What are Infiltrative Lipomas?
Infiltrative lipomas are tumors that are composed of well-differentiated adipose cells. It is difficult to distinguish these tumors from other lipomas. They are generally considered benign as they do not metastasize, however, they are characterized by a high risk of local tissue. These tumors can occur anywhere on the body and can invade into adjacent muscle, fascia, nerves, joints and even bone.
What are the Clinical Signs?
These tumors are often associated with no clinical signs. Usually the owner will report finding a solid mass on their pet’s body. When present, clinical signs are often associated with the site of involvement and tumor invasiveness. As they get large, they may start to ulcerate, or your pet may begin to lick the tumor.
How is an Infiltrative Lipoma Diagnosed?
Fine needle aspiration (FNA) and cytology is often consistent with other benign lipomas. The diagnosis of an infiltrative lipoma comes from evidence of adjacent muscle, fascia, nerves, joints and even bone on advanced imaging (CT or MRI), at the time of surgery or evidence of this invasion on histopathology after surgery.
What are the Treatment Options?
- Surgery: The extent of surgery required for these tumors is dependent on the infiltrative nature of aparticular mass based on advanced imaging done prior to surgery (CT or MRI) and involves wide surgical margin around the tumor. If incomplete surgical resection occurs, the risk of local disease recurrence, when no additional therapy is pursued, is around 30-40% with the most likely time of recurrence happening within 8 months of surgery.
- Radiation Therapy: This therapy is often used in the management of residual microscopic local disease after surgery, or if aggressive surgery is not a viable option. In the event that microscopic disease is left behind at the surgical margins, the goal with definitive radiation is to prevent or delay the recurrence of the tumor.
- Definitive radiation would involve daily treatments for a total of 15-18 treatments, and is associated with a >90% chance of patients being disease-free at the primary tumor site at one to four years when used as adjuvant therapy for incomplete surgical margins. Side effects of radiation therapy include a superficial dermatitis, like sunburn, that would occur on the skin exposed to the radiation field. Side effects are typically encountered later in the treatment and are effectively addressed with supportive topical and/or systemic medications.
- Chemotherapy: As discussed above, infiltrative lipomas rarely metastasize. As such, primary treatment is aimed at local tumor control with surgery, radiation therapy or a combination of the two treatmentmodalities. Chemotherapy is rarely indicated for this tumor type.
What is the Prognosis?
Prognosis for infiltrative lipomas is excellent with adequate local control using surgery alone, radiation therapy alone, or using a multimodality treatment approach by combining these modalities. Survival times reported in the literature of dogs treated with a combination of these two modalities ranged from 6 months to 46 months. Animals who had surgery as part of their treatment regimen seemed to have a better long-term outcome compared to those who didnot.