Sunburst periosteal reaction | Radiology Signs

Answer:
Sunburst periosteal reaction is caused by an aggressive periosteal response to rapidly growing bone lesions, including malignant primary bone tumors such as osteosarcoma (the classic association), metastatic bone disease (e.g. prostate, lung, stomach primaries), or aggressive infections. It is also seen in Ewing sarcoma and, rarely, plasmacytoma. This reaction indicates rapid new bone formation in response to cortical destruction or irritation.

Why is it called so?

The term sunburst refers to the radiographic appearance where new bone spicules radiate outward in a divergent, ray-like pattern from the cortex, resembling rays of the sun emanating from a center point. This X-ray pattern is a key imaging modality indicator of aggressive periosteal bone formation.

Pathophysiology

The sunburst periosteal reaction pattern occurs when the periosteum attempts rapid formation of new woven bone in response to a rapidly expanding lesion. Stretched Sharpey’s fibers ossify perpendicular to the cortex. Because the process is aggressive and rapid, the periosteum cannot lay down a continuous or organized lamellar bone layer. Instead, multiple spicules of bone form perpendicularly and diverge outward in a disorganized fashion, producing radiating linear opacities on radiographic imaging. This represents aggressive periosteal bone formation induced by an underlying rapidly growing tumor or infection.

Alternative names:
Sunray periosteal reaction

Other associated named signs:
Codman triangle (elevated periosteum at the margin of the lesion) and hair-on-end sign (fine perpendicular spiculation, classically seen in thalassemia and some marrow-expanding processes) – other aggressive/expansile periosteal reaction patterns.

References

1. Rana RS, et al. Periosteal reaction. AJR Am J Roentgenol. 2009;193(4):W259-W272.