Retroperitoneal Extraskeletal Osteosarcoma in a Pregnant Woman Incidentally Found on Cesarean Section

Extraskeletal osteosarcoma (ESO) is a rare malignant mesenchymal neoplasm accounting for 1% - 2% of all soft tissue sarcomas that produce osteoid, bone, and/or chondroid material. Here we report the first case of a retroperitoneal ESO in a 35-year-old pregnant woman. A retroperitoneal ESO was incidentally found at cesarean section. Despite treatment, she rapidly died from wide spread dissemination. We focus on histological characteristics including immunohistochemical results and differential diagnosis of ESO.