Lipomas and Liposarcomas (Adipose Tumors)

What is a lipoma?

A lipoma is a common, benign (harmless) fat tumor seen in middle-aged to older animals. These tumors occur often in dogs and infrequently in cats. They can vary greatly in their rate of growth. Some may remain the same size for years, while others may grow very large.

Lipomas can become problematic if they cause discomfort for your pet or if they grow in locations that affect your pet’s ability to walk. Lipomas can grow anywhere (even inside the abdomen), but the most common place is under the skin, where fat is normally present. If they grow between layers of muscle, they are called infiltrative lipomas.

The malignant form of this tumor is called liposarcoma. Fortunately, these tumors usually do not spread to other places in the body, but they require more extensive surgery to control them than lipomas.

What causes this type of tumor?

The reason why a particular pet may develop this, or any tumor or cancer, is not straightforward. Very few tumors or cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary.

How are these types of tumors diagnosed?

Typically, lipomas can be diagnosed by fine needle aspiration (FNA). FNA involves taking a small needle with a syringe and suctioning a sample of cells directly from the tumor and placing them on a microscope slide. The cells on these microscope slides can be examined by a pathologist (cytology).

In some cases, results from FNA may not be clear, and a biopsy may be necessary. A biopsy is the surgical removal of a piece of the tumor, which is then examined by a veterinary pathologist under the microscope (histopathology). Histopathology helps make a definitive diagnosis. Advanced imaging, such as a CT scan, may also be recommended, especially for infiltrative lipomas or suspect liposarcomas.

How do these types of tumors typically progress?

Lipomas are benign, which means they are not cancerous and will not spread (metastasize) to the surrounding tissues or internal organs; however, these tumors may continue to grow and cause your pet discomfort if not removed. Once removed, the likelihood of recurrence is relatively low. However, it is very common for middle-aged to older dogs to have multiple lipomas. Every lipoma is different; some may grow rapidly, and some may take years to grow large enough to be of concern.

Liposarcomas, however, are malignant, and have a much higher recurrence after surgical removal. Multiple surgeries may be required to remove the tumors. In addition, liposarcoma can spread, although this is rare. Liposarcomas are typically treated more aggressively than lipomas.

How will this type of tumor affect my pet?

In cases of benign lipomas, removal of these masses is primarily cosmetic. Some pets develop these tumors in areas that cause discomfort and/or lameness, such as their armpit region, between their legs, or around the neck. You may see your pet exhibit an irregular gait, reluctance to stand, walk upstairs, or go for their normal walks.

How are these tumors treated?

The most effective treatment for lipomas is surgical removal. It is best to remove these masses when they are small; the surgery is usually less invasive, and the incision is much smaller and less painful for your pet. As lipomas continue to grow, surgery may become more difficult for both your veterinarian and your pet.

Liposarcomas and infiltrative lipomas may be best removed by a board-certified veterinary surgeon, as these procedures are more challenging. Post-surgical radiation therapy may be recommended for recurrent, infiltrative lipomas or liposarcomas.

One study shows promise for a steroid called triamcinolone acetate in shrinking tumors. The drug is injected directly into the lipoma to cause shrinkage, but recurrence may occur. This treatment could be considered for dogs if surgery is not an option.