RESUMEN
Extraskeletal osteosarcoma (ESOS) is a rare malignant mesenchymal neoplasm that accounts for less than 4% of all osteosarcomas and approximately 1-2% of all soft tissue sarcomas. The tumor is typically located in the deep soft tissues, without attachment to skeletal bones. Although ESOS has been found todevelop virtually in every organ, its most common locations are the limbs. In the case of abdominal or pelvic lesions the diagnosis can be very difficult, thus it necessarily requires confirmation after exploratory laparotomy and histopathology. Such tumors may reach enormous sizes before detection because the enlarging mass may not be associated with pain. ESOS may be one of the differential diagnoses to be considered in the case of calcified masses arising in retroperitoneal space. Here we describe a bulky, bilateral, metastatic ESOS arising from the retroperitoneum and causing obstructive uropathy with consequent hydronephrosis.