Beyond Mast Cell Tumor Grade: Other Prognostic Factors

In my last blog on mast cell tumor (MCT) grade, I discussed that grade is one of the most important prognostic factors, or predictors, for dogs with MCT. And I also discussed the challenges of using the grade as a predictor: a good percentage of grade 2 MCT behave more aggressively than the rest of grade 2 MCT, and the recent news that the grade from a pathologist might not be reliable. What is a dog Guardian to do?

Well the good news is that there are additional tests besides grade that can help us predict your dog’ outcome.

Mitotic Index

One very important test is the tumor’s mitotic index, which measures how many MCT cells are dividing when the pathologist examines the biopsy under a microscope. Thankfully, this test is now routinely included on the pathology report.

Here’s what the numbers associated with mitotic index mean. In a recent study, it turned out that the higher the mitotic index, the poorer the prognosis.

For example, if the score was over 5 on the mitotic index, the median survival time was only two months, regardless of grade.

If the score was under 5, however, the median survival time was seventy months (over five years), regardless of grade.

So, you see, it is very important to know your dog’s mitotic index for their mast cell tumor. This knowledge can really help your oncologist to plan treatments, and you to understand your dog’s prognosis.

It should be noted that reported cutoffs for these prognoses can vary — from 5 all the way to 10. In the newer 2-tiered system described in this post, the cutoff was 7, not five. I expect that the cutoff will continue to be refined as more studies evaluate mitotic index as it relates to the different grading systems.

MCT Prognostic Panel

There is other prognostic information that can be drawn from a biopsy sample that may be helpful. This is often referred to as the MCT prognostic panel or proliferation panel, and it involves evaluating certain markers related the MCT’s proliferation (how much it multiplies) and how much cell division is happening.

The panel typically measures AgNOR, PCNA, Ki-67, c-Kit, and tests for c-Kit mutation status. This a panel helps me to predict whether the MCT will metastasize in the future, whether chemotherapy is recommended, and what chemotherapy to include in the protocol (based on the c-kit mutation result).

One of the most important numbers in the panel is c-Kit and it’s mutation status. One-third of dogs with MCT have a genetic mutation in the c-Kit gene. When there is a mutation, an enzyme called tyrosine kinase is stuck in the “on” position, leading to uncontrolled growth and survival of these mast cells. Studies have shown the presence of the mutation is a negative predictor, but now there are 2 relatively new chemotherapy drugs that target this mutation. I will discuss the c-kit inhibitors Palladia and Kinavet in a future blog.

Only a handful of laboratories have the special stains and expertise needed to analyze the MCT prognostic markers, so I recommend consulting with your oncologist to see if this information is necessary before adding these expensive tests to the biopsy. I find this panel especially informative for Grade II and grade III tumors.

There are several other prognostic factors for MCT included in the Guide, including rapid growth, size, recurrence and location.

Remember what I said in the 1st blog on MCT: MCT are not one size fits all tumors. Not one test – grade, mitotic index, c-kit mutation, or other prognostic factor – can definitively predict your dog’s outcome. But we can use these to help make good recommendations, so your dog can not only live longer with MCT, but live well.