An osteoblastoma is an uncommon primary neoplasm of the bone that has clinical and histologic manifestations that are similar to those of an osteoid osteoma; therefore, some consider the 2 lesions to be variants of the same disease, with osteoblastoma representing a giant osteoid osteoma. However, an aggressive type of osteoblastoma has been recognized, making the relationship between the lesions less clear. [1, 2, 3, 4, 5, 6]
See the osteoblastoma images below.
Presentation and natural history
Osteoblastoma is typically seen in the second decade of life, with a male predilection.  Approximately 40% of all osteoblastomas are located in the spine. [8, 9, 10] The tumors usually involve the posterior elements, and 17% of spinal osteoblastomas are found in the sacrum. The long tubular bones are another common site of involvement, with a preponderance in the lower extremities. Osteoblastoma of the long tubular bones is often diaphyseal, and fewer are located in the metaphysis. Epiphyseal involvement is extremely rare. Other reported sites include the bones of the hands, wrists, feet and ankles  ; the skull and facial bones [12, 13, 14, 15, 16, 17] ; the ribs [18, 19] ; and the sternum, clavicles, scapulae, patellae, and pelvis. 
Benign osteoblastoma of the cranium affects women slightly more often than men and typically presents in the first 3 decades of life. Cranial osteoblastoma usually presents as a painful, nonmobile, subcutaneous mass or swelling. On CT scan, cranial osteoblastoma generally presents as a well-demarcated, mixed lytic and sclerotic lesion, with enlarged diploe, thinning outer and/or inner tables, and varying degrees of calcification. It is hypointense to isointense on T1-weighted MRI and has variable presentation on T2-weighted MRI. 
The etiology of osteoblastoma is unknown. Histologically, osteoblastomas are similar to osteoid osteomas, producing both osteoid and primitive woven bone amidst fibrovascular connective tissue. Although the tumor is usually considered benign, a controversial aggressive variant has been described in the literature, with histologic features similar to those of malignant tumors such as osteosarcomas. [22, 23, 24, 25]
Patients with osteoblastomas usually present with pain of several months' duration. In contrast to the pain that is associated with osteoid osteoma, the pain of an osteoblastoma is usually less intense, is usually not worse at night, and is not relieved readily with salicylates. If the lesion is superficial, the patient may have localized swelling and tenderness. Spinal lesions can cause painful scoliosis, although this is less common with osteoblastomas than with osteoid osteomas. In addition, lesions may mechanically interfere with the spinal cord or nerve roots, producing neurologic deficits. [26, 27]
The preliminary radiologic workup should consist of radiography at the site of the patient's pain. [1, 2] In most patients, radiographic findings are not diagnostic of osteoblastoma; therefore, further imaging is warranted. Computed tomography (CT) scanning is often necessary to support clinical and conventional radiographic findings that are suggestive of osteoblastoma and to better define the margins of the lesion for potential surgery. [28, 21] CT scans are best used for the further characterization of the lesion with regard to the presence of a nidus and matrix mineralization. [29, 28] A CT scan examination that is performed with the intravenous administration of a contrast agent poses the risk of a patient's allergic reaction to the contrast material.
Magnetic resonance imaging (MRI) aids in the detection of nonspecific reactive marrow and soft-tissue edema. [28, 30, 31] This modality best defines soft-tissue extension, although the finding is not typical of osteoblastoma. The lengthy duration of an MRI examination and a history of claustrophobia in some patients limit the use of MRI.
Bone scintigraphy demonstrates abnormal radiotracer accumulation at the affected site, substantiating clinical suspicion, but this finding is not specific for osteoblastoma.  In many patients, biopsy is necessary for confirmation of the diagnosis.
Although osteoblastomas demonstrate an increased radiotracer accumulation on bone scans, the tumors' appearance is nonspecific, and it is difficult to differentiate these lesions from those due to other etiologies that involve increased radiotracer accumulation in the bone. Therefore, bone scans are only useful in conjunction with other radiologic studies and are best not used alone.
The major diagnostic problem is in differentiating malignant or aggressive osteoblastoma from osteosarcoma on the basis of the pathologic features because the diagnosis markedly changes treatment in patients.
Most of the medical and/or legal pitfalls that are associated with osteoblastomas result from the often nonspecific radiographic appearance of these tumors. In many patients, analysis of biopsy specimens is necessary for a definitive diagnosis. If a radiologist is asked to perform a biopsy in a suspected osteoblastoma, the orthopedic surgeon should be consulted first.
For patient education information, see the Cancer and Tumors Center.
