Small cell extraskeletal osteosarcoma.

Extraskeletal osteosarcoma is a rare malignant mesenchymal neoplasm that accounts for <4% of all osteosarcomas and approximately 1.2% of all soft tissue sarcomas. Among the extraskeletal osteosarcomas, the small cell type is extremely rare. This article describes a 31-year-old man who had small cell extraskeletal osteosarcoma arising from the semimembranosus muscle. An incisional biopsy was performed and the histopathological findings showed many small cells and osteoid formation. The results were reported as a malignant small round cell tumor, consistent with an extraskeletal Ewing's sarcoma or primitive neuroectodermal tumor. Immunohistochemically, the tumor showed reactivity with antibodies against CD99 and neuron-specific enolase, but not with antibodies against S100 protein, CD138, alpha smooth muscle actin, chromogranin, Ki-67, leukocyte common antigen, epithelial membrane antigen, CD30, or desmin. The patient refused neoadjuvant chemotherapy. One week after an open biopsy, a wide marginal resection was performed. The final diagnosis was small cell extraskeletal osteosarcoma. Adjuvant chemotherapy was performed using doxorubicin, ifosfamide, and cisplatin together with a total of 60 Gy of radiation therapy. At 2-year follow-up, the functional Enneking score of the operated lower extremity was 28 points. We performed chest computed tomography, magnetic resonance imaging, and positron emission tomography-computed tomography. There were no regional recurrence and distant metastasis. Presently the patient is disease free.