Extraskeletal osteosarcoma in the rectus abdominis muscle: a case report.

Extraskeletal osteosarcoma is a rare malignant tumor with an osteoid or bone matrix that develops in soft tissues that are not in contact with bone or periosteum. A 47-year-old woman presented with a subcutaneous mass in her right abdomen. A needle biopsy revealed a giant cell tumor of the soft tissue. Denosumab was administered, but the tumor enlarged. Wide tumor resection was performed. Histopathological analysis confirmed the diagnosis of extraskeletal osteosarcoma. The patient remains alive and disease-free one year after the surgery. In this case, based on the results of the needle biopsy, chemotherapy was not administered, and surgery was prioritized. Because the tumor has a poor prognosis, careful follow-up is recommended and will be continued.