The radiographic appearances of osteoblastomas vary. Occasionally, the osteoblastoma appears as a sclerotic lesion, and in other instances, it appears as a lucent expansile lesion. Findings in as many as 25% of patients may demonstrate features that are suggestive of a malignant process, such as cortical thinning, expansion of the bone, and the presence of a soft-tissue mass. 
Radiographic findings are not diagnostic of osteoblastoma in most patients; therefore, further imaging studies are warranted.
Osteoblastomas in the spine usually occur in the posterior elements (see the image below). When a well-defined expansile lesion is identified in this location, a diagnosis of osteoblastoma should always be considered. Approximately 50% of osteoblastomas in the spine contain matrix mineralization.
An osteoblastoma in the skull produces a sharply marginated radiolucent defect that contains central calcification or ossification; this finding is highly suggestive of the diagnosis. Lesions in the mandible are often located near the tooth root.
The varied radiologic appearance of the neoplasm in sites other than the posterior elements of the spine and the skull does not allow a precise diagnosis. The appearance of the lesion may resemble a large osteoid osteoma, with the typical radiographic features of a nidus and a surrounding area of reactive bone. The nidus of an osteoblastoma is larger than that of an osteoid osteoma, with some investigators using 2 cm as a size distinction. If the nidus is eccentrically located in the bone, thick periosteal reaction may be prominent. See the images below.
The lesions may have radiographic features that are similar to those of an aneurysmal bone cyst, eosinophilic granuloma, enchondroma, fibrous dysplasia, chondromyxoid fibroma, or solitary bone cyst. The presence of an osseous matrix within the lesion may suggest an osteoblastoma. In patients in whom osteoblastoma simulates an aggressive tumor, neoplasms such as osteosarcoma and Ewing sarcoma are included in the differential diagnosis.
Osteoblastomas in the long tubular bones may arise from the medullary or cortical bone. These lesions usually appear as geographic lucencies with internal calcification and/or ossification, and they often expand the cortex. The surrounding sclerosis and periostitis, seen in as many as 50% of patients, can simulate an aggressive process and may be misinterpreted as evidence of a malignant neoplasm. Osteoblastomas may be encountered in the small bones of the hands, wrists, feet, and ankles, as well as in the flat bones. See the images below.
CT scans aid in defining the extent of the osteoblastoma and in detecting the presence of matrix mineralization. CT scan findings may support the diagnosis of osteoblastoma that has been made with the use of plain films, increasing the interpreter's degree of confidence. See the images below.
CT scans, similar to conventional radiographs, may demonstrate a predominantly osteolytic and expansile lesion, with or without central mineralization. The images may also show a predominantly sclerotic lesion or a mixed lesion. See the images below.
The medullary or cortical location of the tumor can be well defined. Adjacent bony sclerosis, periosteal reaction, or cortical erosion may be demonstrated. See the images below.
Some investigators have used CT to guide radiofrequency ablation of osteoblastoma. 
Magnetic Resonance Imaging
Similar to CT scans, MRI examinations can aid in defining the extent of the osteoblastoma, as shown below. Although MRI is useful for delineating the extent of osteoblastomas, the appearance of the tumors is usually nonspecific. MRI findings may lead to an osteoblastoma's appearance mimicking that of a malignancy.
MRI is often superior to CT scanning with regard to the detection of a soft-tissue mass, although this is a relatively uncommon feature of osteoblastomas. A typical osteoblastoma has decreased signal intensity on T1-weighted images.  See the images below.
In the authors' experience, the signal intensity of osteoblastomas on T2-weighted images is variable. Although, in general, the tumors are hyperintense relative to marrow on T2-weighted images, osteoblastomas may be heterogeneous with low signal intensity components on non–fat-suppressed T2-weighted images, presumably reflecting the ossific matrix of the lesion. See the images below.
Adjacent cortical thickening may be demonstrated. MRI often reveals inflammatory edema-type changes in the adjacent marrow and soft tissues, which are particularly evident on fat-suppressed T2-weighted sequences. See the images below.
Both the mass and the inflammatory reaction may enhance after the intravenous administration of gadolinium-based contrast material (see the images below).
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.
Although osteoblastomas accumulate radionuclide on bone scintigraphy studies (see the images below), the scintigraphic appearance of the tumors is nonspecific.
Osteoblastomas have a nonspecific increased uptake of bone-seeking agents. This finding adds little to increase the degree of confidence in bone scanning for the diagnosis. Various lesions, such as fractures, osteomyelitis, and other bone tumors, similarly accumulate radionuclide at the site of the lesion as osteoblastomas.