A mast cell tumor is a growth formed when mast cells, a type of immune cell found throughout the body, begin dividing uncontrollably. Mast cells normally help fight infections and trigger allergic responses by releasing chemicals like histamine. When they become cancerous, they form tumors that can range from slow-growing and curable to aggressive and life-threatening. Mast cell tumors are the most common skin cancer in dogs, but they also occur in cats and, rarely, in humans.
What Mast Cells Do in a Healthy Body
Mast cells are part of the immune system’s front line. They sit in tissues throughout the body, especially in the skin, lungs, and digestive tract, waiting to react to threats. When activated, they release histamine and other chemicals that increase blood flow to an area, trigger swelling, and recruit other immune cells. This is the same process behind allergic reactions, hives, and the redness around a healing wound.
Beyond allergies, mast cells destroy pathogens through direct engulfing, break down toxins and venom components, and stimulate the production of collagen during wound healing. They’re useful, versatile cells. The problem starts when genetic changes cause them to multiply without stopping.
What Goes Wrong at the Genetic Level
Most mast cell tumors involve mutations in a gene called KIT, which controls a protein on the cell surface that tells the cell when to grow and when to stop. In a normal mast cell, this protein only activates when it receives a specific signal. Mutated versions of KIT are stuck in the “on” position, sending a constant growth signal that drives the cell to divide over and over.
In dogs, KIT mutations appear in roughly 20 to 30 percent of mast cell tumors. In human systemic mastocytosis, a related condition, up to 80 percent of cases carry a specific KIT mutation. The presence or absence of these mutations matters for treatment, because drugs that block KIT signaling only work when the mutation is driving the tumor’s growth.
Mast Cell Tumors in Dogs
Dogs are by far the most commonly affected species. These tumors typically appear as lumps in or just under the skin, and they can look like almost anything: a small raised bump, a soft mass, or a red irritated patch. Some stay the same size for months before suddenly growing. Others grow rapidly from the start. Because mast cells release histamine, the area around the tumor often swells, reddens, and itches, sometimes making the lump appear to change size from day to day.
Certain breeds face higher risk, including Boxers, Boston Terriers, Labrador Retrievers, Golden Retrievers, and Bulldogs. The tumors can appear anywhere on the body but are most common on the trunk, limbs, and head.
How They’re Diagnosed
The first step is usually a fine needle aspirate, where a veterinarian inserts a small needle into the lump and draws out a sample of cells. This is quick, minimally invasive, and highly accurate for identifying mast cell tumors specifically. The distinctive granules inside mast cells stain a characteristic purple under the microscope, making them relatively easy to recognize.
A fine needle aspirate confirms the diagnosis, but determining how aggressive the tumor is requires a tissue biopsy. The full tumor (or a piece of it) is examined under the microscope and assigned a grade based on how abnormal the cells look and how quickly they’re dividing.
Grading: Low vs. High
Veterinary pathologists use two main grading systems. The older Patnaik system assigns tumors a grade of I, II, or III. Grade I tumors are well-differentiated, meaning the cells still look relatively normal, and carry about a 7 percent mortality rate. Grade III tumors are poorly differentiated and aggressive, with a 94 percent mortality rate. Grade II sits in the middle and is the most common diagnosis, which makes it frustratingly vague in terms of predicting outcomes.
The newer Kiupel system simplifies things into just two categories: low-grade and high-grade. A tumor is classified as high-grade if a pathologist finds a certain number of dividing cells, abnormally shaped nuclei, cells with multiple nuclei, or wide variation in nuclear size within a standard viewing field. This two-tier system tends to be more consistent between pathologists and more useful for making treatment decisions.
Survival and Prognosis
Prognosis depends heavily on grade. Low-grade or grade I tumors that are completely removed surgically have an excellent outlook, and many dogs are effectively cured. The picture is murkier for intermediate tumors. With surgery alone, roughly 40 to 50 percent of dogs with grade II tumors survive the first year. Adding chemotherapy improves those numbers significantly: one widely cited protocol using vinblastine and prednisone achieved a 90 percent survival rate for grade II tumors, and dogs that made it to the one-year mark went on to live at least two years.
High-grade tumors carry the worst prognosis, with a strong tendency to spread to lymph nodes, the spleen, liver, and bone marrow.
Treatment Options for Dogs
Surgery is the cornerstone of treatment for mast cell tumors that haven’t spread. The goal is to remove the tumor with a wide margin of healthy tissue around it. How wide that margin needs to be remains a point of debate in veterinary oncology, and recommendations vary depending on the tumor’s grade and location.
Staging, the process of checking whether the cancer has spread, has become more sophisticated. Traditionally, vets checked the nearest lymph node based on anatomy. Newer sentinel lymph node mapping techniques identify the actual first lymph node that drains the tumor site, which doesn’t always match the anatomically closest one. Studies have found the sentinel lymph node differs from the expected regional lymph node in up to 57 percent of cases for tumors on the trunk and limbs. This means the old approach can miss spread or check the wrong node entirely.
Targeted Drug Therapy
For tumors that can’t be fully removed or have already spread, targeted drugs that block the KIT protein have changed the treatment landscape. Toceranib phosphate (sold as Palladia) was the first cancer drug approved specifically for dogs. In a large placebo-controlled trial, 42.8 percent of dogs with recurrent mast cell tumors showed a measurable response to the drug, including complete disappearance of the tumor in some cases. Among dogs that responded, the median duration of that response was 12 weeks, and the median time before the tumor progressed was about 18 weeks.
Injectable Treatment
A newer option involves injecting a plant-derived compound directly into the tumor. Approved by the FDA for use in dogs, this injectable treatment (tigilanol tiglate, sold as Stelfonta) destroys the tumor from the inside. In clinical trials, 75 percent of treated dogs achieved complete resolution of their tumor by day 28, compared to just 5.3 percent of untreated controls. It’s used for tumors that are accessible for injection and haven’t yet spread, and it offers an alternative for cases where surgery isn’t practical due to the tumor’s location.
Mast Cell Disease in Humans
True mast cell tumors in humans are rare. The more common human condition is mastocytosis, where mast cells accumulate abnormally in one or more organs. Systemic mastocytosis affects about 13 out of every 100,000 people worldwide. It ranges from indolent forms that cause skin flushing, itching, and digestive symptoms to aggressive variants that damage bone marrow and other organs.
A localized form called a mastocytoma can appear in children, typically within the first few months of life. These show up as firm, slightly raised patches 1 to 5 centimeters across, colored red, pink, yellow, or brown, sometimes with a textured “orange peel” surface. One to three spots is typical.
The hallmark diagnostic feature is called Darier’s sign: rubbing the affected skin causes it to redden, swell, and sometimes blister within minutes. This happens because the physical stimulation triggers the abnormal mast cells to dump their histamine, producing an intense localized allergic reaction that lasts about 15 minutes. In children, most solitary mastocytomas resolve on their own by puberty.
Diagnosing systemic mastocytosis in adults typically involves blood tests measuring tryptase (an enzyme released by mast cells), bone marrow biopsy to look for abnormal mast cell clusters, bone scans to check for damage, and genetic testing for KIT mutations.