| Osteochondroma (exostosis) | commonest benign bone tumour; metaphyseal, sessile or pedunculated; cortex and medulla continuous with the host bone; grows away from the joint; cartilage cap | malignant transformation (chondrosarcoma) suggested by cap > 1.5–2 cm in adults, growth after skeletal maturity, pain; solitary ~1%, HME (diaphyseal aclasis; AD, EXT1/EXT2) higher (~1–5%) |
| Osteoid osteoma | nidus < 1.5 cm (lucent ± central mineralisation), dense reactive sclerosis; cortical diaphysis/metaphysis of femur (proximal) and tibia; night pain relieved by NSAIDs (prostaglandins); CT best; bone scan double density sign; intense vascular blush; RF ablation curative | spinal (posterior elements) → painful scoliosis (concave side) |
| Osteoblastoma | > 2 cm, posterior elements of the spine, expansile; less sclerosis | "giant osteoid osteoma" |
| Enchondroma | hand (phalanges) commonest; central lucent, scalloping, ring‑and‑arc calcification; pathological fracture | Ollier (multiple, unilateral), Maffucci (+ soft‑tissue haemangiomas, phleboliths); both increase chondrosarcoma risk |
| Simple (unicameral) bone cyst | < 20; proximal humerus > proximal femur; central metaphyseal, migrates to the diaphysis; thin cortex; fallen fragment sign (pathognomonic) | fluid on MRI, no fluid–fluid levels unless fractured |
| Aneurysmal bone cyst | < 20; eccentric, expansile "blown‑out", thin shell; fluid–fluid levels (CT/MRI); primary or secondary (GCT, chondroblastoma, osteosarcoma) | telangiectatic osteosarcoma mimics (look for solid nodular tissue) |
| Non‑ossifying fibroma / fibrous cortical defect | children; eccentric cortical metaphyseal lucency with scalloped sclerotic margin; regresses (fills in) | "don't touch"; multiple in NF1, Jaffe–Campanacci |
| Fibrous dysplasia | ground‑glass matrix, long lesion in long bone, endosteal scalloping, rind sign, shepherd's crook (proximal femur), leontiasis ossea (skull), ribs; mono‑ or polyostotic | McCune–Albright (polyostotic FD + café‑au‑lait "coast of Maine" + precocious puberty); Mazabraud (+ intramuscular myxomas) |
| Chondroblastoma (Codman tumour) | epiphysis of skeletally immature (proximal humerus, distal femur, proximal tibia); lucent with sclerotic rim, chicken‑wire calcification; marked surrounding oedema | — |
| Chondromyxoid fibroma | eccentric metaphyseal, tibia, "bitten" margin | — |
| Vertebral haemangioma | commonest benign spinal tumour; vertical striations ("corduroy / jailhouse"), polka‑dot on axial CT; high T1 and T2 (fat) | aggressive forms (low T1) may compress the cord; skull: sunburst/spoke‑wheel |
| Osteoma | dense, well‑defined, frontal/ethmoid sinuses, outer table of skull | multiple in Gardner syndrome (FAP + osteomas + epidermoid cysts + desmoids) |
| Intraosseous lipoma | calcaneus (Ward's triangle), fat density with central calcification (target/cockade) | — |
| Bone island (enostosis) | dense spiculated ("brush border") cortical bone in medulla; stable; cold on bone scan | osteopoikilosis if multiple |
| Langerhans cell histiocytosis | see Concept 57 | — |