MCT in dogs most commonly refers to mast cell tumors, the single most common malignant skin tumor in dogs. They account for about 11% of all canine skin cancers and can range from a harmless-looking lump to an aggressive, life-threatening mass. The term “MCT” can also refer to medium chain triglycerides, a dietary supplement used for epilepsy and cognitive decline in dogs, which is covered at the end of this article.
What Mast Cell Tumors Are
Mast cells are part of your dog’s immune system. They normally live in skin and connective tissue, where they release chemicals like histamine during allergic reactions and wound healing. A mast cell tumor forms when these cells begin dividing uncontrollably, usually because of a genetic mutation.
In 10% to 45% of canine mast cell tumors, the cause traces back to a mutation in a gene called KIT, which acts like an on/off switch for cell growth. Normally, the KIT receptor on a mast cell only turns on when a specific signaling molecule binds to it. When the gene is mutated, the receptor stays permanently switched on, telling the cell to keep growing and dividing regardless of any outside signal. Dogs without this specific mutation can still develop MCTs; chronic skin inflammation and exposure to irritating substances have also been linked to tumor development.
Breeds at Higher Risk
Any dog can develop a mast cell tumor at any age, but they appear far more often in adult and older dogs. Certain breeds carry a significantly elevated risk. Shar-Peis have roughly six times the odds of developing an MCT compared to other breeds with skin tumors. Boxers follow closely at about five times the average risk. American Staffordshire Terriers, Labrador Retrievers, French Bulldogs, and Golden Retrievers are also predisposed, with risk levels roughly two to three times higher than average. In Golden Retrievers specifically, researchers have identified gene variants associated with MCT development, suggesting a heritable component in at least some breeds.
What Mast Cell Tumors Look and Feel Like
One of the tricky things about MCTs is that they have no single consistent appearance. A mast cell tumor can show up as a small, firm, seemingly harmless bump under the skin that’s been there for months. It can also appear as a rapidly growing, red, ulcerated mass. Some dogs develop multiple skin nodules at once, which happens in 11% to 14% of cases.
Because mast cells are packed with histamine and other inflammatory chemicals, the tumor can “degranulate,” releasing those chemicals into surrounding tissue. This causes a distinctive reaction: the area around the tumor swells, turns red, or develops hives when touched or bumped. About half of dogs with MCTs show secondary signs from this chemical release. In more aggressive cases, the histamine enters the bloodstream and causes systemic problems like vomiting, diarrhea (sometimes with dark, tarry stools indicating stomach bleeding), loss of appetite, and lethargy.
How MCTs Are Diagnosed
The standard first step is a fine needle aspirate, where a veterinarian inserts a small needle into the lump and collects cells to examine under a microscope. Mast cells have a distinctive appearance with purple-staining granules, making them relatively straightforward to identify on cytology. This quick, in-office procedure is highly reliable for confirming that a lump is an MCT.
Once a tumor is removed surgically, it goes to a pathologist for histopathology, which provides the critical information about how aggressive the tumor is. Two grading systems are used. The older Patnaik system sorts tumors into three grades: grade 1 (well-differentiated, confined to the upper skin layers), grade 2 (intermediate, extending deeper), and grade 3 (poorly differentiated, infiltrating deep tissues). The newer Kiupel system simplifies this into just two categories, low grade and high grade, based on how rapidly cells are dividing and how abnormal they look under the microscope. The Kiupel system is generally considered more accurate at predicting whether a tumor will spread.
In one study of 291 Patnaik grade 2 tumors (the large, ambiguous middle category), 83.5% were reclassified as low grade under the Kiupel system and only 16.5% as high grade. This distinction matters enormously, because the grade drives treatment decisions and outlook.
Treatment Options
Surgery
Complete surgical removal is the primary treatment for localized mast cell tumors. Current guidelines call for removing at least 2 centimeters of normal-looking tissue around the tumor on all sides, along with one layer of the tough connective tissue (fascia) beneath it. Older recommendations called for 3-centimeter margins, but veterinary surgeons now recognize that 2 centimeters is sufficient for most low and intermediate-grade tumors.
If a pathologist finds tumor cells at the edge of the removed tissue, the question becomes whether cancer remains in the dog. Needle aspirates of the surgical scar can help answer this. In one study, scar aspirates had an overall accuracy of about 89% in predicting whether residual tumor cells remained, and dogs with evidence of leftover MCT cells were nearly 22 times more likely to have the tumor come back locally.
Injectable Treatment
In 2020, the FDA approved an injectable drug called Stelfonta for treating non-metastatic mast cell tumors in dogs. It’s injected directly into the tumor, where it activates a protein that destroys the tumor cells from within. In clinical trials of 118 dogs, 75% achieved complete remission after a single injection. Dogs that didn’t fully respond could be retreated about a month later, and 44% of those achieved complete remission on the second round. This treatment is approved for skin-based MCTs anywhere on the body, and for tumors under the skin on the lower legs (below the elbow or hock).
Additional Therapies
Radiation therapy and chemotherapy are typically recommended for high-grade tumors, incompletely removed tumors, or cases where the cancer has spread to lymph nodes or internal organs. For tumors driven by the KIT mutation, targeted therapies that block the overactive receptor can slow or stop tumor growth. Your veterinarian or a veterinary oncologist will tailor the approach based on the tumor’s grade, location, and whether it has spread.
Outlook and Prognosis
Prognosis depends heavily on grade. Dogs with low-grade (Kiupel) or grade 1 (Patnaik) tumors that are completely removed surgically have an excellent long-term outlook, with the vast majority living normal lifespans. Local recurrence after complete removal of a low-grade tumor is uncommon.
High-grade tumors tell a very different story. They’re more likely to have already spread at the time of diagnosis and carry a significantly higher risk of recurrence. Tumors that show signs of degranulation (swelling, redness, ulceration) also tend to behave more aggressively. Multiple tumors, rapid growth, and location on certain body areas like the groin or nail bed can also signal a worse prognosis. Early detection and removal of any new or changing skin lump gives your dog the best chance, since most MCTs caught before they spread are curable with surgery alone.
MCT Oil: The Other Meaning
MCT also stands for medium chain triglycerides, a type of dietary fat that’s gaining attention as a supplement for dogs with epilepsy and age-related cognitive decline. MCT oil provides fatty acids that the brain can use as an alternative energy source when its normal glucose metabolism falters.
For dogs with epilepsy, the evidence is promising. In a randomized, double-blinded study where MCT oil was added to dogs’ regular food (providing about 9% of daily calories), there was a significant 22% reduction in the median number of seizures. Nearly one in five dogs in that study showed a meaningful response, with some becoming seizure-free. Separate studies using commercial diets enriched with MCT oil found that roughly 43% to 47% of dogs experienced more than a 50% reduction in seizure frequency. Beyond seizure control, MCT supplementation in these studies also improved spatial working memory, problem-solving ability, and owner-reported trainability.
For older dogs showing signs of cognitive decline (confusion, disorientation, disrupted sleep-wake cycles), MCT oil may help compensate for the age-related drop in brain glucose metabolism that’s been documented in dogs, primates, and humans alike. Some evidence suggests MCTs improve cognition in dogs with mild cognitive impairments. MCTs are considered a promising add-on to standard treatments because they lack the central nervous system side effects common with conventional anti-seizure medications.