Introduction:
Most common soft tissue tumor (50%)
Superficial type: common locations for superficial type are back, neck, shoulder, abdomen; most common 5-7 decade; usually smaller than 5cm
Deep type: commonly seen in muscles; common 30-60 years; common locations include lower extremity (45%), trunk (17%), shoulder (12%), upper extremity (10%)
Infiltrating type: extend in both intra and intermuscular tissues
Multiple lipoma: about 10%; common in males; common in back, shoulder, upper arms; predilection for extensor surface; common in 5-6 decade; familial in 30% (AD or AR); associated with Bannayan-Zonana syndrome, Cowden syndromes, Fröhlich syndrome, Proteus syndrome
Radiograph:
Small lipomas often normal. Larger show radiolucency. Mineralization is unusual but chondroid or osteoidis seen in 10% and suggested liposarcoma
US:
Hyperechoic. No posterior enhancement. May be heterogenenous
CT:
Homogenous mass; HU –65 and –120. No enhancement except of the fibrous capsule. Thin septa (less than 2mm)
MR:
Isointense relative to fat. No enhancement. Septal enhancement may help to differentiate it from well-differentiated liposarcoma. The lesions with non-enhancing thin septa can be confidently diagnosed as lipoma.
Capsule: Many are encapsulated and capsule may enhance. In intramuscular lipomas, capsule may not be seen, may show irregular margins and interdigitations with skeletal muscle, causing striated appearance. Non-encapsulated lipomas or the one with thin capsules blend with surrounding subcutaneous fat and may be difficult to see on MR , hence placing a marker over palpable mass, clinical correlation, and occasionally comparison with contralateral side may be needed.
May encase neurovascular bundle, commonly seen in popliteal or inguinal region.
Differentiating from well-differentiated liposarcoma:
Septal enhancement, areas of mineralization, absence of striated appearance
References:
Murphey MD et al. Benign Musculoskeletal Lipomatous Lesions. RadioGraphics 2004;24:1433-1466
