All about Mast Cell Tumor Grade

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!