Mast Cell Tumors in Dogs: Signs, Diagnosis and Treatment

What a mast cell tumor is

Mast cells are immune cells in the skin and other tissues that release histamine and other chemicals during allergic reactions. A mast cell tumor is a cancer made of these cells. The Merck Veterinary Manual describes it as the most common malignant tumor seen in dogs, and VCA calls it the most common skin tumor. Most appear in the skin or just under it, but they can also arise in the spleen, liver, intestines or bone marrow.

They are most common in dogs around 8 to 10 years old, though younger dogs can be affected, and males and females are affected equally. Boxers, Pugs, Boston Terriers, Rhodesian Ridgebacks, Bulldogs, Golden Retrievers and Labrador Retrievers are among the breeds at higher risk. Merck notes that Boxers and Pugs often develop more than one, while VCA reports that about 85 percent of affected dogs have only a single tumor.

Signs to watch for

A mast cell tumor is usually a raised lump in or under the skin, most often on the limbs, especially the back of the thigh, and on the lower belly and chest. It may be soft or firm, hairless or furry, red, itchy or ulcerated, or look completely ordinary. Some grow slowly for months; others grow fast. A telltale feature is a lump that swells, reddens or shrinks from day to day, or swells after being touched, because handling makes the cells release histamine.

Histamine released into the body can also cause stomach and intestinal ulcers, so some dogs have vomiting, a poor appetite, lethargy or black, tarry stools. Rarely, a large release causes a sudden allergic-type reaction. Vets call mast cell tumors the great pretenders because they can resemble insect bites, warts or fatty lumps called lipomas, which is exactly why appearance alone means nothing.

When to call your vet today

Book an appointment soon for any new lump, and call promptly if a lump is growing, changing size or color, ulcerating, bleeding or itching, or if it appeared on the feet, gums, lips, eyelids, around the genitals or around the anus, where tumors tend to behave more aggressively. Note the size with a ruler and take photos so you can show your vet how it has changed.

Call the same day if a dog with a known or suspected mast cell tumor starts vomiting, stops eating, becomes lethargic or has black stool. These can be signs of stomach ulcers caused by histamine, which need treatment of their own.

When it's an emergency

Go to your vet or the nearest emergency vet immediately if your dog suddenly develops facial swelling, hives, difficulty breathing, pale gums, collapse or severe vomiting, particularly after a lump was squeezed or knocked. A large release of histamine can cause a dangerous drop in blood pressure.

Vomiting blood, black tarry stool with weakness, or a painful, swollen belly are also emergencies. Tell the team if your dog has a known mast cell tumor so they can plan treatment accordingly.

What you can safely do at home

Leave the lump alone. Do not squeeze, scratch, pop or massage it, and stop your dog licking or chewing at it; your vet may suggest a cone. Measure it every week or two and keep a short record with photos so changes are obvious.

Do not try home remedies, creams or supplements on a lump, and do not give human antihistamines or other medicines unless your vet prescribes them. If your vet does prescribe medicine to protect the stomach or block histamine, give it exactly as directed and ask before stopping.

If your dog keeps licking or scratching the lump, ask your vet whether a dog cone is appropriate until it is checked.

What the vet will likely check

The first test is usually a fine-needle aspirate: a thin needle collects cells from the lump, which are examined under the microscope, often the same day. Mast cells are usually easy to recognize this way. Your vet may then recommend checking nearby lymph nodes, blood work, and an abdominal ultrasound to see whether the cancer has spread, before planning surgery.

After removal, the tissue is sent for a biopsy, which gives the tumor a grade and shows whether the edges were clean. VCA reports that dogs with low-grade tumors have an average survival of more than two years, while high-grade tumors carry a much shorter outlook. Treatment is usually surgery with a wide margin of normal tissue, and depending on the grade and spread may include radiation, chemotherapy or newer targeted medicines, often with a veterinary oncologist's input.

Prevention and follow-up

There is no known way to prevent mast cell tumors, but early detection makes a real difference, because a small, low-grade tumor removed completely is often cured. Check your dog's skin regularly, for example during grooming or a weekly belly rub, and get anything new checked rather than waiting to see whether it grows.

Dogs that have had one mast cell tumor are more likely to develop another, so keep up with rechecks and have every new lump sampled, even if it looks like the harmless ones your dog already has. Breeds at higher risk benefit most from regular skin checks.

This page is general information, not veterinary advice, and it cannot replace an examination by the vet who knows your pet.