The differential for a fat-containing mediastinal mass includes:

• mediastinal lipomatosis
• tumors arising from fat (lipoma, liposarcoma, hibernoma)
• tumors containing fat (teratoma, thymolipoma)

Liposarcomas present more commonly in the anterior than posterior mediastinum. They are usually large and can infiltrate the mediastinum. Patients typically are middle-aged and present with dyspnea or chest pain. Well differentiated liposarcomas have large amounts of fat as well as soft tissue, as in this case (lack of PET uptake suggested low-grade tumor pre-operatively). High-grade liposarcomas do not have significant amounts of fat.