May 1, 2026, 2:13 pm | Read time: 5 minutes
While petting, a small lump under the skin is noticed. The area might be slightly red, feel soft or doughy, or resemble a poorly healing wound. Many owners initially think it’s a harmless change. Because such skin lumps can look so different, they are often underestimated. PETBOOK explains how a mast cell tumor develops in dogs, what symptoms may occur, and why early clarification is crucial.
Function and Disruption in the Body
Mast cells are part of the body’s defense system. They originate in the bone marrow, travel through the blood into the body, and settle in various tissues. There, they play an important role in immune reactions, allergies, and inflammatory processes.
Inside, mast cells contain various active substances like histamine, heparin, and enzymes. These substances are released as needed and trigger reactions in the body, which can manifest as swelling or inflammation.
A mast cell tumor is a collection of degenerated mast cells and is classified as a cancer. In dogs, it is one of the most common skin tumors. It usually appears on limbs, the torso, or the head. Areas like the perineum or genitals can also be affected. Less commonly, there are forms where multiple sites are affected simultaneously or the tumor cells primarily settle in internal organs. Besides dogs, cats and other pets can also be affected by mast cell tumors.
How Does a Mast Cell Tumor Develop?
The exact causes of mast cell tumors in dogs are not fully understood. It is known that they can arise from changes, known as mutations, in the mast cells. The c-Kit gene, which is involved in regulating cell growth, plays an important role. However, this mutation is not detectable in every tumor.
There is a noticeable prevalence in certain dog breeds. Boxers, Dachshunds, Shar-Peis, Rhodesian Ridgebacks, Weimaraners, Labrador Retrievers, and Bernese Mountain Dogs are particularly often affected. Boxers often develop a less aggressive form, which can be associated with a better prognosis. This distribution suggests a genetic component.1
What Does a Mast Cell Tumor Look Like in Dogs?
The question of what a mast cell tumor looks like cannot be answered definitively. This is precisely what makes the disease so difficult to recognize. Essentially, any skin lump can be a mast cell tumor.
Typical appearances include:
- nodular or doughy skin changes
- hairless or hairy lumps
- reddened or ulcer-like areas
- oozing or poorly healing wounds
The tumor usually appears alone. However, in about 11 to 14 percent of affected animals, multiple lumps can occur simultaneously.
The lumps can feel soft or firm and change in size. Touching them can lead to the release of histamine, causing the tumor to swell partially within a short time. This can also result in additional redness or welts. The growth rate of a mast cell tumor varies greatly. A fixed growth rate cannot be specified, but rapid size increase within a short time is possible.
Other possible symptoms indicating a mast cell tumor include:
- itching
- swelling
- tendency to bleed
- wound healing disorders
- vomiting
- loss of appetite
- abdominal pain
- dark-colored stool due to bleeding in the gastrointestinal tract
In rare cases, allergy-like reactions up to shock symptoms can occur.2
Diagnosis by the Veterinarian
Due to the variable appearance, an external assessment is usually not sufficient. The most important examination is fine needle aspiration. The veterinarian uses a fine needle to extract cells from the lump, which are then examined under a microscope.
If the suspicion is confirmed, the tumor is usually surgically removed. A histological examination by a pathologist follows. The result is crucial for further assessment. An additional blood test can help evaluate the general health status and identify possible effects on other organs.
Mast cell tumors are classified by their degree of differentiation, either as Grade I to III or as low-grade and high-grade. Additionally, it is checked whether the tumor has already spread. This involves examining lymph nodes and, depending on the case, conducting ultrasound examinations of the abdominal area or other diagnostic measures.3
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Treatment of Mast Cell Tumors
The primary treatment measure is the surgical removal of the tumor. For completely removed tumors of low grade (Grade I, many Grade II or low-grade), the prognosis is often good. Frequently, the animals are considered cured after the procedure.
If the tumor could not be completely removed, a follow-up surgery or radiation is often necessary. The goal is to destroy remaining tumor cells and prevent regrowth.
For aggressive tumors (Grade III or high-grade), chemotherapy is often additionally used. If metastases are already present, this treatment can improve quality of life and extend lifespan. Medications can also be used to reduce the effects of released signaling substances. These include antihistamines and agents to protect the gastrointestinal tract.4
During the illness, nutrition can play a supportive role. However, it does not replace medical treatment. Tumor cells have an altered metabolism and deprive the body of energy and nutrients. This can lead to weight loss and deficiencies in affected dogs.
The goal of feeding is therefore to meet the energy needs and adequately supply the body with nutrients. A high-energy, easily digestible, and palatable diet is recommended. Adequate supply of essential amino acids, vitamins, and minerals is important.5
How Long Can a Dog Live with a Mast Cell Tumor?
The prognosis depends heavily on the tumor grade and the disease stage of the animal. The location of the tumor also plays a role. For low-grade tumors, the prognosis after complete removal is usually good. Many dogs live more than two years after treatment, sometimes significantly longer.
The situation is significantly worse for high-grade tumors. These grow more aggressively, spread more frequently, and recur more often. The metastasis rate here is about 50 to 90 percent. The survival time is often less than four months.
Targeted prevention is difficult. It is all the more important to take skin changes seriously early and have them examined by a veterinarian.