Imaging description

In the long bones, spine, and flat bones, the majority of aneurysmal bone cysts (ABCs) are eccentrically located whereas in the short tubular bones they are usually central in location. All lesions show expansion and cortical thinning (Figure 88.1). Aneurysmal bone cyst is one of the few lesions that can invade the epiphyseal plate and cross from the metaphysis to the epiphysis causing growth abnormalities. The natural history of ABC is described as evolving through four radiographic stages: initial, active, stabilization, and healing stage. In the initial stage, the lesion is characterized by a well-defined lytic area associated with periosteal reaction. This is followed by the active stage where the lesion grows rapidly and aggressively, producing a “blown-out” radiographic appearance, resembling a malignant lesion (Figure 88.2). This stage is succeeded by a stabilization period which is characterized by the “soap bubble” appearance caused by maturation of the surrounding shell and internal septa between the cystic spaces. The lesion in the healing phase or quiescent phase shows progressive ossification of the bony shell and internal septa. Radiographs typically show an eccentric lytic lesion with an expanded or “blown-out” bony contour. Cortically based lesions comprise about 12–18% of cases, whereas purely surface lesions are less common and comprise about 7–8% of cases. Rarely radiographs may show flocculent densities within the lesion which mimic chondroid matrix.