In the described sample, what best describes the periosteal reaction pattern?

• A. Uniform, smooth periosteal reaction
• B. Progressive periosteal reaction that is irregular to columnar with ill-defined margins at the caudoproximal aspect
• C. Onion-skin periosteal layering
• D. No periosteal reaction

Answer: (B)

Periosteal reactions reflect how quickly and aggressively the periosteum responds to insults to the bone. When the process is slow or benign, you’d expect a smooth, uniform new bone outline. In contrast, an aggressive process pushes the periosteum to lay down new bone in a disorganized, irregular pattern that can appear as columnar spurts with poorly defined, indistinct margins. The description of a progressive periosteal reaction that is irregular to columnar with ill-defined margins at the caudoproximal aspect fits this pattern of an active, invasive process rather than a benign or slow reaction. That’s why it best describes an aggressive lesion—such as a malignant bone tumor or an aggressive osteomyelitis—where the new bone is laid down in a disordered, progressively advancing fashion.