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scan is usually normal, but there are isolated reports of increased uptake of radiophosphate.

Unicameral or Simple Bone Cyst

A bone cyst is an intramedullary cavity lined by fibrous tissue and is usually filled with a clear amber fluid, but it may be multi-chambered. The proximal metaphyseal ends of the humerus and femur are the most frequent sites of occurrence. The condition is seen in children and adolescents and thought to undergo involution as it is much less common in adults. Radiographically, the appearance is that of a large radiolucent lesion, broad at the metaphyseal end and narrow at the shaft end, and does not cross the epiphyseal plate. Lower extremity cysts are more likely to fracture than those in the upper extremity. Bone scans are usually negative, but a pattern of a slight increase in radiophosphate uptake at the margins of the cyst and photopenia in the center may be seen. A fracture will induce an avid concentration of the scanning agent.

Primary Malignant Bone tumors

Radiophosphate scanning is of limited use in the initial diagnosis of primary malignant bone tumors, as these lesions are detected by radiography when the patient first presents with local symptoms. Computed tomography (CT) and MRI are more accurate in defining the extent of the tumor, both in bone and soft tissue. There is a role for scintigraphy in the search for occult bone metastases, as may occur with osteosarcoma and Ewing's sarcoma.

Figure 5. Osteoid osteoma. Posterior planar image of the lumbar spine depicts subtle increased uptake in the 5th lumbar vertebra (arrow). Coronal SPECT images clearly define and localize the lesion.

The radiophosphate manifestations of malignant tumors are dependent on the degree of adjacent reactive bone formation, regional hyperemia and the histological nature of the tumor itself. Tumor osteoid and bone are produced by the malignant connective tissue of osteosarcoma which facilitate an increased concentration of radiophosphate within the tumor itself. Chondrosarcomas feature cartilaginous tumor cells and a chondroid matrix which may undergo calcification and ossification that promote uptake of radiophosphate. "Pure" fibrosarcomas are less likely to exhibit tumor uptake because calcification and ossification are absent.

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