Fatty tumors are called lipomas. They are benign tumors that arise from the growth of fat cells. They are very common in overweight middle-age to older dogs. All breeds of dogs can be affected, but Labrador retrievers are overly represented. Though only one tumor may be present, more often several lipomas develop over time.
The exact cause is unknown. These tumors also can develop in people.
Lipomas are well-defined, soft, oval-to-round growths that can be felt under the skin. They usually feel smooth and soft and can be easily moved around under the skin. Most occur on the trunk of the dog, especially under the chest. They start out small but can become as large as an orange or grapefruit. Most lipomas do not cause clinical signs and are discovered by the owner petting their dog. Some lipomas can cause problems with walking if they involve one of the dog’s joints. In rare instances, lipomas can develop in the abdomen or around the heart, where they can cause more clinical signs of distress.
Lipomas can mimic other more malignant tumors, so it is important for your veterinarian to obtain a sample with a small needle. This is called a fine-needle aspirate. When the sample is examined under the microscope (cytology), fat cells can be identified.
Most lipomas only require monitoring by the owner, as most are slow-growing. However, tumors that are fast-growing or causing lameness can be removed surgically. Some lipomas will invade deeply between muscle tissues. Those are called infiltrative lipomas, and can be difficult to remove totally during surgery, and can often recur after surgery. Recurrence is uncommon for most simple lipomas following surgery.
Monitoring the size and the growth rate of any bump on a dog is important and should be done at regular intervals, depending on the advice of your veterinarian. Report any changes in the size, shape or consistency (firmness) to your veterinarian.
Prognosis is always good with benign lipomas. Infiltrative lipomas often recur and can cause damage to adjacent tissues.
Dr. Karsten Fostvedt is a veterinarian at St. Francis Pet Clinic in Ketchum.