By Dr. Alison Book, DACVIM Oncology

In two previous posts, I’ve focused on different aspects of mast cell tumors (MCT). In Part I, I gave an overview of MCTs, the problems they cause, diagnosis, where they occur and which types of dogs get them. In Part II, I addressed the most common questions dog owners have when their pet is diagnosed with an MCT. In this final post, I am going to explain different treatment options and the expected outcome for dogs with MCTs.

What treatment should my dog have for his/her mast cell tumor?

Surgery: Surgery to remove the MCT is a mainstay of treatment. As you might recall from Part I, MCTs can grow into and disturb surrounding tissues. Because of this, wide margins of visually normal tissue around the tumor should be removed to increase the likelihood of complete removal. If the MCT is located somewhere that makes this technically challenging, it is worth consulting with your veterinary oncologist or a board certified surgeon prior to surgery. An effective first surgery can save a lot of time and money!

Radiation therapy: In the case that a MCT was not completely removed (or could not be due to location), radiation therapy can be used to treat residual tumor cells. In select cases, a more aggressive second surgery is possible instead of radiation therapy. Radiation therapy may also occasionally be used alone for tumor control when surgery is not possible.

Chemotherapy: Systemic chemotherapy is usually reserved for dogs with grade III tumors or other markers of aggressive disease (high mitotic index or other warning signs). Chemotherapy is typically used as a follow-up treatment after surgery to address the potential for systemic spread of the cancer. It may also be recommended to control a primary tumor but responses can be unpredictable. Some commonly used chemotherapy drugs are vinblastine and CCNU (lomustine).

Tyrosine Kinase Inhibitors: Some MCTs have mutations in a certain gene known as c-kit (a type of tyrosine kinase). Drugs have been developed to inhibit c-kit (and some other tyrosine kinase receptors as well), and may be recommended for your dog in some cases. The goal of these drugs is to inhibit MCT growth and/or new blood vessel formation. Commonly used drugs are toceranib (Palladia) and masitinib (Kinivet).

Supportive medications: In addition to treatment of the tumors, some dogs will receive medications that help fight the secondary effects of the tumor. Prednisone (a steroid), Benadryl (an antihistamine) and Pepcid (a gastric acid blocker) are the most common drugs that we will prescribe.

What sort of outcome should I expect with these treatments?

Dogs with completely removed grade I and grade II MCTs with no other concerning signs have an excellent prognosis and many dogs are cured. Prognosis for dogs with incompletely removed grade I and II tumors and treated with radiation therapy after surgery is also excellent and about 90-95% have no recurrence of the MCT within three years. Dogs with grade III tumors have a guarded prognosis because local recurrence and/or spread are more likely. The overall median survival time can vary depending on a variety of factors and ranges from a few months to several years. Your veterinary oncologist can help to more specifically define the prognosis for your dog.

What type of follow-up is needed?

We will sit down and discuss the best follow-up plan based on your dog’s individual needs, but every dog that has had a MCT should be closely monitored for the development of new skin tumors!

Also, check out this video testimony of an owner who treated her dog for an MCT: