Osteoid Osteoma


 

Osteoid osteoma is a benign bone tumor that, though typically small, can be very painful. The tumor is often richly innervated and surrounded by reactive bone. The pain associated with an osteoid osteoma can be intense but is typically relieved by aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), suggesting that the pain is particularly mediated by prostaglandins. Although the majority of patients with an osteoid osteoma are 10-20 years old, some 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 often presents because of pain. The pain can be intense and may be present preferentially at night. The pain is often relieved by aspirin and NSAIDs and can be exacerbated by drinking alcohol. Although the lesion and its associated pain “burn out” over time, the process can take 2-3 years, during which symptoms can become 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 a 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 is 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 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; larger lesions are classified as osteoblastomas.

 

Figure 1: AP radiograph of the right hip demonstrating a lytic lesion in the proximal femur, consistent with an osteoid osteoma.
  

CT scanning, the advanced imaging modality of choice, usually reveals the nidus surrounded by thick reactive bone (Figure 2).

 

Figure 2: Thin-slice axial CT scan of the right proximal femur, demonstrating an osteoid osteoma within the posterior cortex.
 

A bone scan can identify the lesion by the showing intense uptake associated with the reactive bone (Figure 3).

 

Figure 3: Whole body nuclear bone scan demonstrating intense uptake associated with a right femoral osteoid osteoma.
 

MRI imaging is useful in demonstrating the characteristic extensive edema within the surrounding marrow. On MRI, the T1-weighted images show a nidus with low-intermediate signal intensity; reactive sclerosis has low signal intensity. On the STIR sequences, the nidus demonstrates moderate signal intensity, the reactive sclerosis has low signal intensity, and the impressive marrow edema is appreciated (Figure 4).

 

Figure 4: STIR coronal MRI of the left tibia demonstrating an osteoid osteoma of the medial cortex. Extensive marrow edema is shown in red; nidus is shown in green. Cortical thickening around the lesion can be appreciated.
 

 

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. A ring of reactive bone surrounds the nidus, with no permeation of the tumor into surrounding bone (Figure 5). 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 5: Low-power histology slide of osteoid osteoma (stained with H&E).
 

Figure 6: High power H&E histology slide of osteoid osteoma, demonstrating neoplastic bone surrounded by stroma of vascular channels and osteoclasts. The nidus can appear fairly cellular, but there are no malignant cells seen.
 


Differential Diagnosis

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

  1. Osteoblastoma (osteoid osteoma can be distinguished by its smaller nidus, and lack of enlargement over time),
  2. Intramedullary osteomyelitis or intracortical abscess (osteoid osteoma can be distinguished by an absence of permeative edges),
  3. Stress fracture (osteoid osteoma can be distinguished by an absence of a transverse fracture line),
  4. Arthritic subchondral cysts (when osteoid osteoma arises in juxta-articular location),
  5. Metastasis (though osteoid osteoma will not have permeative or moth-eaten margins),
  6. Bone island,
  7. Eosinophilic granuloma,
  8. Osteoblastoma,
  9. Osteosarcoma.

 


Disease Course: Treatment and Prognosis

Osteoid osteoma is first treated with NSAIDs, which typically causes 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 centimeters of a critical neurovascular structure, due to risk of thermal injury. For these lesions and for those that produce recurrent symptoms after ablation, surgical excision is indicated.


Finding the nidus of an osteoid osteoma can be extremely difficult. Also, the dense rim of sclerotic bone surrounding the nidus poses a challenge to excisional curettage. Surgery 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.


 

Things That Tend to Come up on Examinations

  1. Classic appearance on plain radiographs and CT,
  2. The dramatic response to NSAIDS,
  3. Role of radiofrequency ablation and its contraindications.