Mast cells are immune cells found in the skin and other tissues. They take part in allergic and inflammatory reactions [2]. A mast cell tumor is a growth made of these cells [2].
In dogs, mast cell tumors are the most common skin cancer [2]. In cats they are less common than in dogs [1].
Any new lump on your pet should be checked by your vet. Do not squeeze or press on a lump. Mast cell tumors can look like harmless lumps.
For other kinds of lumps, see lumps and bumps on dogs and lumps and bumps on cats.
Mast cell tumors in dogs
Which dogs get them. They are seen most often in older dogs, with a peak at around 8 to 10 years of age [1][2], but young dogs can get them too [1]. Breeds with a higher risk include Labrador and Golden Retrievers, Boxers, Pugs, French Bulldogs, Rhodesian Ridgebacks, Staffordshire Bull Terriers and Weimaraners [1]. Shar Peis tend to get them at a younger age, and more often in an aggressive form [1].
Where. They can grow anywhere on the body. The trunk and the hind legs are the most common places [1]. Pugs often have several at once on the trunk, and these often do well [1]. Tumors in or around the mouth, and tumors on the tail, around the anus or where skin meets a mucous membrane, tend to behave worse [1][2].
What they look like. Most are a single lump in the skin or just under it [2]. The lump may be hairless, and the skin around it can become red and swollen [2]. Some grow slowly, while others grow fast and spread into nearby tissue [2]. In young dogs, a harmless lump called a histiocytoma is common, and a mast cell tumor can look much like it [1]. You cannot tell lumps apart by sight or touch.
More than the lump. Because these tumors can grow into the tissue around them, surgeons remove the lump together with a border of healthy tissue [2]. That is why vets plan surgery carefully.
Effects elsewhere in the body. Mast cells store substances such as histamine [2]. When these are released, they can cause itching, redness and swelling of the skin, and less often vomiting or diarrhea [2].
Mast cell tumors in cats
In cats, skin mast cell tumors are usually found on the head and neck [1]. About one in five affected cats has more than one skin tumor [1].
- Typical form: the form seen in most cats, often on the head and neck [1].
- Atypical form: once called the histiocytic type. Siamese cats under 4 years old may be more likely to get it [1].
Unlike in dogs, mast cell tumors in cats often involve internal organs, most often the spleen. Up to half of cats with mast cell tumors have this internal form [1].
How vets diagnose them
Your vet will examine your pet and ask about the lump’s history. A fine needle aspiration is a useful first test [2]. It takes a few cells from the lump with a thin needle, and the cells are looked at under a microscope.
Knowing before surgery matters, because the surgeon can then plan to remove a border of healthy tissue around the lump [2]. Your vet may also check the nearby lymph nodes [2].
After removal, the tumor is sent to a lab. The lab report gives the grade and shows whether the edges are clear [1][2]. The grade describes how aggressive the tumor looks under the microscope, and clear margins mean no tumor cells were seen at the edges. Your vet will explain your pet’s results.
How vets treat them
The plan depends on the tumor’s grade, where it is and whether it has spread [1][2].
- Surgery: removing the lump with a border of healthy tissue is the usual first choice [2]. When enough healthy tissue is removed, surgery alone is often enough [2].
- Other treatments: if tumor cells remain or the tumor has spread, medicines or radiation may be added [2]. Your vet may refer you to a veterinary oncologist, a cancer specialist.
At home, as directed by a vet:
- do not squeeze, press or rub a lump;
- give all medicines exactly as prescribed;
- check your pet’s skin for new lumps and note the size, place and date of each;
- go back for every recheck.
Outlook
Outcomes vary between dogs and cats and between tumors:
- In dogs, low-grade tumors usually do well after surgery, while high-grade tumors are linked to shorter survival and more regrowth at the same spot [1].
- In dogs, some low-grade tumors stay quiet for years, while others grow fast into nearby tissue [2].
- In dogs, new tumors can appear elsewhere on the skin even after a tumor was removed completely [2].
- In cats, having several skin tumors carries a worse outlook than a single one, the opposite of what is seen in dogs [1]. How the atypical form behaves is still debated [1].
These are statements about groups of animals. They cannot predict how one pet will do. Your vet can explain what this means for your pet.
When to see your vet
See your vet if your dog or cat has:
- any new lump on or under the skin [1][2];
- a lump that changes in size, grows fast, or turns red and swollen [2];
- a lump in or near the mouth, on the tail or around the anus of a dog [1][2], or on the head or neck of a cat [1];
- new lumps after a mast cell tumor was removed [2].
FAQ
Can my vet tell a mast cell tumor by looking at it?
No. Many lumps look alike, so your vet takes a needle sample or a biopsy to find out [1][2].
Are mast cell tumors cancer?
They are tumors that range from low grade to high grade [1][2]. In dogs they are the most common skin cancer [2]. Your vet will explain what your pet’s lab results mean.
Can a mast cell tumor look like a harmless lump?
Yes. In young dogs, for example, it can look like a histiocytoma, a common harmless lump [1].
The bottom line
In dogs, mast cell tumors are the most common skin cancer. In cats they are less common. They can look like any lump, including a harmless one, so only a needle sample or biopsy can tell what a lump is. Surgery with a border of healthy tissue is the main treatment, and a lab report on grade and margins guides what comes next. Have your vet check every new lump and do not squeeze it.
This article is for general information only and does not replace advice from your veterinarian. If your pet has a new lump, see your vet.
Sources
- Schwinn J, Törner K, Beitzinger C, Klopfleisch R, Aupperle-Lellbach H. The Characteristics of 924 Cutaneous Mast Cell Tumours in Dogs ≤ 3 Years of Age, With a Short Literature Review of Feline, Equine, and Human Proliferative Mast Cell Disorders. Veterinary Sciences. 2026;13(5):500. Open access, CC BY 4.0. https://doi.org/10.3390/vetsci13050500
- Zmorzynski S, Kimicka-Szajwaj A, Szajwaj A, Czerwik-Marcinkowska J, Wojcierowski J. Genetic Changes in Mastocytes and Their Significance in Mast Cell Tumor Prognosis and Treatment. Genes. 2024;15(1):137. Open access, CC BY 4.0. https://doi.org/10.3390/genes15010137