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

All about Mast Cell Tumor Grade

Updated: April 13th, 2021

Knowing the grade of your dog’s mast cell tumor (MCT) is important, because the grade tells us a lot about how serious a case your dog has, and what the likely prognosis or outcome will be.

Tumor grade cannot be discovered via aspirate. MCT grade is determined by a boarded pathologist at a lab, who looks at your dog’s surgical biopsy.

Classic Grading System

The pathologist will look at several criteria of the cells under the microscope to determine the MCT grade. The classic grading system, known as the Patnaik grading system, assigns one of three grades to each biopsied tumor: grade I, II, or III. Let’s break down the grades.

  • Grade I, or low grade, tumors account for 33-50% of all MCT cases. Of the three grades, they are the most benign and are generally the easiest MCT to deal with. They do not invade surrounding tissues, are well-differentiated, and rarely metastasize. Grade I tumors usually do not recur once they are surgically removed, with clean margins.
  • Grade II, or intermediate grade, tumors account for approximately 25-45% of all MCT. These tend to be more locally invasive (have gone into the deeper layers below the skin) and are more likely than grade I MCT to disseminate to other parts of the body. They are also more likely to recur, especially with incomplete or narrow margins.
  • Grade III, or high grade, account for approximately 20-40% of all MCT cases. These are usually very malignant, have invaded deep in the skin layers into underlying tissue, and are highly aggressive, with a 55% to 95% rate of metastasis. Grade III tumors are also extremely likely to recur.

The grade II can be challenging to predict a dog’s outcome. Some grade II behave more like grade I but others behave more aggressively like grade III. Based on the original work by Patnaik, there is nearly a 50/50 chance of 5 year survival for grade 2 tumors. Oncologists like me are often frustrated because a diagnosis of grade II MCT means unpredictability for the dog in my exam room.

Another challenge is the reliability of the grade from the pathologist. Unfortunately, tumor diagnosis and grade as determined by a pathologist are not black and white. There is some degree of subjectivity. It has also been shown that agreement among pathologists looking at the same tumors with the classic 3 grades is not always consistent. In one study, concordance among 28 pathologists for 95 tumors was 75% for grade 3 tumors, and 63% for grade 1 and 2 tumors.

In plain English, that means that different pathologists will assign different grades – to the same tumors! For example, in that last study, 37% of the pathologists disagreed with each other about whether a tumor was a grade 1 or a grade 2.

There is also a tendency for pathologists to call a tumor grade 2 when it is borderline for grade 1 and 2. If more pathologists are now calling tumors grade 2, the prognostic value is weakened. We can’t rely on their opinions as much as we should be able to.

New Grading System Makes Prognosis Easier

The potential inaccuracy in the classic system was the motivation for creating a new, two-tiered system for grading MCT tumors.

The new grading system for MCT is based on studies out of Michigan State University. In this system, MCT are graded as either high-grade or low-grade. Simpler, and easier to use.

This grading is based on evaluating the number of mitoses, the presence of multinucleated cells or bizarre nuclei, or karyomegaly (increased nuclear size).

High-grade MCT tumors have been shown to be significantly associated with a shorter time to metastasis, mast cell tumor associated mortality (death due to the tumor), and a shorter overall survival time: less than 4 months.

Low-grade MCT tumors, on the other hand, have a median survival time of more than 2 years.

This simpler system is still quite new, and it’s usefulness should be validated in future studies.

What You Need to Know

I know it is concerning to think about the implications for your dog if the biopsy grade on your pathology report is not a reliable number. The decision about whether and how to treat is often based on that number – so you want it to be as accurate as possible.

That is why I generally recommend biopsy second opinions for most tumors in most dogs. This is especially true as we start using this new system. Hopefully it will address this variation and provide less ambiguity in the future.

Regardless of the grading system used and the reported grade, there are other things I consider when formulating a prognosis for my patients. We will discuss mitotic index and prognostic proliferation markers next.

