Fatty Tumor or Cancer? How to Tell if Your Cat’s Lump is Benign or Malignant


Tumors are relatively common in cats and can be benign (non-cancerous) or malignant (cancerous). It is estimated that 1-5% of cats develop tumors, with the most common types being lymphoma, fibrosarcoma and squamous cell carcinoma [1]. As cats age, their likelihood of developing tumors increases. One study found over 20% of cats over 10 years old had at least one tumor [2].

Being able to correctly identify and classify a tumor as benign or malignant is critically important. Malignant tumors are cancerous, locally invasive and have the ability to metastasize to other parts of the body. Benign tumors tend to grow slowly, rarely spread and can often be surgically removed. Detecting cancer early and determining the right course of treatment can significantly impact prognosis and survival.

This article provides an overview of the key differences between fatty tumors and cancerous tumors in cats. It covers appearance, behavior, diagnostic tests and treatment options to empower cat owners with the knowledge to identify potential warning signs.


Fatty tumors and cancerous tumors can look quite different in cats. Here are some key factors to consider regarding appearance:

Size – Fatty tumors are often small, ranging from less than 0.4 inches to 2 inches across. Cancerous tumors can grow much larger. According to the Merck Veterinary Manual (https://www.merckvetmanual.com/cat-owners/skin-disorders-of-cats/tumors-of-the-skin-in-cats), some malignant cancers can reach 6 inches in diameter or more.

Shape – Fatty tumors tend to have smooth, rounded, or lobulated shapes. Cancerous tumors often have irregular shapes with uneven borders.

Color – Fatty tumors are usually skin-colored, white, or pink. Cancerous tumors may be various colors like black, blue, red, or multi-colored.

Texture – Fatty tumors feel soft and movable under the skin. Cancerous tumors are often firm, immobile, and fixed to underlying tissues according to the Merck Veterinary Manual (https://www.merckvetmanual.com/cat-owners/skin-disorders-of-cats/tumors-of-the-skin-in-cats).

Location – Fatty tumors can develop anywhere on the body, but commonly occur on the trunk, limbs, head or neck. Cancerous tumors may also appear anywhere, but certain types tend to develop in characteristic locations.


There are some key differences in behavior between fatty tumors and cancerous tumors in cats. Fatty tumors typically do not cause pain or mobility issues unless they are in locations that impede movement. Cancerous tumors, on the other hand, often become painful over time as they grow larger and impinge on surrounding tissue. This can cause changes in behavior like irritability, aggression, hiding, and reluctance to move around or jump up on things.

Cats with cancerous skin tumors may excessively groom, lick, bite, or scratch at affected areas. They may seem restless or agitated. Cancer in other areas like the abdomen or chest can cause lethargy, decreased appetite, vocalizing/meowing from pain, and other personality changes as the cat feels unwell.

Mobility can be impacted by tumors on the limbs or near joints. Cats may limp, avoid jumping or climbing, or have difficulty rising from a resting position. Advanced metastatic cancer that spreads to bones and organs causes severe pain, weakness, and rapid deterioration of quality of life.

While the location matters, cancerous tumors in general tend to cause progressive behavior changes correlated with pain and illness over time. Fatty tumors that are not impinging on mobility tend to cause minimal behavior changes. Monitoring for alterations in temperament, activity levels, and pain signs can help distinguish benign fatty tumors from more concerning cancerous ones when combined with other diagnostic steps.

Diagnostic Tests

To diagnose fatty tumors versus cancerous tumors in cats, veterinarians may recommend several diagnostic tests, including:

Fine Needle Aspiration: This involves using a small needle to take a sample of cells from the tumor for examination under a microscope. It allows the vet to determine if the cells are normal fat cells or abnormal cancer cells (Viera East Veterinary Center).

Biopsy: This test removes a small piece of the tumor to be examined by a pathologist. It provides a definitive diagnosis about whether the tumor is benign or malignant (MedVet).

Imaging: X-rays, ultrasound, CT scans, or MRI can be used to evaluate the size and location of tumors. These are especially useful for assessing tumors in the chest or abdomen (Paws & Claws Animal Hospital).

By selecting the appropriate diagnostic tests, vets can determine if a tumor is harmless fat or potentially cancerous tissue requiring treatment.

Fatty Tumors

The most common type of fatty tumor in cats is a lipoma. Lipomas are benign (noncancerous) masses or growths that form in the layer of fatty tissue under the skin (Source 1). They are soft, moveable lumps that feel like bubbles under the skin and are usually painless. Lipomas typically occur in middle-aged to older cats, especially overweight cats.

