Pediatric extraskeletal osteosarcoma.

Extraskeletal osteosarcoma (ESOS) is a rare malignant tumor composed of mesenchymal cells comprising approximately 1% of soft-tissue sarcomas and 4% of all osteosarcomas. Primary osseous osteosarcomas more commonly develop in children and adolescents, but there are very few reports of ESOS occurring in those younger than 40 years. These variants of high-grade osteosarcoma are often characterized histopathologically by the production of an osteoid matrix and bone from malignant osteoblasts that by definition are present in the soft tissue outside the normal skeletal anatomy. We present a 13-year-old girl with a gradually enlarging, painless left thigh mass.