And remember, there is more info on grading and MCT in the Guide. Thanks for reading!


Leave a Comment

  1. […] there’s the traditional Patnaik system. Here, there are three distinct grades indicating severity, with Grade 1 being the least severe and aggressive, and Grade 3 being the most severe and […]

  2. Ece on October 19, 2017 at 9:02 am

    My 5 year old Golden Retriever had 6 MCTs removed over the last 3 weeks with 2 separate surgeries. Three of the MCTs are Grade 1 with a 0 MI, one is a Grade 2 with a 0 MI, one is a Grade 3 with a 1 MI, and one is subcutaneous. The tumors are inconsistent with one another and within themselves (ex: GIII but MI1). We were told that this is a very unusual case and I was wondering if you’ve had an experience with a similar case. If so, what was the prognosis? What was the survival rate? Much appreciate your response.

  3. Tom Thompson on January 21, 2014 at 7:55 pm

    Dr. Ettinger. Thank you for your website. Very informative. My golden retriever, Rowdy, just had a mast cell tumor removed that received a grading of Patnaik grade II, Kiiupel low grade. Margin was 1.5 mm. My vet suggests two options: (1) bringing him back in a month or so (after he’s had a chance to recover a little more from the first surgery) to consider getting a bigger margin; or (2) playing it by ear and monitoring for recurrence. The tumor size was 1.8 X 1.0 X 1.0. Thank you.

  4. Kim C. on October 25, 2013 at 7:25 am

    My dog recd a diagnosis of soft tissue sarcoma high grade after a mammary mass was removed. 6 sections were examined. Amongst other things, It says there are 24 mitotic figures in 10 random 40x HPF with occasional bizarre mitotic figures. We’ve seen an oncologist and are trying Kinavet but I’m curious what you’re interpretation of what I’ve shared means in terms of prognosis.

  5. Susan Kazara Harper on October 15, 2013 at 5:16 am

    Hello Lucy’s Mon, Is Lucy your dog’s name? I’m so sorry you’re going through all of this. In short, there is a lot you can do, starting with her diet. Go to and download the free ebook which will help you put together the best foods to give her. Nutrition is one of hte most important tools we have in giving our dogs the best fuel to deal with whatever they’re dealt. The Dog Cancer Survival Guide book ( is an excellent reference for all the possible options of caring for a dog with MCT, as well as getting yourself through this. And there are many nutraceuticals, including Apocaps ( or which can help you and your wonderful dog through this. Most of all, find a way to manage your panic and all the terrible stories going through your mind. Today is today. Your girl tells you every day how she feels, and that is what really counts. Stay positive, stay strong, and get educated about this, for her. You are her champion, and your love, and hers for you, will get you through. All the best.

  6. Lucy's Mom on October 14, 2013 at 4:53 pm

    I have a 8 year old Austrailian Shepard that was diagnosed with Stage 3 Mast Cell cancer. It first started with a large growth inside her ear. The vet removed the growth and in days she formed a larger more aggressive growth on the outside of the same ear. It all went so fast, I came home from work and she had a large growth on her neck the size of my hand. When the ancology report came back, that was when I was made aware of her cancer and its severity.
    With speaking with the vet she stated that the only advise she would give me was what she would do if it was her pet which was to love her and I will know when she is no longer enjoying her life. I feel so helpless, I feel so horriable doing nothing but I also do not have another $2000 to $3000+ dollars to try to fight this.
    Is there anything I can be doing, is there any hope at this point?

  7. Ms. Charlie W. on November 14, 2012 at 9:03 am

    Dr. Ettinger, I was researching the type of cancer my precious dog, Dude, was diagnosed with, the soft tissue Fibrosarcomas oral cancer on the upper jaw. The suggested treatment is doxorubicin chemo 1 tx so far with piroxicam then Palliative radiation therapy (six treatments- he is on his 3rd tx) then return to doxorubicin.

