Dedifferentiated parosteal osteosarcoma with rhabdomyosarcomatous differentiation.

Dedifferentiated parosteal osteosarcomas are characterized histologically by the combination of low-grade fibroblastic osteosarcoma admixed with a high-grade component that typically has the appearance of malignant fibrous histiocytoma or osteosarcoma. Herein we report a case of dedifferentiated parosteal osteosarcoma of the distal femur, in which the high-grade component consisted of rhabdomyosarcoma. To our knowledge, a rhabdomyosarcomatous component has not been described previously in a dedifferentiated parosteal osteosarcoma. The clinical, radiologic, and pathologic features of this rare type of surface osteosarcoma are described.