Localization: Anywhere there is fat. Most common in thigh, retroperitoneum, inguinal region, and popliteal fossa. Never in the hands, feet, and neck.

Clinical: Insidiously growing, deep-seated, ill-defined mass that usually reaches a large size. Pain, tenderness, and functional disturbances occur in 10–15 % of cases, but these are usually late complaints. Tumor may be painful due to compression on the nerve or cause edema of the limb due to venous occlusion. Retroperitoneal tumor may cause hydronephrosis, intestinal compression, inguinal hernia, and edema of the lower limbs.

Imaging: On X-ray: in more differentiate varieties, well-delineated radiolucency clearly distinguished from the surrounding muscles. Occasionally calcification, little vascularity. In more undifferentiated cases, less well-defined masses with greater density. On CT scan: in lipoma-like type, well-marginated mass mimicking lipoma. Frequently, thickened linear, nodular septa that enhance after contrast. In myxoid type, encapsulated, septated, slightly heterogeneous mass mimicking a cyst. Density of water. On MRI the myxoid type shows a homogeneous low signal intensity (dark) on T1 and high (white) on T2, strong enhancement, and minute foci of fatty tissue; small bright streaks on T1 in an overall dark lesion are typical. Nonspecific appearances are displayed by the other varieties: heterogeneous signal intensity on T1, white and dark areas on T2, and enhancement of all components. In dedifferentiated liposarcoma, a homogeneous mass of low signal intensity on T1 converts to a lesion with bright and intermediate areas on T2. Often the tumor is adherent or erodes adjacent cortical bone that responds with modest periosteal reaction. On bone scan liposarcomas are inordinately hot in contrast to other malignant soft tissue tumors. On angiography: richly vascular with intense intra- and peritumoral neoangiogenesis.

Histopathology: Usually very large, multilobulated, and delimited by a thin reactive tissue (pseudocapsule) interrupted by some tumor nodules. Satellites are present.