[A case of extraskeletal osteosarcoma of the retroperitoneum].

A case of extraskeletal osteosarcoma of the retroperitoneum in a 57-year-old woman is reported. A rapidly growing intra-abdominal tumor was noticed eight months before her death. The resected tumor, weighing 1.9 kg, consisted of a large amount of solid mucinous tissue with significant bony materials. Histologically, the tumor showed fibrosarcomatous, chondrosarcomatous and osteosarcomatous growth patterns. A diagnosis of osteosarcoma was made. The CT scan and autopsy findings confirmed that the tumor did not originate from the preexisting bones, but from the retroperitoneum. Differences distinguishing retroperitoneal osteosarcoma from other soft tissue sarcomas were also described from the histological point of view.