Giant cell tumor of the bone is a relatively uncommon tumor that is characterized by the presence of multinucleated giant cells. This type of tumor is usually regarded as benign. In most patients, giant cell tumors have an indolent course, but they can recur locally in as many as 50% of cases. Metastasis to the lungs may occur. The radiologic features of giant cell tumors are demonstrated in the images below.
Cooper first reported giant cell tumors in the 18th century; in 1940, Jaffe and Lichtenstein defined giant cell tumor more strictly to distinguish it from other tumors. Giant cell tumors usually occur de novo but may also occur as a rare complication of Paget disease of the bone. [1, 2]
Most giant cell tumors occur in the long bones (see the images below), and almost all are located at the articular end of the bone. Metaphyseal involvement may occur in skeletally immature patients. Common sites include the proximal tibia, distal femur, distal radius, and proximal humerus, although giant cell tumors have also been reported to occur in the pubic bone, calcaneus, and feet.
Giant cell tumors may also occur in the vertebrae (as seen in the image below). Giant cell tumors are 3-4 times as common in the sacrum as they are in the rest of the spine. Sacral tumors may be so extensive that they involve the entire sacrum. Rarely, the tumor may extend across the sacroiliac joint to involve the adjacent ilium or may extend across the L5-S1 disk to involve the posterior elements of the L5 vertebra.
The location of giant cell tumors within the spine can vary, and the most commonly involved areas are the vertebral body and the vertebral arch. Rarely, giant cell tumors develop in the ribs, as the CT (computed tomography) scan below demonstrates.
Giant cell tumors typically occur in adults aged 20-40 years. Patients often complain of pain and swelling at the affected site. Pathologic fracture (seen in the images below) is present in 10% of patients.
Vertebral giant cell tumors may extend into the spinal canal and compress the spinal cord, resulting in neurologic symptoms. Giant cell tumors are rarely multicentric. This condition should be considered when patients present with giant cell tumors in the hands, because the incidence of tumors in the small bones of the hand and sacrum is increased. Multicentric tumors are seen below. [3, 4, 5, 6, 7, 8]
The radiographic appearance of giant cell tumors is often characteristic. Magnetic resonance imaging (MRI) is sensitive for the detection of soft-tissue changes, intra-articular extension, and marrow changes. MRI is the best method for assessing subchondral breakthrough and extension of tumor into an adjacent joint. The diagnostic accuracy of MRI is high, especially when MRIs are interpreted in conjunction with plain radiographs. CT scans and bone scans are usually less useful than are other examinations. [6, 7, 9, 10, 11, 12]
On radiographs, typical giant cell tumors are usually easily distinguished from other bone tumors. Giant cell tumors are lytic, subarticular, and eccentric, and they are often lacking a sclerotic rim; however, unusual variants may make the radiographic diagnosis difficult.
The disadvantages of MRI are its relatively high cost and limited availability. In addition, some patients experience claustrophobia during the examination and may require sedation. MRI is also contraindicated in patients with cardiac pacemakers, orbital foreign bodies, and noncompatible aneurysmal clips.
The most important radiographic findings of giant cell tumor are the location of the tumor, its lytic nature, and the lack of a host response.
Typically, giant cell tumors are expansile, osteolytic, radiolucent lesions without sclerotic margins and usually without a periosteal reaction. Septa, found in the image below, may be seen in the lesion in 33-57% of patients; these represent nonuniform growth of the tumor rather than true septa. The tumors are typically in the range of 5-7 cm in diameter when they are discovered. 
Most giant cell tumors occur in the long bones; approximately 50% are located in the bones around the knee. Location is important in the diagnosis of giant cell tumor. Most tumors are eccentric and are seen in a subarticular location (see the first 2 images below); however, the tumor originates in the metaphysis, and the common epiphyseal involvement is the result of the patient's skeletal maturity (see the third image below).
Early lesions may lie solely in the metaphysis. A narrow zone of transition with a lack of sclerosis at its margins is a distinctive finding and strongly suggestive of the diagnosis. When sclerosis of the tumor margins is present, it is seldom complete. Periosteal reactions are not usually seen; the lack of a host-reactive response is typical of giant cell tumors.
Giant cell tumors in the spine (seen below) are uncommon and account for only 5% of giant cell tumors. The sacrum is the most common location. Patients with these tumors tend to be slightly younger than those with tumors in the appendicular skeleton. The location in the vertebrae can vary, but the tumor most commonly involves the vertebral body. On radiographs, the tumors may be seen in areas of destruction of the vertebral body with invasion of the posterior elements. The tumor can cause vertebral collapse and spinal cord compression, especially when it involves the posterior elements.