    Eight days after the chemo (also started piroxicam at least a week before the chemo) Dude got very very ill with severe diarrhea and extreme lethargy and just wanted to lay as far away from the house in theback yard as possible. I had to carry him into the house. He was given medicine for diarrhea but the oncologist wasn’t sure why he got so sick 8 days after chemo. They didn’t believe it was the chemo and thought it might be the piroxicam. My fear is whatever it was nearly killed him so I’m hesitant to return to the chemo with piroxicam.

    What has been your experience with the above treatment and side effect?

    I was reading a book “Cancer and your pet” by Debra Eldredge and Margaret Bonham and they reported some positive outcomes combining chemo with NSAIS’s and were so excited about great outcomes with bladder cancer and developed a new and relatively safe treatment for dogs and tried in on canine oral cancers that can’t be operated on.

    This book was dated 2005 is there more current research on this? What are more current outcomes? Also Can’t my dogs tumor be removed with surgery even if it means removing the upper jaw bone? Nobody seems to think so.

    Dude is 12 years old and show no signs of illness, he is my best friend ever and he just loves me and I him, he is the best dog ever, so I want to leave no stone unturned in finding the best possible chance of him beating this cancer!

    Your feedback would be invaluable.

    Ms. Charlie W.

    • Dr. Susan Ettinger on November 24, 2012 at 5:21 pm

      Ms Charlie W.
      Most dogs have chemo-related side effects after doxorubicin about 1 to 5 days post chemo, so at day 8 it could be due to the chemo, but less likely. Was blood work run? Piroxicam’s most common side effects are kidney or liver toxicity and stomach irritation/ulcers. Blood work would help look at liver and kidney values.
      The mechanism of the NSAIDs is thought to be anti-angiogenesis, targeting tumor blood vessels, and it is a good approach. It is also known as metronomic chemo when combined with low dose oral chemo, like cyclophosphamide. I am assuming the the mass is not a candidate for surgery due to the location. There is info on metronomic chemo for STS (like fibroscarcomas) after surgery to delay recurrence, but not specific info for measurable tumors. I talk about metronomic chemo in the Guide. More recently, oncologists are using Palladia (instead of NSAIDs) with low dose oral cyclophosphamide for metronomic chemo.
      All my best, Dr Sue

  8. […] my last blog on mast cell tumor (MCT) grade, I discussed that grade is one of the most important prognostic […]

  9. Mary Emmons on September 4, 2012 at 12:44 pm

    I was reading the blog today and saw your note to the Dr. Please don’t feel quilty, because I know that as good dog parents we are doing everything we can, especially if we are here learning all we can about cancer in our fur babies. My heart goes out to you, as it is tough when we can’t make it go away and save our babies from the pain of cancer. Hugs!

    • Dr. Susan Ettinger on September 7, 2012 at 4:57 pm

      I agree, Mary. We should not beat ourselves up, but focus on thr good memories and the joy our pets gave us during their time with us.
      All my best, Dr Sue

  10. Jeannette Botza on September 1, 2012 at 11:22 am

    I have been waiting to longest time to read something about Hemangiosarcoma,
    but never see anything. My dog died from even after doing 4 sessions on cheom. saw no signs of it for three months, and then it was in the spleen
    and liver, can you please write about this cancer……Ultrasound was totally
    clear..but nonethe less it returned. Please give me some peace mind.I
    thought I did everything. acupunture, diet and herbs. I have guilt that more
    could have been done…….

    • Dr. Susan Ettinger on September 7, 2012 at 4:43 pm

      Hi Jennette,
      I am so sorry to hear about your dog. Hemangiosarcoma is an aggressive and frustrating cancer, and it sounds like you treated it aggressively and did all you could.
      No I have not blogged about this cancer yet. I do have so much to share about on so many topics. In the meantime, there is a chapter in the Guide, and also a ton of other helpful info that may help you with your grieving. I too have a hole in my heart after the loss of my beloved black lab Paige, and it’s been four years. It does get easier with time, but the sadness still lingers for me.
      With sympathy, Dr Sue

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