Dedifferentiated liposarcoma of the retroperitoneum with osteosarcomatous component: report of two cases.

We describe two cases of ossified soft tissue tumors of the retroperitoneum. Computed tomographic and magnetic resonance imaging both revealed retroperitoneal masses consisting of two components -- densely ossified and lipid-rich components. In one case, a 50-year-old man, a histological diagnosis of dedifferentiated liposarcoma with osteosarcoma was made based on the needle biopsy of the two components. In another case, a 54-year-old man, surgical resection of the complex perirenal mass was performed and the same diagnosis was made. Although an ossified component represent high-grade lesion, the fatty component is an important clue to the diagnosis of dedifferentiated liposarcoma. The imaging features may be similar to those of malignant mesenchymoma, which is not a currently used term.