Radiographic criteria to determine aggressiveness (which somewhat correlates with malignancy) in a bone lesion/tumor:
1) Size - big is worse than small
2) Margins - indistinct is worse than sharp/distinct
3) Cortical violation - penetration of cortex indicates more aggresive
4) Soft tissue mass - growth out of the bone into surrounding soft tissues indicates more aggressive

This radiograph shows an osteosarcoma of the proximal tibia.

Ken Yaw