Imaging characteristics of deep-seated lipomatous tumors: intramuscular lipoma, intermuscular lipoma, and lipoma-like liposarcoma

Background. Lipoma-like liposarcomas mimic deep-seated lipomas in regard to imaging as well as histological findings and occasionally cause problems concerning diagnosis and treatment. The differences in the imaging findings among these lesions are not well defined. The purpose of this study was to elucidate the differences among the deep-seated adipocytic neoplasms including intramuscular lipoma, intermuscular lipoma, and lipoma-like liposarcoma. Methods. The imaging and clinicopathological findings of 40 intramuscular lipomas, 27 intermuscular lipomas, and 22 lipoma-like liposarcomas were evaluated, and the possibilities in the differential diagnosis were assessed. Results. Although the most frequent symptom was a palpable mass, swelling was a common symptom of intramuscular lipomas and lipoma-like liposarcomas. Imaging studies revealed dumbbell-shaped appearances among intermuscular lipomas, whereas spherical masses were characteristic of intramuscular lipomas and lipoma-like liposarcomas. Computed tomography and magnetic resonance imaging revealed fatty lesions containing streaky structures in benign lesions, and CT revealed foci of hazy amorphous density, representing spindle cell proliferation, in lipoma-like liposarcoma. Although streaky structures corresponding to entrapped muscle fibers were thick and occasionally interrupted in intramuscular lipomas, the streaky structures corresponding to areolar fibrous tissue were thin and were usually not interrupted in intermuscular lipomas. In lipoma-like well-differentiated liposarcomas, thick streaks represented entrapped muscle fibers, and thin streaks represented fibrous tissue or neoplastic spindle cell proliferation. Conclusions. The imaging findings are helpful and often afford almost pathognomonic evidence of these lesions and could help with the selection of appropriate surgery.