Lipomas can develop anywhere on a cat’s body, but commonly form on the chest, abdomen, legs, and armpits. They range in size from a pea to several inches across. Most lipomas in cats are small, solitary growths that do not cause problems. However, larger lipomas may interfere with movement or become irritated. The vast majority of feline lipomas are benign fatty masses that do not turn into cancer (Source 2).

Cancerous Tumors

Cancerous tumors in cats can be carcinomas, sarcomas, or mast cell tumors. Carcinomas start in the epithelial cells and are commonly found in the mouth, nose, thymus, thyroid, mammary glands, and skin. Squamous cell carcinoma is the most common oral cancer in cats. Sarcomas arise from connective tissues like fat, cartilage, and bone. They often form rapidly growing masses under the skin. Common locations for sarcomas include the mouth, skin, and legs. Mast cell tumors originate from mast cells in the body and can be found in the skin, mouth, and gastrointestinal tract. They may appear raised and ulcerated. According to the University of Illinois Veterinary Teaching Hospital, lymphoma, mammary tumors, mast cell tumors, and squamous cell carcinoma are among the most common cancers seen in cats [1].

When to See the Vet

If you notice any lumps or bumps on your cat, it’s important to have your veterinarian examine them as soon as possible. Some key signs that indicate a vet visit is warranted include:

Monitoring changes in the lump’s size, shape, color or texture. Benign fatty tumors tend to grow slowly, while malignant tumors often grow more rapidly.

Rapidly growing lumps or bumps should always be checked by a vet, even if they appear harmless. Growths that develop quickly could be cancerous.

Getting an ultrasound or fine needle aspirate of any suspicious lumps or masses. These diagnostic tests allow the vet to analyze cells from the growth to determine if it is benign or malignant.

Ultrasound is recommended to evaluate internal organs for masses that are not visible from the outside. It provides important information about the tumor’s size, shape and location.

Any time there are significant changes or you have concerns about a lump or bump on your cat, it’s vital to schedule a prompt veterinary visit for proper evaluation and testing.


Treatment for fatty tumors and cancerous tumors in cats often involves surgery. Fatty tumors that are small and not bothering the cat may just be monitored, but larger fatty tumors or ones in locations that irritate the cat can be surgically removed [1]. For cancerous tumors, surgery is often the first line of treatment to fully remove the tumor if possible.

Chemotherapy may be recommended after surgery for cancerous tumors, especially if they are larger or there is a concern that some cancer cells were left behind. Chemotherapy drugs like carboplatin, doxorubicin, and others can be used to target remaining cancer cells. Radiation therapy is another option that can be used along with surgery and chemotherapy to treat cancerous tumors in cats [2].

Cats tend to tolerate chemotherapy and radiation therapy well, but side effects like gastrointestinal issues and bone marrow suppression can sometimes occur. Your veterinarian will determine the best combination of treatments for your cat based on the type, size, and location of the tumor.


The prognosis for benign fatty tumors like lipomas is generally good. Lipomas are harmless growths and do not turn cancerous, so they pose little health risk to cats (https://petcureoncology.com/lipomas-in-cats/). As long as the lipoma does not grow large enough to restrict movement or impair quality of life, it can be left untreated. Even large lipomas that require removal have an excellent prognosis after surgery.

The prognosis for cancerous tumors depends on the type of cancer, location, size and stage. Lymphoma, mast cell tumors, melanoma and other cancers can vary greatly in aggressiveness. In many cases early detection and treatment lead to better outcomes. Regular vet exams and diagnostics tests can help identify concerning lumps early, when treatment is most effective. Working closely with an oncologist is important for determining the best treatment options and prognosis for each individual case of feline cancer (https://www.nevccc.com/site/blog/2021/12/30/cat-lipoma).


There are some steps cat owners can take to try to prevent tumors in their cats:

  • Regularly monitor your cat for any new lumps or bumps on their skin by petting them and feeling for abnormalities. Early detection of tumors gives the best chance for successful treatment.
  • Limit your cat’s exposure to the sun. Sun exposure increases the risk of certain skin cancers. Keep cats indoors during peak sun hours and apply pet-safe sunscreen before letting them outside.
  • Feed your cat a high-quality diet with plenty of antioxidants. Poor nutrition can weaken the immune system and increase cancer risk. Choose a diet low in carbs and high in protein and omega-3 fatty acids.
  • Avoid exposing your cat to second-hand smoke, which contains carcinogens. Take precautions when using lawn chemicals, flea/tick medication, and cleaning products around your cat as well.
  • Have your cat spayed/neutered before 6 months of age, as this can reduce the risk of some reproductive cancers.

While you can’t eliminate the risk of cancer entirely, focusing on early detection and minimizing carcinogen exposure can help reduce your cat’s chances of developing tumors.

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