Fatty tumors are called lipomas. Ninety-nine percent of fatty tumors are benign and arise from the growth of fat cells. They are most common in overweight, middle-aged to older dogs. The breeds most often affected are the Labrador retriever, Doberman pinscher, miniature schnauzer, Cocker Spaniel, dachshund, and Weimaraner. Although only one tumor may be present, more often several of them develop over time. The exact cause is unknown. It is not well-defined what impact obesity has on these tumors, but in both conditions the numbers of fat cells in the body are increased. Lipomas are well-defined, oval or round growths that can be felt under the skin in the subcutaneous area. They typically feel smooth, soft or rubbery, and can be easily moved around under the skin. Most occur on the trunk of the body, especially under the chest. They can also occur on the legs and occasionally in the neck region. Lipomas start out small but can become quite large. Most lipomas do not cause any clinical signs and are discovered by petting or feeling the dog. Some lipomas can impede walking if they develop in certain areas. In rare instances, lipomas can develop in areas other than the skin such as in the abdomen, around the heart, or behind the eye. If lipomas develop in these areas they may cause serious clinical signs. Lipomas can mimic other, more malignant tumors, so obtaining a sample with a small needle can be helpful in identifying them. When the sample is examined under the microscope, oily material is seen, and often fat cells can be identified. Malignant lipomas are called liposarcomas. There are rare but can occur. Many lipomas require only monitoring, since most of them are benign and slow-growing. However, tumors that are more deeply attached, are rapidly growing, or very large are usually removed. Some lipomas penetrate or invade deeply into the surrounding muscle tissues. These are called infiltrative lipomas, and they can cause more problems and be more difficult to remove. This type of lipoma may recur after surgery, whereas recurrence is uncommon for most other lipomas. Strict weight control can be tried to prevent the development of more lipomas. Monitoring the size and growth rate of any bump on the dog is important and should be done at regularly scheduled intervals. Report any change in tumor size, shape, or firmness and any changes in the skin overlying the tumor to your veterinarian. Prognosis is generally good with benign lipomas. Infiltrative lipomas often recur and cause destruction of underlying tissues.

Dr. Karsten Fostvedt is a veterinarian at St. Francis Pet Clinic in Ketchum.

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