# Diagnostic Radiology/Musculoskeletal Imaging/Tumors Basic/Osteoid ostemoa

Osteoid osteoma is a benign osteoblastic (bone forming) tumor. It consists of a highly vascularized osteoid core. The growing core induces a peripheral zone of sclerosis and periosteal reaction.

## Clinical Presentation

The hallmark is pain which is classically increased at night and often relieved by low doses of salicylates.
Typically occurs in patients age 5–25 years old. Can also occur in the very young and very old.
Male to female ratio is 3:1.

## Imaging Findings

Classic plain film/CT description: Centrally located oval or round nidus, < 2 cm in diameter, with a uniform peripheral zone of sclerosis.
Unfortunately, appearance can be highly variable because the nidus can be located in the cortex, intrameduallary space, or periosteum of a bone. Description above is classic for an intracortical lesion. Diagnosis can also be more difficult when there is intraarticular extension or occurs in the spine.

### Additional pearls

* Radiolucent nidus, < 2 cm in diameter.
* Nidus may contain calcified bone matrix.
* Nidus surrounded by sclerosis. (Sclerosis sometimes is so extensive that the nidus can't be detected on plain films.)
* Nidus located in the metaphysis or diaphysis of long bones. (Ephysis is very uncommon.)
* Periosteal reaction.
* Monostotic or polyostotic. One or multiple niduses. Niduses may be clustered.
* Lesion is located on the concave side of bone in patients with painful scoliosis.
* Synovitis commonly occurs, especially if there are periarticular or intracapsular lesions.
* Limb overgrowth in children.

### Distribution

* Intracortical—80%
* Tibia/femur—55%
* Hands/feet—20%

## Images

Plain film imaging is the initial study in evaluating patients with suspected osteiod osteoma.
Fig. 1 Plain film imaging is the initial study in evaluating patients with suspected osteiod osteoma.
CT is preferred study for identifying and counting niduses.
Fig. 2 CT is preferred study for identifying and counting niduses.
Focus of increased radiotracer uptake on bone scan (very hot).
Fig. 3 Focus of increased radiotracer uptake on bone scan (very hot).
Nidus shows dense blush on angiography.
Reactive bone marrow edema on MRI.

## Differential Diagnosis

The classically described intracortical lesion is almost an Aunt Minnie.
However, since these lesions may occur in the intramedullary space or periosteum as well, the appearance can be highly variable. Sometimes all that can be seen on plain film is extensive sclerosis.
Consider differential diagnoses for bone forming tumors, sclerotic lesions, bone lesions with radiolucent centers, and periosteal reaction. Consider including infection in the differential. Remember that these usually occur in 5-25 year old patients. Ask if the patient has pain. Narrow the list based on specific clinical information and imaging findings.
Often the differential diagnosis includes:
* Stress Fracture ( Linear, radiolucent center with callus formation.)
* Abscess
* Osteoblastoma (This is the main differential when suspect a benign bone forming tumor.)
Osteoblastomas: Rare, can be expansile, either lytic or > 2 cm in diameter—in which case called a giant osteoid osteoma, have variable sclerosis, and frequently located in the posterior elements of spine.
Osteoid osteoma: common, not expanisle, < 2 cm in diameter, always has peripheral sclerosis, and frequently located in femur/tibia.
* Osteosarcoma
* Hemangioma
* Osteoma

## References

* Osteoid Osteoma by Erin Gensch, M.D., Ph.D., University of Washington Department of Radiology.
* Weissleder, et al. The Primer of Diagnostic Imaging. Third edition. (2003). p. 419-420.
* Resnick, et al. Diagnosis of Bone and Joint Disorders. Second edition. (1988). Volume 6. p. 3621-3635.