Extra osseous osteosarcoma of the retroperitoneum: an unusual entity.

Extra-osseous osteosarcomas constitute about 1-1.2% of all osteosarcomas. The most common sites are the extremities, thorax, and the abdomen. Retroperitoneal osteosarcomas are rare and very few cases have been reported. They are similar in their biology to high grade soft tissue sarcomas. R0 resection appears to be the best possible treatment for these tumors. All three variants of conventional osteosarcoma--osteoblastic, chondroblastic, and fibroblastic have been described in these tumors. Chemotherapy has been attempted with adriamycin-based regimens with poor results. Unlike extremity osteosarcomas, these tumors have been found to be chemoresistant. The 5 year survival has ranged from a dismal 12% to about 25%. We report a 46-year-old male who presented with a kidney tumor infiltrating the descending colon, but turned out to be an extra osseous osteosarcoma. An R0 resection was done and adjuvant chemotherapy given.