Lipomas in Domestic Animals

Tiffany S. Nation, DVM, MS; Kenneth S. Latimer, DVM, PhD; Paula M. Krimer, DVM, DVSc, Tracy Gieger DVM

Class of 2003 (Nation) and Department of Pathology (Latimer, Krimer), and Oncology Service, Department of Small Animal Medicine (Gieger), College of Veterinary Medicine, The University of Georgia, Athens, GA 30602-7388

Introduction

Lipomas are benign neoplasms composed of lobules of well differentiated adipocytes. These neoplasms are commonly encountered in dogs and budgerigars, occasionally identified in cats and horses, and rarely observed in other domestic species.¹

Incidence and Predisposition

Lipomas generally occur in adult dogs (average age ~ 8 years old) and the incidence of neoplasms increases with age.⁵ A breed predilection has not been demonstrated. In dogs, lipomas tend to be more common in obese females than in males.⁵ Lipomas are common in budgerigars, but also may be observed in larger psittacine birds. This may be due to genetic factors, obesity, and consumption of an high oil seed diet (e.g., sunflower seeds). Obesity does not seem to be a predisposing factor for lipoma development in cats.

Infiltrative lipomas are a subclass of lipomas that have a more aggressive clinical behavior. Infiltrative lipomas occur in middle aged to older dogs, with females being affected more frequently than males.¹ They occur much less frequently than simple lipomas in the dog and are rare in cats.¹

Site of Origin

Lipomas may occur anywhere in mammals, but commonly arise in the subcutis of the chest, abdomen, legs, and axillae (Fig. 1).⁶

Figure 1. Lipoma on the ventral abdomen of a dog (© Noah's Arkive, the University of Georgia).

Some simple lipomas may occur between muscles in the caudal thigh region of dogs (on the back of the rear leg), predominantly between the semitendinosus and semimembranosus muscles. Lipomas occasionally develop within the thoracic or abdominal cavity of mammals (e.g., mediastinum, pericardial sac, and mesentery). In birds, lipomas commonly involve the subcutis of the breast or abdomen (Fig. 2). Infiltrative lipomas may invade fascia, myocardium, bone, joint capsule, and nerve, but do not metastasize (Fig. 3). ⁴

Figure 2. Lipoma on the lower abdomen of a psittacine bird (© Noah's Arkive, the University of Georgia).	Figure 3. Infiltrating lipoma that is dissecting the skeletal muscle of a dog (© Noah's Arkive, the University of Georgia).

Clinical Appearance and Associated Signs

Simple lipomas are single to multiple subcutaneous growths of variable size and shape. They generally grow slowly and are well circumscribed.⁶ They generally feel soft to spongy on palpation, but may feel more solid due to the presence of fibrous connective tissue, necrosis, or inflammation.⁵ When located in the subcutis, the skin is freely movable over the tumor and the tumor is usually not attached to underlying fascia and muscle.⁵ These lipomas enlarge by expansion and are noninvasive. Simple lipomas that arise between muscles give the impression of muscle swelling associated with a large, firm to fluctuant mass. These intermuscular lipomas usually are not infiltrative and have an excellent prognosis after surgical excision.⁹

Infiltrative lipomas appear as poorly defined, soft to firm, nodular to diffuse swellings.^1,6 They are generally grow slowly but may exhibit periods of rapid growth.⁵ They are not very mobile on palpation because of their infiltrative nature. Infiltrative lipomas also may cause pain, pressure atrophy of muscles, and interfere with movement resulting in lameness.

Diagnosis

A fine-needle aspirate is the traditional method used to diagnose lipomas, although aspirates of normal fat may have a similar appearance.⁵ Grossly, aspirates from lipomas and subcutaneous fat will appear oily and glisten. The cytologic smears may fail to air dry and much of the sample may be lost during fixation as lipid is solubilized by alcohol.⁶ Romanowsky-stained aspirates often contain only residual abundant free lipid from disrupted lipocytes (adipocytes, fat cells) admixed with rare intact cells. Intact lipocytes have a balloon-like appearance with voluminous cytoplasm and a small peripherally displaced nucleus (Fig. 4). Lipocytes will range from very large cells that are fully distended with fat to collapsed cells with lacy cytoplasm.⁶ They generally stain poorly with Romanowsky stains and fail to stain with aqueous stains such as new methylene blue. Occasionally, clusters of intact lipocytes may be observed in some aspirates (Fig. 4). Foci of fat necrosis may appear as chalky white material.⁵ Cytologic examination is adequate for a diagnosis of lipoma when a mass is present, but cannot differentiate a simple lipoma from an infiltrative lipoma because the lipocytes are identical cytologically. A definitive diagnosis of infiltrating lipoma can only be made histologically.

Figure 4. Fine-needle aspirate of a canine lipoma. Neoplastic lipocytes appear as balloons that are arranged in lobules (Wright stain).

