Image Description
Intramuscular lipomas present as slow-growing, painless masses involving the large muscles of the thigh, shoulder and upper arm in adults. When the involved muscle contracts, they become more apparent as a spherical mass. They are uncommon in children and difficult to distinguish from diffuse lipomatosis and lipoblastomatosis in that age group. The image shows mature fat cells diffusely infiltrating skeletal muscle.
About the Disease
Solitary lipoma is the most common soft tissue tumor. It is slow-growing and rarely poses significant clinical problems. About 5% of patients have multiple lipomas, many of which have hereditary predisposition. Most lesions arise in the subcutaneous tissues of upper back, neck, shoulder, abdomen, buttocks, upper thigh and upper arm. Infrequently, they occur in the deep soft tissues or specific anatomic sites. Both CT and MRI are excellent imaging modalities for the diagnosis of lipomas. On MRI, they show high signal intensity on T1-weighted images. Grossly, superficial lipomas are small, well-circumscribed, round to oval masses with a thin capsule. The deeper lipomas are larger and more irregular in shape. The cut surface is homogenous, greasy and pale yellow to orange. Microscopically, they show lobules of variably-sized mature fat cells and a delicate vascular network. There is no nuclear hyperchromasia. Lipoma variants contain mesenchymal elements such as fibrous tissue (fibrolipoma) or myxoid stroma (myxolipoma). Most lipomas show structural rearrangements of 12q13-15, usually in the form of translocation t(3;12)(q27-28;q13-15), targeting the HMGA2 gene. Less commonly, the translocations involve 12q13-15 and other chromosomes (1p, 2p, 2q, 5q, 11q, and 12p).