Osteoid osteoma

Osteoid osteoma accounts for approximately 10% of benign primary bone tumours and is almost always accompanied by pain. The pain increases at night and is usually relieved by salicylates.

Epiphyseal involvement is extremely rare. It is usually diaphyseal in origin although extension to the metaphysis does occur.

Common sites include the femur and tibia which account for 60% of the lesion sites. 20% of lesions arise in the hands and feet. It occasionally occurs in the posterior elements of the spine usually in the lumbar region, although involvement of the vertebral body can occur. In the spine the tumour is usually associated with a scoliosis and is located at the concave surface.

Typically a rounded central lucency is present measuring less than 1 cm. This nidus contains variable amounts of punctate calcification. There is eccentric bone expansion and surrounding dense sclerosis and periosteal reaction.

Osteoid osteoma typically exhibits moderate tracer uptake in the zones of bony sclerosis with marked uptake in the region of the nidus. This pattern of uptake is called the "double density" sign. On CT the nidus enhances after administration of IV contrast.