Technicium diphosphonate bone scans are extremely valuable in identifying occult lesions and diagnosing metastatic disease. Whereas nearly 30% of bone mineral must be lost for a lesion to appear on plain radiograph, bone scans show disease much earlier. This test (1) is an essential part of cancer staging for skeletal metastases, (2) identifies sites of symptomatic disease, and (3) identifies potential sources of referred pain. Certain cancers such as lung and melanoma grow rapidly and evoke little reactive bone formation, leading to false-negative scans. Multiple myeloma is also notorious for having false-negative bone scans.
Bone scan can be used to evaluate the response to chemotherapy, hormone therapy, or radiation therapy. Much more sensitive than plain radiographic evaluation, it reflects the biology of the lesion and the extent of the host response. The method has been most useful in evaluating the treatment of breast cancer patients. Up to 15% of patients will have an initial increase in activity, the so-called flare phenomenon. This reflects new bone formation around the quiescent lesion. Over time, the surrounding bone can heal, and osteoblast activity, estimated by the bone activity, will diminish. Development of “new” scintigraphic lesions early in treatment does not necessarily reflect disease progression; sometimes it reflects healing and ossification of areas where the tumor did not evoke a response initially.
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One limitation of this technique is that it merely measures metabolic activity and does not evaluate the structural integrity of the skeleton. Biologic control of the tumor does not translate into mechanical restoration of the skeleton in all cases. Therefore, the bone scan findings must be evaluated in parallel with plain radiographs, CT scan, or both.
Radioimmunoisotope scanning holds promise to improve both the sensitivity and specificity of bone scans in patients with metastatic disease. As better, humanized antibodies become available, both the diagnosis of occult disease and the monitoring of established disease will become more reliable. Preliminary results in imaging breast cancer metastases are encouraging.
Computed tomography is very effective in evaluating the three-dimensional integrity of bone and to better visualize abnormal lesions identified on bone scan. This is helpful to confirm the presence of metastatic disease, particularly when evaluating tumors in the pelvic and shoulder girdles. Spine lesions can also be seen by CT, but are probably better evaluated by magnetic resonance imaging (MRI) in most circumstances. In all locations CT demonstrates the bone mineral content and cortical integrity better than MRI, at present. These characteristics thus reflect the structural integrity of the diseased bone. Commonly MRI will show extensive marrow involvement in a bone while the structural strength is preserved. CT helps to discriminate between the presence of cellular and structurally significant disease.