1. A type of malignant tumor that arises from fat cells in deep soft tissue; such as inside the thigh.

Most frequently seen in older adults (age 40 and above), liposarcomas are the most common of all soft-tissue sarcomas (a group of malignant tumors that involve connective tissue).

2. A malignant neoplasm of adults that occurs especially in the retroperitoneal tissues and the thigh, usually deep in the intermuscular or periarticular planes.

Peritoneal tissues include the serous membrane that lines the walls of the abdominal and pelvic cavities.