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Case Report: Infiltrative Lipoma in a Crossbred Dog

A two-year-old male, neutered crossbred (Labrador/ Greyhound) presented for assessment of a large mass situated over the right hemi pelvis. The mass was first noted one month previously and had not increased significantly in size since. Radiology had revealed a large, soft-tissue density mass lateral to the greater trochanter. Needle aspirates were consistent with adipose tissue. The dog had remained clinically well, with no evidence of gait abnormalities.

On presentation, the dog was bright and comfortable with no visible lameness.  General physical examination revealed a large, firm, fixed, non-painful mass over the right hemipelvis extending as far caudally as the ischial tuberosity and 3cm cranial to the greater trochanter. Rectal examination was unremarkable. There was no pain or discomfort associated with manipulation of the limb.

fig.1 - T1W dorsal slice of proximal femur: a large, lobular, high signal intensity mass is visible lateral to the greater trochanter, extending into the intertrochanteric fossa.

MR-scans of the area (fig.1) revealed a large, lobulated mass, with a high T1W signal intensity, infiltrating into the surrounding muscle tissue, bisecting the biceps femoris muscle throughout most of its length distally, consistent with fatty tissue.  The mass did not appear contiguous with the sciatic nerve.  However, given its infiltrative nature, a provisional diagnosis of malignant liposarcoma was offered. 

In the first instance, local excision was planned, with radical resection (amputation/ hemipelvectomy) a reserve option if adequate margins were not achieved.  The lateral aspect of the right hemipelvis was exposed and the mass dissected away from its attachments (fig. 2). The surgical findings closely mirrored those of the MRI as the mass appeared to have originated from the ischial tuberosity.   Tissue was submitted for surgical margins.  An active drain was placed and the wound was closed routinely.

The dog was hospitalised until drain removal three days later, after which it was discharged on a regime of restricted exercise to minimise the risk of seroma formation.

Histopathology identified the mass as an infiltrative lipoma, an uncommon neoplasm that histologically resembles a lipoma but behaves more like a sarcoma.

Although distant metastasis was considered unlikely, local recurrence was a strong possibility.  Due to the nature of the tissue, it was hard to determine surgical margins histologically.  The risk of damage to intrapelvic structures with follow-up radiotherapy was considered to be low, but the owners declined further treatment.  Follow-up was conducted by telephone four months post surgery, which suggested no evidence of recurrence to date.

Discussion

fig.2 - intraoperative photograph showing dissection of the mass away from the biceps femoris, vastus and gluteal muscle groups.

Tumours of adipose tissue can be divided into benign (lipomas) and malignant neoplasms (liposarcomas). Although they may resemble one another macroscopically, they are histopathologically distinct.1 Lipomas normally occur subcutaneously but can also occur in the abdominal or thoracic cavity as well as other areas of the body.As with other soft tissue sarcomas, liposarcomas tend to be locally invasive and metastasis is rare.  Infiltrative lipomas are uncommon tumours that histologically resemble lipomas but behave like liposarcomas in that they are locally invasive and aggressive.There appears to be some controversy regarding nomenclature of these tumours and it has been suggested that these masses should be called well differentiated liposarcomas based on their biological behaviour.3

CT-imaging of infiltrative lipomas has been described in the literature but differentiation from normal fatty tissue can be problematic.7 Precise anatomical location is therefore not easily described.  Reports detailing MR-imaging of these masses are scant in the literature but from our experience, this form of imaging is very helpful in precisely identifying the tumour’s location, extent and proximity to vital structures such as, in this case, the sciatic nerve.

Surgical removal is the treatment of choice for these tumours with local recurrence being a common problem.4  A reported recurrence rate of 36% and a median time to recurrence of 239 days is cited in one paper.Radiotherapy does appear to have some benefits either alone, for non-resectable masses or in combination with surgery.6  A reported median survival time of 40 months is given for the combined approach; hence why it was considered in this case.

Although there has been no reports of re-growth so far, this case remains under regular review.

 References

1. Baez JL, Hendrick MJ, Shofer FS, Goldcamp C and Sorenmo KU (2004) Liposarcomas in dogs: 56 cases(1989-2000). JAVMA 224 887-891

2. Liptak JM and Forrest LJ (2007) Soft tissue sarcomas. In Withrow SJ and Vail DM Small animal clinical oncology, Missouri, WB Saunders.

3. Saik JE, Diters RW and Wortman JA. (1987) Metastasis of a well-differentiated liposarcomas in a dog and a note on nomenclature of fatty tumours. J Comp Pathol. 97 369-73

4) McChesney AE, Stephens LC, Lebel J, Snyder S and Ferguson HR (1980) Infiltrative lipoma in dogs. Vet Pathol. 17 316-22

5) Bergman PJ, Withrow SJ, Straw RC and Powers BE (1994) Infiltrative lipoma in dogs: 16 cases (1981-1992) JAVMA 205 322-4

6) McEntee MC, Page RL, Maudlin GN and Thrall DE (2000) Results of irradiation of infiltrative lipoma in 13 dogs. Vet Radiol Ultrasound 41 554-6

7) McEntee MC and Thrall DE (2001) Computed tomographic imaging of infiltrative lipoma in 22 dogs. Vet Radiol Ultrasound 42 221-5