Imaging of atypical lipomas of the extremities: report of three cases.

Atypical lipomas are soft tissue neoplasms which differ from simple lipomas in that they consist of mature fat cells interspersed with occasional areas containing a variable admixture of multinucleated cells, collagen bundles, and adipocytes with large, hyperchromic nuclei. Although these histopathologic features resemble those of liposarcoma, atypical lipomas occurring in the extremities differ from liposarcoma because they have no tendency for distant metastases. Three patients with atypical lipomas involving the posterior compartment of the thigh are presented. The lipomas were imaged with computed tomography (CT) and magnetic resonance imaging (MRI), using a 0.15-T whole body imager. Although all lesions appeared largely lipomatous, the three lesions simulated liposarcoma on both CT and MRI. On CT, each lesion demonstrated small areas of confluent soft tissue density; these areas were hyperintense with adipose tissue on strongly T2-weighted MRI. We conclude that atypical lipoma of the extremities may not be distinguishable from liposarcoma on imaging and that biopsy is necessary for differentiation.