Extraskeletal osteosarcoma arising in the subcutaneous tissue of the lower leg: A case report and literature review.

Extraskeletal osteosarcoma (ESOS) is a rare soft tissue sarcoma accounting for 1-2% of all soft tissue sarcomas. ESOS originating in the superficial (cutaneous-subcutaneous) tissue is extremely rare, and only 17 cases with subcutaneous ESOS have been reported in detail to date. The aim of the present study was to report an additional case of subcutaneous ESOS of the lower leg and review previous reports of subcutaneous ESOS, focusing on the clinical characteristics, including the MIB-1 labeling index, treatment methods and outcomes. A 79-year-old healthy man presented with a 3-year history of a painful, slowly growing mass in his right lower leg that measured ~5 cm in greatest dimension. Excisional biopsy was performed, and ESOS was diagnosed based on the histopathological findings. A wide resection was performed when local recurrence developed. Six months after the wide resection, lung metastasis was detected. Considering the patient's age, stereotactic radiotherapy was performed without chemotherapy. The patient showed no evidence of local recurrence or new distant metastases for 2 years after the second surgery. We herein present this case of subcutaneous ESOS and review the previous 17 reported cases of subcutaneous ESOS. The 5-year survival rate of patients with subcutaneous ESOS was 78.6%, which was better compared with that of ESOS cases arising in deep soft tissue. Therefore, patients with subcutaneous ESOS may have a better prognosis compared with those with deep-seated ESOS, although the mean MIB-1 labeling index of subcutaneous ESOS was 24%.