We report a case of extraskeletal osteosarcoma (ESOS) arising in the subcutaneous tissue of the right buttock. The patient, a 32-year-old woman, noticed the mass formation in the right buttock two years ago. The tumor had gradually enlarged and she consulted our hospital. MRI showed a round and well-circumscribed mass, which had a low intensity area on T1-weighted images, a high intensity area on T2-weighted images, and peripheral enhancement in T1 Gd-enhanced images. An incision biopsy was performed and ESOS or MFH was suspected. We performed a wide resection and diagnosed the mass as ESOS. However, chemotherapy was not given since patient consent was not obtained. After 11 monthes, there was no recurrence and no distant metastasis.
ESOS is a rare malignant tumor, which accounts for 1-2% of all soft tissue sarcomas. It is usually characterized as a high grade matrix-producing neoplasm with clinically aggressive course. The tumor in this case was comparatively low grade (pathological malignancy was grade 2) but MIB-1 was about 30%, a high rate, so it is necessary to carry out careful follow-up.
