A lipoma is a soft, round, moveable lump or bump of fatty tissue under the skin. Because they do not cause pain, infection, or hair loss, they’re usually spotted by owners when petting or grooming their pets.
Lipomas are common in dogs and are seen occasionally cats. Lipomas are usually harmless. They grow slowly and stay in one place. The lipomas that tend to cause problems are the ones that are large, or that interfere with movement. (If the lipoma is in an area such as an armpit, it can hamper movement or become irritated by movement.)
Lipomas occur more often in middle-aged dogs, overweight female dogs, and old cats, but some younger dogs can get lipomas. Dog breeds that are more likely to have these include cocker spaniels, dachshunds, poodles, and terriers. We don’t know what causes lipomas, but we do know that cats that have feline leukemia are a bit more likely to get lipomas than cats that do not.
Once a dog or cat has had a lipoma, it is likely to develop others. However, each new lump should be examined by your veterinarian to ensure that it is a lipoma and not a malignant growth. A fine needle aspirate (FNA), in which a thin needle is used for a quick in-clinic microscopic look at the lump’s cells, should be done for each lump. A biopsy may still be necessary if the results of the FNA are equivocal.
Lipomas can be removed surgically. However, if your pet’s lipoma is only a cosmetic issue, your veterinarian will likely take a wait-and-watch approach. (Your veterinarian will check the lump on a regular basis to make sure it hasn’t changed.) The lipoma can be removed when your pet is scheduled for anaesthesia for another issue. However, if the lipoma interferes with movement or is so large it’s irritating or bothering your pet, your veterinarian will probably recommend that the surgery be scheduled sooner rather than later.
Occasionally, some lipomas will “invade” the surrounding tissues, such as muscles and bones, and cause functional compromise (depending on the anatomic location). These are called infiltrative lipomas. They are more common in the extremities (limbs), but other sites have been reported (including the thoracic and abdominal body walls, the head, and the perianal regions). Though infiltrative lipomas are locally invasive and more difficult to control than standard lipomas, they do not metastasize (spread) to other areas of the body. Treatment, ideally following advanced imaging (e.g., CT scan), includes surgery or surgery plus radiation therapy. The use of radiation therapy improves long-term local control in cases of incomplete resection (removal). (If the infiltrative lipoma is not completely removed, it will regrow in over 50% of the patients.) Chemotherapy is of no use with infiltrative lipomas.
Liposarcomas are uncommon malignant tumors of fatty tissue. They behave like other soft-tissue sarcomas, and are locally invasive in the surrounding tissues. They can affect any area of the body, and are not necessarily the progression of a previously benign lipoma. With biopsy and histopathology (analysis of the tumor tissues under microscope), the grade of the tumor can be determined. With low- or intermediate-grade liposarcomas, the risk of dissemination (spread) to other parts of the body is low, less than 20%. With high-grade liposarcomas, the risk is much greater. The main method of treatment is surgical resection. This is sometimes followed by radiation therapy to improve local control and to decrease the risk of recurrence. Chemotherapy is occasionally used on high-grade liposarcomas.