Degree of confidence
The degree of confidence is high for radiography in the appendicular skeleton. In the spine, the degree of diagnostic confidence is not high, as giant cell tumors usually cannot be differentiated from other types of tumors. Tumors in the sacrum are recognizable, and these may be diagnosed on the basis of their appearance and location.
Unusual forms of certain tumors may mimic giant cell tumors.
Telangiectatic or fibrogenic variants of osteosarcoma may not produce visible ossifications or calcifications. These variants may be eccentric and may extend to the subarticular surface, mimicking a giant cell tumor.
Malignant fibrous histiocytomas occur in a similar age group and can also mimic a giant cell tumor.
Brown tumors of hyperparathyroidism are well known in the differential diagnosis of giant cell tumors. 
Chondroblastomas may be mistaken for giant cell tumors because of their subarticular location; however, careful review of the radiographs usually reveals that the epicenter lies in the epiphysis rather than in the metaphysis. The presence of chondroid calcifications further supports the diagnosis of chondroblastoma.
Aneurysmal bone cysts may be only slightly expansile in the early stages, and they can extend to the subarticular cortex, mimicking a giant cell tumor. These cysts usually occur in younger patients. Approximately 29% of aneurysmal bone cysts are reported to be associated with some other solid bone lesion, 39% of which are giant cell tumors.
CT findings are similar to radiographic findings for giant cell tumor of bone. The CT-scan characteristics of giant cell tumors are demonstrated in the images below.
Marginal sclerosis, cortical destruction, and soft-tissue masses are seen more clearly on CT scans than on radiographs. Fluid-fluid levels are occasionally seen but are not specific. 
Degree of confidence
The degree of confidence is high when CT is used in conjunction with radiography. CT does not usually add much diagnostic information to the radiographic results. CT scans are more useful in complex-shaped bones, such as the vertebrae or pelvic bones, because the details of the lesion may not be depicted well on radiographs. CT is also useful in surgical planning.
Magnetic Resonance Imaging
On T1-weighted images, giant cell tumors may show heterogeneous or homogeneous signal intensity characteristics. The signal intensity is usually low or intermediate, but areas of high signal intensity, caused by recent hemorrhage, may be noted.
On T2-weighted images, heterogeneous low to intermediate signal intensity is seen in solid areas of the tumor (see the image below). Areas of low signal intensity may be exaggerated on T2-weighted, spin-echo images, and these may be even more exaggerated on gradient-echo weighted images because of the presence of hemosiderin. Hemosiderin is detected in more than 63% of giant cell tumors, and its presence is probably the result of extravasated red blood cells coupled with the phagocytic function of the tumor cells. 
Cystic areas are common and are seen as areas of high signal intensity on T2-weighted images. Fluid-fluid levels may be seen, as in the image below. Peritumoral edema is uncommon in the absence of a fracture. The tumor is usually heterogeneously enhancing with the intravenous administration of contrast material.
Degree of confidence
The degree of confidence in imaging the appendicular skeleton is high for MRI. The modality is sensitive in the detection of soft-tissue changes, intra-articular extension, and marrow changes. MRI is the best method for assessing subchondral breakthrough and the extension of tumor into an adjacent joint. Its diagnostic accuracy is high, especially when MRIs are interpreted in conjunction with plain radiographs.
In the spine, tumors such as osteoblastomas and aneurysmal bone cysts, as well as metastases, may be found in the same location as giant cell tumors, and they may have overlapping MRI characteristics.
Uptake in giant cell tumors is usually diffuse in all phases. The degree of uptake is not correlated with the grade of the tumor or the malignancy. Bone scanning is not usually required in the evaluation of a giant cell tumor, except for the rare case in which multicentric giant cell tumors are suspected. [10, 13]
The degree of confidence is low for nuclear medicine studies. Giant cell tumors cannot be confidently differentiated from other tumors and diseases by using bone scans alone. 
Angiography is not usually required in the evaluation of a giant cell tumor. Neovascularity is demonstrated in 80% of giant cell tumors, along with an intense, inhomogeneous capillary blush. Unfortunately, overlap in the angiographic features of malignant bone tumors, benign tumors, and nonneoplastic lesions precludes the use of angiography in making the differential diagnosis.
Although angiography can be used to assess the intraosseous and extraosseous extent of a tumor, which is useful in planning surgery, MRI has largely replaced it in surgical planning.