Well-Differentiated Extraskeletal Osteosarcoma Arising from the Retroperitoneum That Recurred as Anaplastic Spindle Cell Sarcoma

Abstract EN

Extraskeletal osteosarcoma is an uncommon high-grade malignant soft tissue sarcoma.

Well-differentiated extraskeletal osteosarcoma is thought to have a better prognosis than classical extraskeletal osteosarcoma, but dedifferentiation after recurrence has also been reported.

We present a case of a primary retroperitoneal extraskeletal osteosarcoma in a 62-year-old Japanese woman.

Abdominal CT revealed a large mass with diffuse calcification in the right retroperitoneal space and tumor resection was performed.

The histopathological diagnosis was well-differentiated retroperitoneal extraskeletal osteosarcoma.

She was followed up by CT every 6 months without adjuvant radiotherapy and chemotherapy for 31 months until anaplastic high-grade spindle cell sarcoma recurred in the retroperitoneum.

Our case is the seventh reported description of well-differentiated extraskeletal sarcoma, and the first to arise in the retroperitoneum and recur as an entirely dedifferentiated spindle cell sarcoma.