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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. D on March 18, 2019 at 3:43 pm

    Our beloved boarder collie cross mastiff has come up with a mass lump on the side of his neck within a week or 2, took him to the vets which i wasnt very happy with they put him to sleep to drain it to see if it was an infection but no puss or anything came out, also took a sample of it apparently but no cells present, she mentioned its squishy surrounding but firm in the middle and doesnt know if its a tumour or fatty lump etc so have put him on 10 day antibiotics to see if the lump shrinks, if not they want to put him under fully and open up to see if they can remove it and then send the sample off to find out what it is. My question is if its cancer does this mean its spread already or can he make a recovery hes happy runs around eats barks etc only thing i noticed thats different for awhile is when he eats he drools like crazy and really trys to chew with his head to the side as well as ” hoicking” like trying to get rid of something in his throat

    Please help

    • Molly Jacobson on March 18, 2019 at 4:45 pm

      Hi D, thanks for writing, and we’re sorry to hear about your pup. It’s impossible to know yet whether it is cancer or not, and whether it has spread or not. You just have to wait for those lab results after they remove it (if they decide that’s the next best thing). It stinks that we have such a hard time diagnosing things, but the reality is that we just have to go through the process of eliminating things. Vets (and doctors) look for the most common thing that could be wrong, and treat for that, hoping that it’s what is happening. Only after they have eliminated the most common cause of the symptoms do they look for a different disease. Hopefully the antibiotics will help shrink it, if it’s an infection. While you have those ten days of antibiotics, don’t hesitate to call your veterinarian if anything causes you concern. The throat symptom and drool could be because of an infection, of course, but if they get worse or you see other symptoms develop, or if the lump doesn’t shrink (or gets bigger), call your vet! You don’t have to wait the whole ten days — if those antibiotics don’t seem to be helping, let them know so they can try something else or move on to the next step. If it turns out to be cancer, Dr. D’s book will really help you to decide how to handle it.

  2. judie lafrance on February 3, 2019 at 3:00 pm

    My 11 year old bichon poodle has a had an intramuscular MCT in her right bicep removed 2 months ago.They removed 1/3 of her bicep muscle and reattached the tendon, she is walking beautifully. Although the MCT was encapsulated, the soft tissue surgeon was unable to get all clean boarders . The MCT was graded as stage 2, The mitotic level was 0. No involvement in lymph nodes, liver, spleen. There is no reasonable place within 300 miles to follow up with radiation. The oncologist is suggesting chemo. I know an intramuscular MCT is extremely rare.
    My question to you, please, is would it be more favorable to have a second surgery performed with the possibility that my dog may loose function of her leg and or amputation as opposed to chemo?
    I so appreciate your input. Thank you Judie Lafrance

  3. Ginny Lombardi on August 23, 2018 at 3:17 am

    Hi. I’m in desperate need of information. I walk a 17 yr old Jack Russel who looks & acts more like a 10 yr old. Her mom is 90 & can generally run circles around me. The pup has an oozing sore on her right lower side so mom brought her to her vet. He did a biopsy – I assume aspiration- and diagnosed cancer. Devastation all around. Vet recommended surgery to remove growth. Mom is worried that, because of her age, pup wouldn’t survive the surgery. Family & Friends suggested a second opinion from a veterinary oncologist. Mom took pup & the doctor looked at the records from first vet and immediately said to mom, “this dog must have surgery immediately, and then chemotherapy and radiation.“. Mom told vet she had granddaughters wedding in a few weeks. Can they wait? Dr said, “the dog will die if you wait”. Dr did some tests. Mom was sick in trying to make decision. Next week, dr calls mom with test results. Says “no surgery, no chemo, no radiation. Go to wedding & have a good time”. Confusion reigns. Back to her vet who still wants to remove tumor. Question: does it make any sense to do surgery on a 17 yr old Jack Russell?

    • DogCancerBlog on August 23, 2018 at 6:11 am

      Hello Ginny, thanks for writing and we are so sorry to hear about your Jack Russel. We are not veterinarians here in customer support so we can’t offer medical advice, however, we can offer information based on Dr. Dressler’s writings 🙂

      There is an amazing article on the Dog Cancer Blog that discusses the topic of whether or not a dog is too old for cancer treatment. Here’s the link:

      There are many aspects that need to be taken into consideration when determining a treatment plan for a dog with cancer– age can be just one of those factors. Another is whether or not your senior dog would need more recovery time, or have a harder time healing after surgery.

      Whether or not you decide to treat with conventional treatments, there are many things you can do to help your dog. As Dr. D says in the Dog Cancer Survival Guide, a cancer diagnosis is not an automatic death sentence. Most dogs– of any age–can benefit from making dietary changes, using specific nutritional supplements, lifestyle adjustments, and brain chemistry modifications– increasing their happiness.

      Many factors have to be taken into consideration when deciding on a treatment plan for a dog with cancer and it’s up to us as dog guardians to make the right choice for our dogs because we know them best.

      We hope this helps 🙂

  4. Marilyn Thomas on April 8, 2017 at 10:45 am

    Mast Cell: Google “Vera’s boxer cancer”
    Follow her directions exactly! It’s helping us.
    Good luck and share.

  5. Ashlin on March 18, 2016 at 6:59 pm

    Love your advise.

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