A Rare Pediatric Example of Subcutaneous Extraskeletal Osteosarcoma: A Case Report and Review of the Morphologic Differential Diagnosis.

Primary extraskeletal osteosarcoma is an exceedingly rare malignant neoplasm that accounts for approximately 1% of soft tissue sarcomas and most often occurs in the deep soft tissues of adults. Extraskeletal osteosarcoma is characterized by the production of osteoid, bone, and/or chondroid matrix. The diagnosis of extraskeletal osteosarcoma requires careful radiologic and clinical correlation to ensure that the patient does not have an underlying bone primary. This is a case report of primary subcutaneous extraskeletal osteosarcoma arising in the thigh of a 15-year-old girl with a complex karyotype, and the morphologic differential diagnosis is reviewed.