Which condition is characterized by an onion-skin periosteal reaction?

Ewing's sarcoma is characterized by an onion-skin periosteal reaction, which is a radiological finding that appears as layers of new bone growth beneath the periosteum. This pattern is typically associated with aggressive bone lesions, especially in pediatric and young adult populations, which reflects the rapid proliferation of the tumor and the resultant reaction from the surrounding bone.

The onion-skin appearance arises from the periosteum being stimulated to lay down new bone in layers; this is a response to the underlying pathology, which in the case of Ewing's sarcoma is the presence of an aggressive tumor. Clinically, Ewing's sarcoma presents with pain and swelling, typically in the long bones or pelvis.

In differentiation, while osteosarcoma can exhibit various types of periosteal response, its more typical radiographic finding is an aggressive, sunburst or "hair-on-end" appearance rather than the onion-skin pattern. Codman's triangle is a specific type of periosteal reaction seen in osteosarcoma when the periosteum is lifted away from the bone due to a tumor, resulting in a triangular area of bone. Pseudopalisading tumor cells refer to a histological feature often seen in glioblastomas or