Figure 1. (a) Simple bone cyst in the os calcis in an adult. Geographic bone destruction, well-defined, sclerotic margin and narrow zone of transition all imply slow growth. (b) Distal ulnar giant cell tumour (GCT). Geographic bone destruction, well-defined, non-sclerotic edge and narrow zone of transition imply more rapid growth rate than in (a). (c) Distal radial GCT. Geographic bone destruction, poorly-defined edge with no marginal sclerosis, and a narrow zone of transition imply a more aggressive, rapidly-growing lesion. Trabeculation and cortical expansion are non-aggressive features, and commonly seen in GCTs. These lesions are often seen in the distal radius. (d) Metastasis in the distal humerus. Geographic bone destruction with an ill-defined edge and moth-eaten appearance imply rapid bone lysis due to an aggressive lesion. (e) Primary bone lymphoma in the proximal femur. Moth-eaten bone destruction with a wide zone of transition and no identifiable edge—an aggressive, rapidly-growing lesion. (f) Ewing's sarcoma in the proximal humerus. Permeative bone destruction with small lucencies visible in the proximal diaphysis. There is widespread cortical abnormality and an extraosseous mass. A multilamellated periosteal reaction ("onion-skin" periosteal reaction) is noted.