Small cell osteosarcoma: a case report.

Introduction: Small cell osteosarcoma (SCO) is a rare bone tumour representing 1.3% of all osteosarcomas. This rare variety of osteosarcoma tends to arise in the metaphysis of long bones and may extend secondary to epiphysis. By histopathology, the tumour is composed of small round cells with a variable degree of osteoid production. We report a new case of SCO in the distal femur with epiphyseal involvement. We also present the clinical, radiologic and therapeutic features of SCO with particular emphasis on the pathologic features that allow differentiation of this neoplasm from other small round cell tumours.
Observation: A 14-year-old girl presented with a 6-month history of a painful tumefaction of the left knee with motor deficit. Imaging analysis of the knee demonstrated a lytic lesion of the metaphysis in addition to epiphysis of the distal femur with cortical destruction and invasion of soft tissues. Histological examination of a biopsy specimen showed sheets of neoplastic small round cells simulating Ewing's sarcoma. Osteoid was focally present. A diagnosis of SCO was made. The patient received 2 cycles of adjuvant chemotherapy with ifosfamide, adriamycin and cisplatin. MRI showed no change in tumour size. An en bloc, wide-margin resection of the lesion was performed. Histological examination showed a viable tumour with few necrotic foci. The patient received adjuvant chemotherapy with Holoxan and VP16. The clinical response was favourable.
Conclusion: SCO is a rare clinical entity with a high grade of malignancy that must be distinguished from other round cell tumours, particularly Ewing's sarcoma, in order to optimise treatment protocols.