Osteoid Osteoma

Osteoid osteoma is a benign bone tumor that, though typically small, is usually quite painful. The tumor is often richly innervated and surrounded by reactive bone. The pain associated with an osteoid osteoma can be intense but is classically relieved by aspirin and other nonsteroidal anti-inflammatory drugs, suggesting that the pain is particularly mediated by prostaglandins. Although most patients with osteoid osteoma are 10-20 years old, occasional patients are old enough to have degenerative joint disease, a diagnosis to which osteoid osteoma symptoms can be initially attributed in error.

Epidemiology

Osteoid osteoma is the third most frequently encountered benign bone tumor (behind osteochondroma and non-ossifying fibroma), representing approximately 10% of all benign bone tumors. The age at presentation ranges from about 5 to 40 years of age, but most patients are 20 years of age or younger. Approximately 70% of patients with osteoid osteoma are male.

Clinical Features

Osteoid osteoma usually presents as pain. The pain can be intense and constant or may be present preferentially at night. The pain is usually relieved by aspirin and other NSAIDs and can be exacerbated by drinking alcohol. Although the lesion and its associated pain may “burn out” over time, this process can take 3-4 years or more, during which time symptoms may be intolerable.

Osteoid osteomas most commonly affect the proximal femur and tibial diaphysis (about 50% of cases) but can also arise in bones of the spine, hand, and foot. Spinal osteoid osteoma (about 15% of cases) can cause rigid scoliosis, a finding thought to be caused by paraspinal muscle spasm. Osteoid osteoma in the hand is less commonly seen (about 10% of cases), but when it is present it may be difficult to diagnose, as it may be either asymptomatic or will produce symptoms that may be confused with those of DeQuervain’s tenosynovitis. When the affected bone is superficial, there may be tenderness to palpation, localized swelling and erythema.

Radiologic Features

Osteoid osteoma usually develops eccentrically within the cortex of the bone. It is characterized radiographically by a small radiolucent nidus, which represents the actual neoplastic cells, surrounded by a halo of reactive sclerotic bone at the periphery (Figure 1). By definition, the nidus is less than 2 centimeters in diameter and in fact, usually much smaller. Larger lesions which otherwise resemble osteoid osteoma are frequently found to be osteoblastomas. CT scanning with thin cuts is the advanced imaging modality of choice and usually reveals the nidus surrounded by thick reactive bone (Figure 2). A bone scan can identify the lesion by showing intense uptake associated with the reactive bone.

Figure 1: Lateral radiograph of an osteoid osteoma of the radial tuberosity.

Figure 2: Thin-slice axial (left) and coronal (right) CT slices of osteoid osteoma of the radial tuberosity.

MRI imaging is useful in demonstrating the characteristic extensive edema within the surrounding marrow but may fail to identify the nidus itself. On MRI, the T1-weighted images show a nidus with low-intermediate signal intensity while reactive sclerosis also has low signal intensity. On the fluid-sensitive (STIR) sequences, the impressive marrow edema is appreciated (Figure 3).

Figure 3: STIR axial MRI of bilateral hips, demonstrating extensive edema associated with an osteoid osteoma within the calcar bone of the left proximal femur.

Pathology

Osteoid osteoma is a highly vascular lesion and thus the gross specimens are cherry red. An excised specimen may be surrounded by a dense, sclerotic reactive bone, obscuring the red region.

On histologic examination, osteoid, osteoclasts, vascular channels, and neoplastic bone formation are found within the nidus, representing the true neoplastic component of the tumor (Figure 4). A ring of reactive bone surrounds the nidus, with no permeation of the tumor into surrounding bone. Notably, a ring of reactive bone is absent in epiphyseal lesions, owing to the lack of periosteum in this area. Cartilage is not found in osteoid osteomas. The lesions usually have only minimal fibroblasts and collagen; capillaries, however, are abundant.

Figure 4: High power H&E histology slide of osteoid osteoma, demonstrating neoplastic bone surrounded by a benign stroma of vascular channels and osteoclasts. The nidus can appear fairly cellular, but no malignant cells are seen.

Differential Diagnosis

Although osteoid osteoma has a fairly distinct radiographic appearance, it can sometimes be confused with the following entities:

Osteoblastoma (osteoid osteoma can be distinguished by its smaller nidus, lack of enlargement over time and pain relief with NSAIDs).
Intramedullary osteomyelitis or intracortical abscess (osteoid osteoma can be distinguished by an absence of permeative edges).
Stress fracture (osteoid osteoma can be distinguished by an absence of a transverse fracture line and a differing pain pattern).
Arthritic subchondral cysts (when osteoid osteoma arises in a juxta-articular location)

Disease Course: Treatment and Prognosis

Osteoid osteoma is first treated with NSAIDs, which typically cause a dramatic but transient decrease in symptoms. If the lesion is refractory to NSAID treatment, percutaneous radiofrequency ablation under CT guidance is performed. Although almost all patients report relief with percutaneous radiofrequency ablation, there is a 10-15% recurrence rate. Note that radiofrequency ablation is contraindicated for lesions that are subcutaneous, or within a few millimeters of a critical neurovascular structure such as the spinal cord, due to risk of thermal injury. For these lesions and for those that produce recurrent symptoms after ablation, surgical excision can be considered.

Finding the nidus of an osteoid osteoma can be extremely difficult surgically, but in any case, radiofrequency ablation has essentially replaced open surgery in most cases. Also, the dense rim of sclerotic bone surrounding the nidus poses a challenge to excisional curettage. Surgery, if indicated, may be assisted by preoperative CT-guided needle localization or intraoperative fluoroscopy. Frozen section analysis can help confirm intralesional location. Successful surgery requires resection of the nidus, but not the reactive rim of sclerotic bone.

Key Test Topics

Osteoid osteomas appear as a small cortically based radiolucent nidus surrounded by thick sclerotic bone on radiographs.
They respond dramatically to NSAIDs with temporary decrease in prostaglandin mediated pain
If patients fail NSAID therapy, as most do, then radiofrequency ablation can be performed under CT guidance. Lesions that are subcutaneous or near critical neurovascular structures should undergo surgical excision due to the risk of thermal injury from the radiofrequency ablation.