What Is The Difference Between A Mast Cell Tumor And A Lipoma In Dogs?

Lipomas and mast cell tumors are two common skin masses found in dogs. Lipomas are benign fatty tumors that usually form under the skin, while mast cell tumors can be cancerous growths of mast cells. Both types of growths may initially appear similar on physical exam, but there are key differences between the two.

This article provides an overview of mast cell tumors and lipomas, including how to identify, diagnose, and treat them. It also covers the main differences between these common canine skin masses.

What are Mast Cell Tumors?

Mast cell tumors (MCTs) arise from malignantly transformed mast cells. Mast cells are a type of white blood cell that are involved in the body’s immune response. They reside in tissues like the skin, respiratory tract, and gastrointestinal tract, and contain granules filled with substances like histamine, heparin, and proteases. When activated, mast cells release these substances, which cause allergy symptoms and inflammation. Mastocytosis refers to a condition where there are too many mast cells accumulating in the body. While mastocytosis itself is typically benign, it can sometimes progress to mast cell cancer or leukemia. MCTs represent a cancer of mast cells, where mutations cause them to proliferate and form a tumor. These tumors most commonly affect the skin in dogs, appearing as raised lumps or bumps on the surface. However, they can develop internally as well. MCTs have highly variable behavior, ranging from benign to aggressively malignant. Their appearance alone does not indicate how they will act, so staging and grading are important for determining prognosis and treatment [1].

Symptoms of Mast Cell Tumors

Mast cell tumors (MCTs) can develop anywhere on a dog’s body, but are most commonly found on the trunk, chest, legs, and mouth. They often appear as raised lumps or bumps on or just under the skin and may vary greatly in appearance. MCTs can be small, solitary growths or larger masses. Their color can range from red, ulcerated masses to small bumps that are colorless or the same color as the surrounding skin.

A key symptom of MCTs is itchiness and inflammation around the tumor site. This is caused by the release of histamine and other chemicals from the malignant mast cells in the tumor. Histamine is responsible for typical allergy symptoms like itchy skin, hives, and swelling. As mast cell tumors grow, they release more histamine leading to more severe symptoms. Dogs may excessively lick, chew, or scratch at MCTs in an attempt to relieve the itchiness and irritation. Severe MCTs can even lead to symptoms like vomiting, diarrhea, loss of appetite, lethargy, and breathing difficulties.

According to the Blue Pearl Veterinary Partners article on MCTs, some symptoms to look out for include:1

  • Decreased appetite
  • Increased respiratory rate
  • Vomiting
  • Diarrhea
  • Sluggish activity level

Diagnosing Mast Cell Tumors

The diagnostic process for mast cell tumors involves multiple steps to confirm the diagnosis and determine the grade and stage of the tumor. The first step is usually a fine needle aspiration or biopsy to collect samples for analysis.

The samples are examined under a microscope by a veterinary pathologist to identify the tumor cells and confirm they are mast cells. According to the VCA, mast cells have distinctive staining features that allow them to be identified.[1]

Once a mast cell tumor is confirmed, the next step is to grade it based on how aggressive it appears under the microscope. There are three grades with grade III being the most aggressive. According to the National Institutes of Health, the grade gives an indication of how likely the tumor is to spread.[2]

Finally, staging involves looking for evidence that the tumor has already spread to the lymph nodes or other parts of the body. This includes blood tests, imaging tests, and potentially surgical lymph node biopsies. Staging indicates the extent of the tumor and helps determine the treatment plan.[3]

Treating Mast Cell Tumors

The primary treatment for mast cell tumors in dogs is surgery to remove the tumor (VCA Animal Hospitals). The goal is to remove the entire tumor with clean margins. Additional treatments like radiation therapy or chemotherapy may be recommended after surgery, especially if the tumor was not completely removed, had spread to the lymph nodes, or if the margins were narrow.

Radiation therapy can be used to kill remaining cancer cells after surgery. It involves a series of radiation treatments to shrink and destroy mast cells (Univ of Penn). This is often used when surgical removal is not possible.

Chemotherapy may help treat mast cell tumors that have spread or have a high risk of metastasis. Common chemotherapy drugs used include vinblastine, prednisone, and lomustine. The goals are to kill cancer cells, shrink tumors before surgery, and prevent metastasis.

What are Lipomas?