Histologically, lipomas may appear similar to lobules of normal adipose tissue but the presence of a neoplasm is indicated by the history of a mass and a fibrous component or capsule. The lipocytes are well differentiated and masses of these cells are divided into variably-sized lobules by septa of fibrous connective tissue (Fig. 5). Subtypes of lipomas have been described that contain abundant connective tissue stroma (fibrolipomas), cartilaginous stroma (chondrolipomas), or a prominent vascular component (angiolipomas).^1,5 Foci of necrosis and dystrophic calcification may occur in lipomas that are traumatized or strangulated.⁷

Figure 5. Surgical biopsy of a common lipoma composed of lobules of well differentiated lipocytes that have delicate cellular membranes; small, peripheral, indistinct nuclei; and nonstaining cytoplasm (hematoxylin and eosin stain).

The histological features of infiltrative lipomas are similar to those of common lipomas except for the invasion of fascia, muscle, myocardium, bone, joint capsule, or nerve by neoplastic lipocytes (Fig. 6). ⁴ In the dermis and muscle, collagen fibers and myocytes, respectively, are entrapped within the infiltrating lipoma; some tumors may degenerate from ischemic necrosis.³ The neoplastic lipocytes often are arranged in single rows between muscle fibers and collagen fibers.⁵ Infiltrating lipomas have minimal fibrous connective tissue stroma and the neoplasm is not encapsulated. ⁵

Figure 6. Surgical biopsy of an infiltrating lipoma. Neoplastic lipocytes dissect the muscle fibers (hematoxylin and eosin stain, (© Noah's Arkive, the University of Georgia).

Treatment

Small lipomas that are devoid of clinical signs may be left untreated, but are observed periodically for growth. Furthermore, the risk of anesthesia and surgery in geriatric patients may exceed the risk associated with an untreated lipoma.⁷ Despite their benign nature, lipomas should not be ignored because they tend to enlarge over time and simple lipomas cannot be grossly differentiated from infiltrative lipomas or liposarcomas (a malignant fatty neoplasm, Fig. 7).¹

Figure 7. Surgical biopsy of a liposarcoma composed of pleomorphic cells with vesicular nuclei and variably-sized cytoplasmic vacuoles that are filled with lipid (hematoxylin and eosin stain).

Surgical excision of a lipoma is warranted if it interferes with function and mobility, the neoplasm is rapidly growing, or if it is aesthetically bothersome to the owner.⁶ In dogs, caloric restriction several weeks before surgery may allow for better definition of the tumor margins.¹ Surgical excision generally is curative.⁶ Intermuscular lipomas that are not infiltrative also have an excellent prognosis after surgical excision.⁹ In birds, hemorrhage during surgery is a major concern. Both CO₂ laser and Cavitron ultrasonic surgical aspirator (CUSA System 200, Valleylab, Boulder, CO, USA) have been used successfully to excise lipomas in birds with minimal hemorrhage.¹⁰

Another form of treatment is the injection of 30% calcium chloride (CaCl₂) directly into small subcutaneous tumors (i.e., lipomas with a diameter of < 4 cm).³ Caution should be used to prevent overdosage with calcium salts or seepage of the calcium chloride solution into extralesional subcutaneous sites that may result in sloughing of the skin.³ The injection of 30% CaCl₂ results in adipocyte necrosis, which induces acute inflammation. ³ Inflammatory cells are probably responsible for the removal of dead tissue debris.³

Infiltrative lipomas are best treated by aggressive surgical excision. Complete resection of infiltrative lipomas is difficult because the tumor margins are often indistinct and the neoplasm may infiltrate muscles that are essential to normal function.^3,8 If surgical excision is incomplete, the tumor eventually will recur.³ Tumor recurrence has been observed in ~50% of patients within 3 to 16 months following attempted surgical excision.⁸ Radiation therapy may be considered postoperatively, especially if residual tumor tissue is suspected.^4,8 However, radiation may be of limited usefulness because it only destroys mitotically active cells at the time of treatment.

References

1. Aiello SE: Merck Veterinary Manual. Merck & Co., Inc., Whitehouse Station, NJ, 1998, pp. 702-703.

2. Albers GW, Theilen GH: Calcium chloride for treatment of subcutaneous lipomas in dogs. J Am Vet Med Assoc 186:492-494, 1985.

3. Kramek BA, Spackman CJ, Hayden DW: Infiltrative lipoma in three dogs. J Am Vet Med Assoc 186:81-82, 1985.

4. McEntee MC, Page LR, et al. Results of irradiation of infiltrative lipoma in 13 dogs. Vet Radiol Ultrasound 41:554-556, 2000.

5. Moulton JE: Tumors in Domestic Animals, 3^rd ed. University of California Press, Berkeley, 1990, pp.31-33.

6. Ogilvie GK, Moore AS: Managing the Veterinary Cancer Patient. Vet Learning Systems, Trenton, NJ, 1995 pp. 486-487.

7. Theilen GH, Madewell BR: Veterinary Cancer Medicine. Lea & Febiger, Philadelphia, 1987, pp. 292-293.

8. Thompson JP: Infiltrative lipoma. In: Tilley LP, Smith FWK (eds): THe 5-minute Veterinary Consult, Canine and Feline, 2^nd ed. Lippincott Williams & Wilkins, Philadelphia, 2000, p. 900.

9. Thomson MJ, Withrow AJ, et al. Intermuscular lipomas of the thigh region in dogs: 33 cases. J Am Anim Hosp Assoc 35:165-167, 1999.

10. Wilson H, Rawlings C, Latimer K, (unpublished data), 2002.

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