Extraskeletal osteosarcoma is a rare malignant tumor of the soft tissue. In addition, there are only several reported cases of osteosarcoma of the retroperitoneum as a primary site. We report here on a case of extraskeletal osteosarcoma arising in the retroperitoneum. An 80-year-old man visited our hospital because of a known renal mass. Abdominal computed tomography showed a 10cm sized huge renal mass. We then performed radical nephrectomy. The mass revealed a solid and ivory-colored cut surface with multifocally ovoid cystic spaces and hemorrhage. Microscopically, the tumor consisted of polygonal stromal cells, giant cells and varying amounts of neoplastic new bone; mitotic figures were frequently observed. There was no distant metastasis or regional lymph node involvement.
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