Figure 9. (a) Lateral radiograph of the knee. A densely ossified mass is seen at the posterior aspect of the distal femur, and there is sclerosis projected over the medulla. The cortex of the femur, which cannot be separated from the mass, appears intact. (b) Axial CT obtained prior to biopsy (note skin markers). The mass is diffusely dense, inseparable from posterior femur, and extends through the cortex. No unossified soft tissue component is seen. (c) Coronal T₁ and (d) axial T₂ MR images show the most densely ossified component of the lesion as low signal on both sequences (arrows). Diffuse ossification in the extraosseous mass is suggested by hypointensity on T₂ images. Tumour abuts the popliteal vessels. Diagnosis: parosteal osteosarcoma.