Periosteal osteosarcoma involving femoral diaphysis in 14-year-old boy who presented with painful thigh mass

(A) Anteroposterior radiograph of femur shows cortical thickening (with Codman triangles [solid arrows] superiorly and inferiorly) that is eroded by a broad-based soft-tissue mass, with involvement of the underlying preexisting cortex (open arrow). Perpendicular periosteal reaction extends into the soft-tissue mass (arrowheads).
(B) Bone scintigram (anterior projection) shows marked and eccentric increased radionuclide uptake (*).
(C) Coronal T1-weighted MR image (650/17) also shows thickened cortex, which is extrinsically eroded centrally (r) by the broad-based intermediate-signal-intensity soft-tissue mass (m). The marrow shows focal areas of decreased signal intensity (arrows) that are separated from the soft-tissue mass by normal intervening cortex.