For surface-based osteosarcoma, what imaging feature most helps distinguish it from central OSA?

• A. Periosteal reaction
• B. Large soft tissue mass
• C. Intact or minimally lytic underlying cortex
• D. Loss of cortical continuity

Answer: (C)

The key idea is where the tumor interacts with the bone cortex. Surface-based osteosarcoma starts on the periosteal surface and tends to spare or only minimally involve the underlying cortex early on. This results in an intact or minimally lytic cortex on imaging, which helps differentiate it from central (intramedullary) osteosarcoma that arises inside the marrow and frequently disrupts the cortex and extends medullarly. So, seeing an intact or only slightly affected cortex is the most informative clue that the lesion is surface-based rather than central. Other features like periosteal reaction or a soft-tissue mass can be present in both, making cortex integrity the strongest distinguishing factor.