Osteosarcoma Arising in Fibrous Dysplasia of the Long Bone: Characteristic Images and Molecular Profiles.

Fibrous dysplasia (FD) is a benign fibro-osseous lesion that frequently involves the craniofacial bones and femur. Malignant transformation of FD is a rare occurrence. We report a 38-year-old woman with osteosarcoma (OS) arising from FD of the femur. Magnetic resonance imaging revealed a well-defined lesion in the medulla of the femur, with cortical thinning and local bone destruction. Wide excision of the femur was performed. Grossly, the inner part of the mass was hard and tan-gray in color, and the outer part of the mass adjacent to the cortex showed myxoid discoloration with infiltrative borders. Microscopically, most of the tumor consisted of curvilinear woven bone and fibrous stroma with bland spindle cells, which transitioned to the outer portion of the tumor, showing cellular proliferation of pleomorphic cells with frequent mitotic activity. Next-generation sequencing revealed GNAS and TP53 mutations, and the diagnosis of OS arising from FD was strongly supported. This case highlights the characteristic images and molecular features of the malignant transformation of FD.