History:
A 12 y/o boy suffered from medial thigh pain (left) that is worse at night but which is relieved by aspirin and nonsteoidal anti-inflammatory drugs.

Questions:

1. What are the findings?
2. What is the differential diagnosis?
3. What is the diagnosis?

Answers:

1. Plane radiography of the pelvis and left hip: A radiolucent nodule with reactive sclerosis near the lesser trochanter of the left femur.
   CT of the bilateral upper thighs without contrast: An intracortical radiolucent nodule (< 1 cm) with exuberant reactive sclerosis at the near the lesser trochanter of the left femur.
2. Osteoid osteoma, stress fracture, Brodie's abscess, osteosarcoma
3. Osteoid Osteoma

Discussion
The history and radiologic appearance of this lesion make the diagnosis of osteiod osteoma. Stress fracture may occur at this particular location and may be evident as cortical thickening, but there should not be a focal lesion at the center of the cortical reaction, and pain would be relieved at night but worse with activity during the daytime. Osteoid osteoma are relatively common bone-forming tumor that consists of a circumscribed nodule of woven bone and osteoid (called the nidus) and a surrounding reactive zone of thickened cortical or trabecular bone and loose fibrovascular tissue. Patients complain of a peculiar pain syndrome that is virtually unique among bone tumors: nocturnal pain that is relieved by aspirin and other prostaglandin inhibitors. Unlike other bone tumor, osteoid osteomas have abundant nerve fibers, particularly in the reactive zone, and this innervation appears to correlate with the pain syndrome. The reactive zone results in a flare of bone and soft tissue edema on MR imaging, and there is typically an intense reactive periosteal response that is disproportionate to the small size of the nidus (1 cm or less). Treatment of osteoid osteoma is ablation, either percutaneously or by surgery. Cost and morbidity are both lower using CT-guided percutaneous ablation rather than open surgery.

Reference
1. J. X. O'Connell, S. S. Nanthakumar, G. P. Nielsen, A. E. Rosenberg. Osteoid osteoma: the uniquely innerverted bone tumor. Mod Pathol 1998; 11: 175-80