks for listening!! GOD BLESS!!

  2. Susan Kazara Harper on July 24, 2014 at 1:27 pm

    Hi Silke,
    I’m sorry to hear about your girl. She sounds like a fighter.
    Palladia is the first chemotherapy drug approved by
    the FDA specifically for use in dogs (all other chemotherapy drugs are human drugs that we have learned how to use effectively in dogs). Palladia is approved for mast cell tumors, specifically those of grade II and III, with or without regional lymph node involvement. Palladia is a chemotherapy drug, but can be given at home, in tablet form.
    There is also a new drug called Masitinib (or Kinavet CA-1) which can also be given at home. It has also been developed to help fight MCT in our dogs. It has some similar properties to Palladia, and some of the same possible side effects. Dogs ocasionally develop resistance or intolerance to Palladia, in which case they may be helped by masitinib. You may want to ask your vet, or your vet oncologist if you have one, to investigate both Palladia and Masitinib and help you choose one if it is appropriate for your dog. Nutrition is also paramount. If you don’t have it yet, the Dog Cancer Diet is available from the main blog page as a free pdf download, and the full diet is in the Dog Cancer Survival Guide Book. Sometimes dogs with MCT itch as well. If so, you may want to alter her nutrition even further by feeding her low histamine foods. We have a great blog on this at I really hope this helps and that your vet is staying on top of her pain with some good meds. Give your gorgeous Beagle a cuddle for me. Good luck!

  3. Silke Schoeman on July 23, 2014 at 7:57 pm


    Our 11 year old beagle has gotten mast cell for the third time now in two years. The previous times we got it surgically removed and also put her on chemo therapy. She’s more in pain for the treatment than having the actual cancer, so I’m not convinced that going this route for the third time is the way to go.

    I have heard about treating this with “Palladium”. What are your thoughts?

    Please help.

  4. Lauren on November 15, 2012 at 8:32 am

    Our beautiful adopted Shepherd/Rottie who is about 9 years old, as diagnosed two years ago with MCT, grade 3, very aggressive. We decided against chemo and radiation and chose to give her a more holistic approach — quality of life over quantity of life.

    We started feeding her Dr. Harvey’s Canine Health. It’s all organic and easy to make. We have seen a huge difference in her coat which is soft, shiny, darker and richer in color, and not so brittle and dry before. We were feeding her a premium dry dog food but switched to this “miracle food” and I do think it’s a miracle. We know it’s a matter of time for her but it’s been nearly two years with minimal issues. We had surgery twice to remove the growth on her muzzle, hoping it might not come back, but it did. We can’t keep cutting off part of her nose. She is slowing down and often has to be on prednisone, but only when necessary. She is such a good girl and we are glad we made this choice, a difficult one. Quality over quantity. It’s never enough time for our beloved pets, but we want what’s best for her. God bless you all and your pets with health issues. Cherish the small moments and give lots of hugs and kisses. That’s all we can do.

    • Dr. Susan Ettinger on November 24, 2012 at 5:28 pm

      Good advice on cherishing the moments and giving lots of love. Good luck with your dog!
      All my best, Dr Sue

  5. Eric Boles on August 25, 2012 at 3:00 am

    My maltese was diagnosed with mast cell cancer, grade 3, three years ago. It was a spot on his foot but removed with not so clean margins because the area was so small, he is only 4 pounds. He has had a couple other spots removed over the past 3 years but he is hanging in there. Originally he was only given 6 months to live but with prayer and love he is still with me.

    I have two questions.
    First, there is a newer drug out thats by BA Science called Kinavet (Masitinib). It is suppose to inhibit the receptors in mast cell tumors and slow down the growth. What do you think about this drug and do you think it will help if the cancer has spread to other organs, (which I am not sure of but having it for 3 years theres a good possibility).

    Second, there is a diet called Budwig Diet that is a mixture of flax seed oil, flax seed, and cottage cheese. I have been giving him this daily since he was diagnosed with cancer. Are you familiar with this diet and what is your take on it.

