Definition

This term refers to soft tissue tumors with two or more lines of differentiation. Both benign mesenchymoma and malignant mesenchymoma cases have been reported. Of note, mesenchymomas of the bone have also been described.

The World Health Organization suggests that this should not be considered a separate nosological entity but rather as a group of neoplasms with a prevalent differentiation with coexistence of additional cell elements of different origin.

Examples are liposarcoma with cartilaginous metaplasia and/or osteogenic areas, malignant peripheral nerve sheath tumor with heterologous elements (which can be observed in about 15% of cases; → see Chap. 244), the (rare) leiomyosarcomas with osteosarcoma-like or rhabdomyosarcomatous zones, and the (rare) embryonal rhabdomyosarcomas presenting with focal cartilage.