History
A 15-year-old castrated male domestic shorthair cat was evaluated because of a mass on the caudomedial aspect of the right thigh. The cat was evaluated 9 days earlier by the referring veterinarian, which was 1 day after the owners first noticed the mass. Fine-needle aspirates of the mass were obtained, but findings on cytologic evaluation were nondiagnostic. The cat had a history of early renal disease, pancreatitis, and a grade II of VI right parasternal heart murmur but did not, at that time, require medical treatment.
On physical examination, a large, fixed, subcutaneous mass was palpated on the caudomedial aspect of the right hind limb associated with the semimembranosus and semitendinosus muscles. Findings on cytologic evaluation of multiple fine-needle aspiration specimens were consistent with adipocytes. Regional ultrasonography was performed and revealed a 4.38-cm-diameter, ill-defined, mixed echogenic mass in the caudomedial aspect of the right thigh 1 cm deep to the skin. A region of hyperechoic lines and specks was present along the margin of the mass and in the surrounding musculature. Differential diagnoses for the mass included myositis, injection-site sarcoma, or other neoplasia. Computed tomography and CT-guided biopsy were performed (Figure 1).
Transverse (A) and sagittal (B) CT images (slice thickness, 1.25 mm) of the hind limbs of a 15-year-old castrated male domestic shorthair cat with a mass on the caudomedial aspect of the right thigh. For imaging, the patient was placed in sternal recumbency with hind limbs extended caudally. The transverse image was made at the level of the distal aspects of the femurs.
Citation: Journal of the American Veterinary Medical Association 245, 1; 10.2460/javma.245.1.43
Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →
Diagnostic Imaging Findings and Interpretation
A well-defined oval mass (8.3 × 4.4 × 6.2 cm) is present in the musculature on the caudomedial aspect of the right thigh, causing an asymmetry and enlargement of the right hind limb (Figure 2). The mass extends from the level of the proximal femoral diaphysis to the level of the proximal tibial diaphysis and is positioned between the biceps femoris and adductor muscles. In the right hind limb, the biceps femoris muscle is laterally displaced and compressed; also, the semimembranosus and semitendinosus muscles of the right hind limb are medially displaced. Most of the mass is fat attenuating (−100 to −120 Hounsfield units). Numerous soft tissue attenuating bands are present along the proximal and lateral aspects of the mass (up to 50 Hounsfield units) in contact with the biceps femoris muscle. An infiltrative lipoma was considered most likely on the basis of the CT findings.
Same images as Figure 1. Notice the striations throughout the proximal and lateral aspects of the mass (arrows). These soft tissue striations represent the fat infiltrating the adjacent muscle.
Citation: Journal of the American Veterinary Medical Association 245, 1; 10.2460/javma.245.1.43
Treatment and Outcome
Results of histologic evaluation of biopsy specimens of the mass obtained via CT guidance were consistent with mature adipocytes without cellular atypia. Histologic and CT findings supported the diagnosis of an infiltrative lipoma. Surgical resection of the lipoma was not pursued by the owners, and the patient was lost to follow-up.
Comments
Infiltrative lipomas are locally invasive and aggressive, displacing and disrupting the normal musculature and surrounding tissues.1 Cytologic evaluation of aspiration or biopsy specimens alone cannot be used to accurately diagnose infiltrative lipomas because those means of evaluation cannot distinguish infiltrative lipomas from a simple lipoma.1–3 An accurate diagnosis is important prior to surgery because infiltrative lipomas can recur after surgery at a rate of 36% to 50%, whereas simple lipomas recur < 2% of the time following excision.1,4 Advanced imaging, such as CT, is often necessary to diagnose an infiltrative lipoma and offers the benefit of assessing the complete extent of the disease prior to surgery.1,3 An accurate diagnosis of a lipoma is achievable on the basis of density of the mass (−100 to −120 Hounsfield units) and adjacent muscle infiltration, seen as soft tissue striations.3
Infiltrative lipomas are rare in cats, and information regarding their imaging appearance on CT is not present in the veterinary literature. McEntee et al5 have described infiltrative lipomas in dogs as infiltration of fat between muscle fibers. Infiltrative lipomas should be considered as a differential diagnosis in cats when cytologic evaluation of aspiration specimens reveals adipocytes. Histologic evaluation and CT should be used to rule out other differential diagnoses such as liposarcoma and a simple lipoma. Excision is a viable treatment option but is often challenging because of the invasiveness into surrounding tissues. Radiation therapy has been shown to be effective in controlling recurrence of incompletely excised infiltrative lipomas in dogs.5
1. McEntee MC, Thrall DE. Computed tomographic imaging of infiltrative lipoma in 22 dogs. Vet Radiol Ultrasound 2001; 42: 221–225.
2. Thomson MJ, Withrow SJ, Dernell WS, et al. Intermuscular lipomas of the thigh region in dogs: 11 cases. J Am Anim Hosp Assoc 1999; 35: 165–167.
3. Morgan LW, Toal R, Siemering G, et al. Infiltrative lipoma causing spinal cord compression in a dog. Vet Radiol Ultrasound 2007; 48: 35–37.
4. Bergman PJ, Withrow SJ, Straw RC, et al. Infiltrative lipoma in dogs: 16 cases (1981–1992). J Am Vet Med Assoc 1994; 205: 322–324.
5. McEntee MC, Page RL, Mauldin N, et al. Results of irradiation of infiltrative lipoma in 13 dogs. Vet Radiol Ultrasound 2000; 41: 554–556.
