What is a lipoma?A lipoma is a term to describe a very common benign tumor of fat seen in middle-aged to older animals. These tumors can vary in terms of their rate of growth (some may remain the same size for years) but become problematic for your pet when they grow in locations that impact their ability to walk or cause discomfort. Lipomas can grow anywhere (even inside the abdomen), but the most commonplace is under the skin where fat is normally present. They may grow in between layers of muscle or may grow in a location that causes your pet discomfort whenever they walk. When these tumors grow in between muscle layers, they are typically called infiltrative lipomas.
The malignant form of this tumor is called a liposarcoma. These tumors tend not to spread to other places.
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 and 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, these types of tumors can be diagnosed by fine-needle aspiration.
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. A veterinary pathologist then examines the slide under a microscope.
In some cases, results from FNA may not be entirely clear and biopsy may be necessary. A biopsy is a surgical excision of a piece of the tumor. Pieces of the tumor are then examined by a veterinary pathologist under the microscope. This is called histopathology. Histopathology is helpful to make a definitive diagnosis. Advanced imaging (such as a CT scan) may also be recommended.
How do these types of tumors typically progress?Although lipomas are benign, meaning they are not cancerous and will not spread (metastasize) to the surrounding tissues or internal organs, without surgery tumors may continue to grow, causing your pet discomfort. Once removed, the likelihood of recurrence is relatively low. However, it is very common for middle-aged to older dogs to have multiple masses suspected to be lipomas. Every lipoma is different; some may grow rapidly and some may take years to grow large enough to be of concern.
"Although lipomas are benign, without surgery tumors may continue to grow, causing your pet discomfort."
Liposarcomas, however, are malignant and have a much higher recurrence after treatment meaning multiple attempts at treatment may be required. In addition, the spread of liposarcoma is possible, although rare. Typically, liposarcomas are treated more aggressively.
How will this type of tumor affect my pet?In cases of benign lipomas, removal of these masses is more cosmetic than anything. Some pets will develop these tumors in their armpit region, between their legs, or around the neck which can cause discomfort and lameness. You may see your pet exhibit an irregular gait, reluctance to stand or walk upstairs, or go for their normal walks.
What are the treatments for these types of tumors?The single 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 will be much smaller/less painful for your pet. As lipomas continue to grow, the surgery may become more difficult for both your veterinarian and your pet.
Infiltrative lipomas and liposarcomas that have recurred after surgery commonly recur and need to be surgically removed again. Post-surgical radiation therapy may be recommended for recurrent liposarcomas or infiltrative lipomas.

This client information sheet is based on material written by Christopher Pinard, DVM© Copyright 2018 LifeLearn Inc. Used and/or modified with permission under license.