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Figure 3. Giant cell tumor of bone. This 26 year old woman complained of increasing right knee pain following minor trauma associated with progressive swelling and inability to weight bear. Radiographs show a large lytic lesion in the lateral condyle of the tibia (white arrow). A three-phase bone scan shows marked hyperemia corresponding to the lytic lesion (black arrow) but reduced tracer accumulation centrally (arrowheads). This appearance is seen with hypervascular destructive bone tumors. (Case provided by Dr. W.D. Leslie.)

Figure 3. Giant cell tumor of bone. This 26 year old woman complained of increasing right knee pain following minor trauma associated with progressive swelling and inability to weight bear. Radiographs show a large lytic lesion in the lateral condyle of the tibia (white arrow). A three-phase bone scan shows marked hyperemia corresponding to the lytic lesion (black arrow) but reduced tracer accumulation centrally (arrowheads). This appearance is seen with hypervascular destructive bone tumors. (Case provided by Dr. W.D. Leslie.)

cartilage. MRI is the preferred modality to diagnose osteochondromas as it clearly demonstrates this cartilage cap. Growth is by enchondral ossification, and it ceases at the time of epiphyseal closure. Osteochondromas commonly present as a painless mass in patients between 10 and 20 years of age, or are found incidentally at all ages. In children, these tumors show an avid concentration of radiophosphate due to the ongoing enchondral ossification. Uptake wanes with fusion of the adjacent epiphysis. Reactivation or persistence of high abnormal uptake by the exostosis after skeletal maturation should be viewed as suspicious for malignant transformation. The frequency of transformation to chondrosarcoma is low in solitary exostosis ( less than 1%) but is higher in hereditary multiple exostosis, exceeding 10% in some reports. Absent uptake of radiophosphate virtually excludes malignant change.

Osteoid Osteoma

Osteoid osteoma is a painful ("pain at site, worse at night") benign bone lesion which consists of a small core, or nidus, surrounded by a zone of sclerotic bone. The nidus is less than 1 cm in diameter and is made up of varying proportions of osteoid and spicules of newly formed osseous tissue in a bed of highly vascularized osteogenic connective tissue; it is surrounded by a margin of dense sclerotic bone. Over 50% of the lesions occur in the femur and tibia, about 10% in the vertebral column and the rest scattered between the ribs, carpals, tarsals, scapulae, patellae, calvarium and mandible.

The reactive sclerosis is most prominent in long-bone osteoid osteomas and, together with its lucent nidus, is readily appreciated by plain radiography. Lesions of the spine, intracapsular portions of the hip and small bones of the hands and feet often elude radiographic detection because of a paucity of reactive sclerosis and presence of overlying bony structures. It is at these sites where radiophosphate imaging is most useful (Fig. 4). SPECT can enhance detection of lesions in the spine (Fig. 5). Radiophosphate imaging can disclose the presence of osteoid osteoma with a sensitivity approaching 100%, and a normal bone scan virtually excludes the diagnosis. The bone scan has been used to guide surgical resection of the complete nidus, which is necessary to relieve the pain. When the lesion is located in the spine or hip scintigraphic guidance will limit the amount of bone required for removal. This is accomplished by injecting radiophosphate prior to surgery and scanning the resected bone intraoperatively to verify that it contains the nidus.

Radiography of the painful area should be performed first in the diagnostic workup as about two-thirds of the osteoid osteomas will be identified as such. The bone scan should be reserved for normal or equivocal radiographic findings. Some patients will present with referred pain and imaging should be continued above and below the painful site. For example, an osteoid osteoma in the intertrochanteric region could have pain referred to the knee.

Osteoma

The osteoma is a well-defined benign slow-growing tumor composed of osseous tissue and is found almost exclusively in the skull, paranasal sinuses and mandible. Histologically, it may be eburnated in type containing quiescent lamellae of bone, or spongy in type showing cellular, vascular connective tissue separating spicules of newly formed bone surrounded by rows of osteoblasts. They measure about 2 cm in diameter, are very dense on the radiograph and do not become malignant. The bone

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