-Differential diagnoses for surface bone lesions. -Imaging appearances of surface bone lesions. -Differentiating imaging features for surface bone lesions.

-Surface bone lesions are bone lesions arising peripheral to the medulla. -The surface of bone consistsof cortex and periosteum. -The periosteum has an outer fibrous part and an inner part consisting of mesenchymal cells (which can differentiate into osteoblasts or osteoclasts). -A primary surface lesion of bone may arise from any of the mesenchymal elements that normally reside there, or it may be the product of the pleuripotential cells of the parosteal tissues. Thus, a surface lesion may contain osteoid, cartilage, fibrous tissue, fat, or...

All cases below were collected from our public and private clinical practice over the last 12 months. Fig. 1 & 2 High Grade surface osteosarcoma (histologically proven). Clinical: 20 year old male with pain in distal left femur. Case imaging findings:X-ray showed partially calcified surface based mass with spiculated periosteal reaction indicating an aggressive lesion. MRI demonstrated a heterogeneous, high signal, avidly enhancing surface based mass. A partially calcified/low signal matrix, and adjacent spiculated/sunburst periosteal reaction were present. Cortical destruction, medullary oedema/enhancement were also demonstrated....

The differential for surface based bone lesions includes neoplastic and non-neoplastic lesions. Non-surface based mimics also need consideration. Imaging findings may help differentiate between lesions. CT and MRI are most helpful in assessing such lesions. Biopsy should only be performed after consultation with a surgical team.

Dr Matthew J Sampson is a Musculoskeletal Radiologist from Adelaide, South Australia. He works in the public and private setting and is published in Clinical Radiology and AJR. Dr Dean G Topham is currently a first year Radiology Registrar at Flinders Medical Centre, South Australia.

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