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Understanding Infiltrative Lipomas in Dogs

Infiltrative Lipomas in Dogs

Infiltrative lipomas in dogs, although benign, are more problematic than regular lipomas. If your vet found an infiltrative lipoma on your dog, you may have several questions about what it is and what can be done about it. Even though these invading growths are considered benign, these types of lipomas can turn out being quite problematic, in some cases, even requiring surgical excision when feasible. Following is some information about infiltrative lipomas in dogs.

What are Infiltrative Lipomas in Dogs?

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Infiltrative lipomas in dogs do not spread, but they are still problematic.

Lipomas in dogs are benign fatty deposits that are often found in older dogs. Regular lipomas are simply deposits of well-encapsulated fat found just under the dog's skin.

Infiltrative lipomas in dogs, on the other hand, although benign (meaning that they don't spread to organs) are not encapsulated, their fatty cells fatty cells are mixed throughout normal tissue and invade muscles, joint capsules and nerves. In some cases, they may even surround bones.

When lipomas are meshed with muscle or connective tissue, they may become troublesome at times and their behavior may not be easy to predict. These growths are fortunately not very common, compared to regular, simple lipomas.

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It may happen for instance, that the growth ends up compressing the spinal cord causing neurological deficits or perhaps it may grow to an extent to impair movement.

The tissue sample taken from the liver is checked under a microscope.

The tissue sample taken from the liver is checked under a microscope.

At the Vet's Office

Usually, when a dog presents with what feels like a fatty lump under the skin, the vet will start by palpating the mass and performing a fine needle aspiration. This is an important first step in diagnosis especially considering that, what looks like a lipoma, may actually turn out being something else.

There are cancerous growths that may look like an innocent lipoma, namely mast cell tumors (also known as the great imitators) and liposarcomas, the malignant version of lipomas, which are fortunately though not very common.

Visually inspecting a liposarcoma can present as indistinguishable from a lipoma. The only way to completely differentiate the two is by having a biopsy done and sending it out to a pathologist. Even then though, things can be challenging. Liposarcomas can look like lipomas and sometimes the sample taken may come from a section missing the characteristic differences that would trigger a pathologist to diagnose it as an aggressive liposarcoma, explains veterinarian Dr. Kara.

Differentiating a regular lipoma from an infiltrative one can also pose a few challenges. Usually, in the case of a suspected infiltrative lipoma the growth feels attached to the underlying tissue when picked up and away from the muscle layer.

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A fine needle aspirate may not provide sufficient information on whether the lump is a regular lipoma or an infiltrative one. An MRI though may be more insightful as it can provide information pertaining the depth of mass. Often though, the best diagnosis of the presence of an infiltrative lipoma is made by the surgeon, upon visually seeing invasion into muscle during surgery.

"Diagnosis of an infiltrative lipoma is largely based on finding fatty infiltration of muscles and fascia at the time of surgery and the biopsy will also confirm the gross surgical findings. "~Dr. Daniel A. Degner, Board-certified Veterinary Surgeon

Infiltrative Lipoma Treatment 

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Treatment for an infiltrative lipoma may vary based on several factors such as size, location and behavior. In some cases, these lipomas can grow quite large, become painful upon palpation and impair movement.

For example, an infiltrative lipoma growing quickly on a dog's elbow joint impairing the dog's ability to walk may require surgical removal and possibly amputation. Also, as already mentioned, sometimes infiltrative lipomas may also compress the spinal cord causing neurological issues such as loss of sensation.

Infiltrative lipomas are difficult to remove, requiring aggressive surgery with removal of muscle or connective tissues that are invaded with the tumor. The margins may be difficult to assess.

Seeing a specialist for surgical removal is often recommended. It is very important to obtain clear margins. Infiltrative dog lipoma surgery costs can be therefore quite substantial compared to regular lipoma removal, and the growths may recur some time after excision (expected recurrence rate is 30 to 50 percent).

Radiation therapy may also be recommended. External beam irradiation may be given alone or in combination with surgery. A study found that median survival times for dogs with infiltrative lipomas treated with radiation were 40 months.

Another study also found the use of steroid injection guided by ultrasound helpful. According to the study, an infiltrative perineal lipoma was injected with steroids and resulted in a reduction in size of about 70 percent; however, a mild increase in size was reported after 10 months when the owners decided to have the mass removed surgically.

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  • Canine Lipomas Treated with Steroid Injections: Clinical Findings, Barbara Lamagna et al.
  • Vet Radiol Ultrasound. 2000 Nov-Dec;41(6):554-6.Results of irradiation of infiltrative lipoma in 13 dogs. McEntee MC, Page RL, Mauldin GN, Thrall DE.
  • Bergman PJ, Withrow SJ, Straw RC, Powers Be. Infiltrative lipoma in dogs: 16 cases (1981-1992). J Am Vet Med Assoc. 1994;205(2):322-4.

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