    Many thanks
    Eric Boles

    • Dr. Susan Ettinger on August 27, 2012 at 4:44 pm

      Hi Eric,
      Before starting chemo, I would see if the cancer has spread, starting with an abdominal ultrasound. I have used Kinavet, it’s in the same class of drugs as Palladia, and both are good options especially if your dog’s tumors are positive for the c-kit mutation. There is more info on MCT in the Guide, and more posts to follow soon. Your oncologist can review the chemo options as they will review the full medical record and do an exam – so important!
      As for the diet, I like the one included in the book from Dr Dressler, but you can download a free copy from the site. If your dog is doing well on his current diet, you can stick with that. Or consult with a nutritionist.
      All my best, Dr Sue

  6. Kristen on August 22, 2012 at 4:23 pm

    My black lab has mast cell tumors. Sorry, I forgot a most important piece of information.

  7. Kristen on August 22, 2012 at 4:22 pm

    My black lab will be 11 in a month. In April we learned that he may have cancer. In May it was confirmed. 2 tumors in his right lung, 2 in his spleen and the lymph nodes at his bronchial tubes are almost cutting off his airway. I wanted to start slow so he started on chloramucil 5mg every other day. There was a decrease is the size and density of the tumors and lymph nodes. One vet recommended changing to lomustine and another to using vinblastine. Each vet had less side effects with the chemo they recommended compared to the opposite vet. My vet agreed to complete the vinblastine treatment for the cost of meds and labs only. Thank God because our funds are past dried out. The next recommended treatment is using Palladia if we can afford it and if we cannot we would go back to chloramucil. Prednisone and Benadryl have been taken along with this as well as a multivitamin, healthier diet and k9 immunity with transfer factor.

    Can you or anyone recommend a different cancer treatment or have comments on his current treatment and some suggestions? This dog is the other half of me and so far he still has quality of life.

    • Dr. Susan Ettinger on August 27, 2012 at 4:33 pm

      Hi Kristen,
      All the drugs you mentioned can be effective for MCT. If there is progression on vinblastine, I agree it is time to change protocols. Palladia is a ckit inhibitor and would a a good choice if your dog’s tumors has the c-kit mutation. There is no perfect or “right” protocol. There are pros and cons to each drug, so I encourage you to discuss them with an oncologist. As I wrote in my blog, these tumors, and the chemo options, are not one size fits all.
      All my best, Dr Sue

  8. shellie on August 17, 2012 at 6:03 am


    I have a 5 year old Ridgeback that has 7 Mast Cells removed since the age of two. Until this year they were curtaneous wart like, all but one punch hole bio worked and got good margins I find them when size of a head of a pin, however she had two removed Jan 2012 that were subcutaneous now she has another on rear thigh at the fatty fleshy part the problem is finding it. All have been grade 0-1 to date. This new one causing a real issue as it swells with histomine and will reduce sometime within hours or up to 12 hours but all you can feel is a thickning of the skin not a lump or tumor itself so the vet cannot cut something out they cannot id. My experience with them is 3 years and onoging no doubt it mast cell and vet agrees. It’s becoming quite a issue as it swells sometime 2-3 per day or 2-3 per week so effecting her activity of daily living.

    If we cannot note an epa center to needle bio or feel any suggests ?

    Can ultrsound id a very small mast cell tumor ?
    Should the pup be taken off her antihistomine ( 50mg reactine per day) etc let it swell and go without restriction of antihistomine to try and id an center/lump ?

  9. Sandy on August 16, 2012 at 10:43 am

    My 9yr old flat coat recently had aspiration of a lump sent for pathology with suspicion of MCT. It shrunk with prednisone & is now in the weaning stages. My question is: can the aspirate determine if the cells are malignant or benign? Is surgery the only option? I’d like to avoid surgery as the vet is suggesting he also has laryngeal paralysis & should have that corrected at the same time.

  10. Eileen Tredway on August 14, 2012 at 9:24 am

    My 9 year old chow chow mix has had a large MCT removed from his side. It was determined to be grade II, but with well differentiated cells and a mitotic index of 2. He healed well and seems to have no other symptoms. After surgery had healed we had his hair cut off so as to better see what is going on. Discovered another MCT (confirmed by in house aspiration) near his chest. Unclear whether it is new or undiscovered previously.
    Vet is suggesting Palladia with Carafate 1G without removing the 2nd lump. Is this a reasonable course of action? It costs over $500.00 for each lump removal. We’ve just had one done, but my reading suggests the first tx of choice is surgery.

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