Lipomas are benign fatty tumors that are commonly found in dogs [1]. They appear as soft, movable lumps under the skin and are usually not painful [2]. Lipomas consist of fat cells surrounded by a thin layer of connective tissue and often feel doughy or rubbery. They are most commonly found on the chest, abdomen, legs and armpits, but can develop anywhere on the body [1]. Lipomas are typically small, ranging from the size of a grape to a golf ball, though they can grow larger [3]. They are usually rounded or oval in shape and move freely within the surrounding tissue [2]. While lipomas generally grow slowly, most tend to remain unchanged over years. They are one of the most frequently encountered tumors in middle-aged and older dogs [2].

[1] https://www.webmd.com/pets/dogs/what-to-know-lipoma-in-dogs
[2] https://vcahospitals.com/know-your-pet/adipose-lipoma-tumors
[3] https://www.denvervet.com/site/blog/2022/08/31/fatty-tumor-lipoma-dog

Diagnosing Lipomas

A veterinarian will typically diagnose a lipoma through a physical exam. The vet will feel for soft, movable lumps under the skin that have defined edges. Lipomas are usually soft, painless masses that feel like a bulge under the skin. They are not attached to underlying tissues and can easily be moved around. The vet may be able to make a diagnosis just by feeling the mass.

If the diagnosis is unclear, the vet may recommend a fine needle aspiration and cytology exam to confirm it is a lipoma. This involves using a small needle to suction out cells from the mass for examination under a microscope. Fat cells from a lipoma will have a distinctive appearance under the microscope. https://www.petmd.com/dog/conditions/skin/c_multi_lipoma

In some cases, a biopsy may be recommended to differentiate a lipoma from a cancerous mass. This involves surgically removing all or part of the mass for pathology testing. A biopsy can definitively determine if the mass is benign fat cells or something more sinister. https://vcahospitals.com/know-your-pet/adipose-lipoma-tumors

Treating Lipomas

The most common treatment for a lipoma is monitoring and observation. Since lipomas are typically benign, noncancerous masses, they often do not require treatment unless they are impairing your dog’s mobility or quality of life. Vets may recommend monitoring the lipoma every few months to check for any changes in size or texture.

If the lipoma is in an area that risks impairing your dog’s movement or activities, surgically removing the fatty mass may be recommended. Surgery can completely remove the lipoma and provide a biopsy to confirm it is benign. This is typically done under general anesthesia as an outpatient procedure with minimal recovery needed afterwards. According to the VCA, surgery is most effective at removing lipomas when they are still small in size (1).

In some cases, if the lipoma is very large or in a difficult location, the vet may recommend partial removal or debulking surgery to reduce its size. After surgical removal, recurrence is uncommon but can happen if some fatty tissue remains or regenerates (2).

Key Differences Between Mast Cell Tumors and Lipomas

While mast cell tumors and lipomas are both skin masses found in dogs, there are some important differences between these two types of growths:

Malignancy – Mast cell tumors can be benign or malignant, while lipomas are always benign. Mast cell tumors have the potential to spread to other areas of the body, while lipomas do not.

Symptoms – Lipomas are usually soft, movable lumps under the skin, often on the chest or legs. They are generally asymptomatic. Mast cell tumors can vary in appearance and may be reddened, ulcerated, or swollen. They may itch or cause other symptoms.

Diagnosis – A biopsy is required to definitively diagnose a mast cell tumor and determine if it is benign or malignant. Lipomas can often be diagnosed through physical examination, but biopsy may be recommended if the vet is unsure.

Treatment – Treatment for mast cell tumors depends on the grade, location, and spread and may include surgery, chemotherapy, or radiation. Lipomas typically do not require treatment unless they impact movement or quality of life, in which case surgical removal may be done.

Overall, mast cell tumors have the potential to be cancerous and spread, while lipomas do not. Careful diagnosis and treatment is important for mast cell tumors, while lipomas rarely require intervention unless causing discomfort.


In summary, mast cell tumors and lipomas are two distinct types of skin tumors that can develop in dogs. While they may appear similar on initial examination, there are key differences in their cellular origin, typical appearance, common locations, diagnostic methods, and treatment options.

Mast cell tumors arise from mast cells in the skin and tend to be bumpy and ulcerated. Lipomas are benign fat cell growths that are soft and movable under the skin. Mast cell tumors are most concerning due to their variable behavior and metastatic potential, requiring staging and grading to determine prognosis and guide treatment. Lipomas are almost always benign and surgical removal is curative.

The takeaway is that any new mass on a dog’s skin should be evaluated by a veterinarian. While many lumps will be harmless lipomas, identifying and treating malignant mast cell tumors early is key to the best outcome. Being aware of the differences between these two common skin masses in dogs helps owners and vets recognize when a growth could be more serious.

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