Written by: Dr. Chip Coombs
Most dogs as they get older are going to develop lumps of some sort. One of the most common and least inclined to cause any concern to an owner is a lipoma. Lipomas are soft, movable masses that often lie just below the skin on potentially any part of body and can be quite small (2 cm) to very large (30 cm). Usually, an owner will discover one on the lower trunk or on a leg. Although some breeds are somewhat more inclined to have them, like Labradors, they can occur in any breed and most often in middle-aged to older dogs. Being overweight is believed to be a predisposing factor, but the truth is no one knows for sure what causes them, and slim, trim dogs have certainly been known to develop them as well. Although lipomas can occur in cats as well, they are very uncommon.
Lipomas consist of the same fat cells that are present throughout the body, but as a tumour they have been stimulated to grow into a centralized mass or lump. They can also grow inside the chest or abdominal cavity, but the majority of them sit just under the skin and the surface of the skin looks completely normal. In situations where the lipoma develops below (or even inside) muscle layers, they no longer feel soft and pliable, but rather quite firm. These lipomas deserve more attention, for if they continue to grow, particularly in the groin, shoulder or chest area, they can impede a dog’s ability to walk as it can grow to a large size.
Dogs can have more than one lipoma on their body, but these are independent tumours. Usually, lipomas are benign tumours and so not a form of cancer. That said, it’s possible to have variations of a lipoma that can act much more aggressively and can attach to and infiltrate surrounding muscle, although they don’t metastasize or spread to other parts of the body. If your dog had a lipoma removed, because it was quite firm and deep in muscle tissue and then it reoccurred, then an infiltrative (invasive but benign) form of the lipoma may be the reason. To a pathologist looking at the fat cells under a microscope, the cells look very much the same between the two tumours, but if your veterinarian submits a piece of the attached muscle, the pathologist can actually see the infiltration. In an extreme form, although very uncommon, there is a form of the infiltrative lipoma called a liposarcoma which is a form of cancer, although it would be rare for it to metastasize. This latter tumour may require radiation, as well as surgery to keep under control. It is important to remember, though, that the vast majority of lipomas are not the focus of concern for dog owners.
Diagnosing a lipoma is very straightforward. Your veterinarian can simply stick a small needle into the soft mass, aspirate out some of the cells into a syringe, place a tiny amount onto a microscope slide and examine it (fine needle aspirate – FNA). Fat is easy to recognize under a microscope. In many cases, no treatment is necessary other than monitoring. It is worthwhile for owners to keep a journal of all lumps on their dog and monitor their size every few months. If it becomes clear that the lipoma is continuing to grow, it is certainly easier to remove when it is fairly small vs huge. These tumours are not painful and unless they are interfering with gait, they are essentially asymptomatic to the dog.
It is important to appreciate that not every soft, movable mass under the skin is a lipoma. Other tumours (like mast cells) can mimic the outside appearance of a lipoma, yet these growths could be quite nasty. So, if your dog develops a lump, it is very worthwhile to have your veterinarian do an FNA to determine what it is and to give you piece of mind, if it is discovered to be a lipoma.