Mast cell tumor treatment: surgery and radiation
Updated: October 11th, 2018
We’ve spent a number of recent blogs understanding how MCT behave, how to confirm the diagnosis, MCT grade, what staging tests to consider, and what the prognostic predictors are. Now let’s talk treatment.
First, let’s think about the three main conventional tools oncologist use to treat tumors: surgery, radiation, and chemotherapy.
In general, it is best to think about surgery and radiation as local treatment options. By “local”, I mean dealing with the primary tumor: we either remove the cells with surgery or irreparably damage them with radiation.
On the other hand, chemotherapy is systemic therapy. It is given orally or intravenously (into a vein) and the drug circulates through the body systems. In general, chemotherapy is not the treatment of choice for the primary tumor. Instead, it is usually recommended as a follow up treatment for a primary tumor that is at high risk of metastasis, or spreading through the system to other organs.
Let’s more specifically look at using surgery and radiation therapy to treat MCT — and I’ll save chemotherapy for another post.
Surgery is the treatment of choice for MCT, and often the only treatment needed. The goal is to completely excise (remove) it and prevent its recurrence. The surgeon should aim for a minimum margin of two centimeters all the way around the tumor, including at least one deeper tissue layer. Then, a surgical biopsy must be submitted to the lab to determine the tumor’s grade, and to determine whether the margins are complete.
What if the tumor is too large for surgery?
This may be the case with large tumors on the legs, face, but also on the body trunk. In those cases, I might try to shrink the tumor before the surgery, so that we can then remove it more easily. For this, I use chemotherapy or radiation treatments.
What if the surgical margins are narrow (less than 2 cm), or dirty (have detectable cancer cells)?
If the scar is in an area of the body that can handle a second surgery, I recommend a scar revision — second surgery — to remove more tissue. The reason for this is that a completely removed tumor is less likely to recur at the surgical site.
For some MCT, especially those located on the lower leg, surgery alone may not achieve the necessary clean and wide margins, because there just wasn’t sufficient tissue surrounding the tumor. In these cases, radiation therapy will almost always be recommended.
If there are microscopic cells remaining after surgery, a follow up with radiation treatment provides excellent long term local control. The vast majority (85% to 95%) of dogs with low- or intermediate-grade MCT remain tumor-free two to five years after radiation treatment. Even high grade MCT cases can benefit: in a recent study, 70% of dogs with grade III MCT were still alive one year after radiation treatment.
Remember, radiation, like surgery, only treats the primary tumor site (that’s why we call it a local treatment) — it doesn’t prevent metastasis. The radiation is typically directed to the area three centimeters around the surgical scar to prevent recurrence, and is usually scheduled two to three weeks after surgery.
Radiation instead of surgery?
Using radiation after surgery is helpful, but radiation therapy is less successful as a primary treatment for a measurable tumor. Only 50% of dogs were still alive after one year in one study. If radiation is being considered as a primary treatment, it is often helpful to add chemotherapy and steroids for an improved outcome. I consider this a palliative approach for non-resectable, or inoperable, tumors. Radiation therapy may also effectively shrink an inoperable tumor, which may allow more complete removal of the remaining tumor in a future surgery.
In the next blog, we will discuss the systemic treatment option for MCT: chemotherapy. Remember, there is more in the Guide in addition to non-conventional approaches to supplement your dog’s care, including supplements, nutrition, and diet.
Sue Ettinger, DVM. Dip. ACVIM (Oncology). Dr. Sue is a boarded veterinary medical cancer specialist. As a Diplomate of the American College of Veterinary Internal Medicine (Oncology), she is one of approximately 400 board-certified veterinary specialists in medical oncology in North America. She is a book author, radio co-host, and an advocate of early cancer detection and raising cancer awareness. Along with Dr. Demian Dressler, Dr. Sue is the co-author of The Dog Cancer Survival Guide: Full Spectrum Treatments to Optimize Your Dog’s Life Quality and Longevity.
Hello! I have a 13yo Boston Terrier. He has a grade I/II MCT on the inside of his back leg at the knee cap. We had it surgically removed in January, only to have it grow back twice the size within a matter of months. It is now about half the size of a baseball, still growing and not able to be surgically resected. I’m getting conflicting info on whether chemo or radiation is the best option to try and shrink it for either surgery again or possibly Stelfonta. Want to avoid removing his leg if at all possible! Will both options shrink it about the same? I can do chemotherapy locally in town, but would have to travel 2.5 hours for radiation. I would prefer to start with the best option to ensure we aren’t wasting money and know we’ve done what we could before taking other meaures.
Hi Kelsey — there’s no way to really tell ahead of time which will work best for YOUR dog, of course. But your oncologist is the best person to ask about whether chemo or radiation is more likely to be effective. My understanding is that generally, chemo is not used as often for mast cell tumors. This is the perfect question to take to your vet oncologist and really dig in. What does he/she expect to happen with either treatment? How much likely is that to happen? What are the possible side effects, and how are they managed? Chapter 24 has more questions for you, and of course, chapter 30, the chapter on mast cell tumors, will also help.