Visiting Vet: Mast cell cancer

Terry has a challenging situation.

At first we thought the swelling on Terry’s elbow might be an injury, or perhaps an allergic reaction to an insect bite. It wasn’t painful. The little terrier wasn’t limping. Still, in order to be sure it wasn’t anything serious, I suggested a fine needle aspirate. This quick and easy test, commonly abbreviated FNA, can be done right during the exam. “It’s simple,” I said. “I quickly poke the mass, aspirate some cells with a syringe, spread them on slides and send them to the pathologist.” FNA does have limitations. It doesn’t provide an intact section of tissue for the pathologist to evaluate, it’s more just a schmear of cells sucked through a needle, but it’s a good first step, minimally invasive and done without anesthesia.

Terry’s FNA report indicated a suspicion of mast cell cancer, but added that there was insufficient sample to definitively differentiate whether it was cancer or an allergic reaction. What is a mast cell? Why might mast cell cancer look like allergic swelling? Good questions. Mast cells are a normal part of the immune system. They occur primarily in the skin, lungs, and gut, where their main job is to fight off foreign invaders, like parasites. They do this in a unique way by releasing granules of biochemicals such as histamine that are toxic to the invaders, but also cause redness, swelling, and itching. Sometimes mast cells get confused and release histamine granules in response to stuff like pollen, thus causing annoying allergies … and sometimes mast cells go haywire and turn into tumors.

Mast cell cancer (MCC) is one of the most common types of skin tumor in dogs. It can present in many different ways on the surface or under the skin. It has the unusual characteristic that the tumors sometimes seem to come and go, appearing and disappearing spontaneously. Breeds that are predisposed include boxers, dachshunds, English bulldogs, Labrador retrievers, Rhodesian ridgebacks, pugs, shar-pei, Weimaraners, and several different terriers, including Boston, fox, and American Staffordshire, but MCC can occur in any dog. Definitive diagnosis is made with FNA or biopsy, but here’s where things get tricky. Any time we manipulate a mast cell tumor (MCT), we run the risk of stimulating release of those biologically active substances. This can lead to anaphylactic shock and/or clotting problems and hemorrhage.

Why do an FNA? Why not just remove the tumor completely, right off the bat? A few reasons. MCC is notoriously invasive, sending little microscopic fingers deep into surrounding tissue, making it impossible to identify the true tumor margins. Surgical excision must be extremely wide and deep if we hope to remove it all. We would not normally cut that extensively on other cancers, so it behooves us to confirm the diagnosis before wielding the scalpel. Pretreatment with antihistamines minimizes risk of anaphylaxis. We must also be prepared for other potential surgical complications, such as excessive bleeding.

Using a method called the Patnaik system, veterinary oncologists classify MCTs as Grade I, II, or III. About half of all MCTs are Grade I. These are basically benign, and curable with surgery alone. Grade II MCTs are unpredictable. Prognosis depends largely on how fast the mass grows. Median survival ranges from 5 months to 70 months. Treatment may be surgical or include radiation and/or chemotherapy. About one-fourth of MCTs are Grade III — nasty, rapidly growing malignancies that spread to lymph nodes, liver, spleen, and bone marrow. These require aggressive treatment. Prognosis is guarded.

An additional method for classifying MCTs, called the Kiupel, or two-tier high/low, system, assesses a tumor’s “mitotic index” by counting the number of actively dividing cancer cells seen in 10 high-power microscope fields. The more dividing cells, the faster the tumor may grow. High index is bad, with life expectancy often as short as four months. Low index is less bad, with many dogs surviving two years or more.

Terry has a challenging situation — little dog, big swelling, tough location. After discussing pros and cons, we decided to get a small “incisional” biopsy to confirm the diagnosis and assess the grade and mitotic index. Terry was fasted, pretreated with antihistamines, then anesthetized. The elbow area is anatomically complex, with various muscles, tendons, ligaments. What was tumor, what was normal tissue? It was impossible to tell for sure. I took small samples from areas that seemed representative. There was a fair amount of bleeding, as expected with MCC. I ligated some small vessels. The bleeding lessened. I packed the site with sterile hemostatic material. All looked good. I closed, bandaged the leg, and moved Terry to recovery. She woke up smoothly and seemed fine, but as the day progressed, blood seeped through the bandage. Those mast cell granules were doing their thing, causing continued bleeding — too much to risk not addressing it aggressively.

In 35 years of practice I have twice seen similar situations. In one case, we were able to stop the hemorrhage, and the patient recovered. In the other case, the bleeding persisted. The dog developed a clotting disorder called disseminated intravascular coagulation, and eventually passed away from complications related to MCC. We anesthetized Terry again, reopened the site, and, using multiple techniques, were able to stop the bleeding. I closed, applied a pressure bandage, and called her owner. I was hopeful all would be well, but if the bleeding recurred, advised transferring her immediately to the care of specialists off-Island.

Much to everyone’s relief, Terry was fine and did not need to take a ferry ride that day, but the biopsy confirmed the diagnosis. MCC, Grade II, low mitotic index. Because of the location, complete excision would require leg amputation, not an ideal solution. Radiation therapy is another possibility. Or her owner might decide not to pursue any treatment. The next step is an oncology consultation, then taking time to consider what feels right. Each MCT is different. Each owner is different. Each dog. There is no right or wrong. There is only what is right for Terry.