Certain disorders affecting bone are discussed elsewhere (hyperparathyroidism, metastatic tumor in bone). The commonest diagnostic problems involve fractures, osteomyelitis, and metastatic tumor. The nonradiologic procedures most frequently used in bone disease are serum alkaline phosphatase and bone scanning.

Strontium 85 was the first agent to be widely used in bone scanning. Its relatively high patient radiation dose and problems with excretion by the colon led to replacement by fluorine 18, which, in turn, has been superseded by the technetium-labeled phosphates. Bone scan abnormality is due to increased osteoblastic activity, whether neoplastic, reactive, reparative, or metabolic. Local hyperemia is also a factor.

Bone scan provides important information in the evaluation of trauma and unexplained pain in areas where an occult fracture is a possibility. Whereas some fractures are immediately evident on x-ray film, in many cases the fracture line cannot be seen, especially in the spine, ribs, face, and smaller bones of the extremities. The radiologist is then forced to look for secondary changes produced by healing, which will not become evident before 10-14 days and in some cases may never be detectable. On bone scan, many fracture sites become abnormal by 3 days after trauma, and the great majority in 5 days. Once evident, the abnormality persists for a variable length of time, the average being approximately 2 years. A fracture site revealed on x-ray film but not on scan at least 7 days after trauma usually represents an old healed injury. Severe osteoporosis or severe malnutrition retards osseous reaction and may result in an equivocal or false negative scan. As noted previously, various types of joint disease can be visualized on the bone scan and may on occasion present problems in differentiation of the disease from possible fracture in the neighborhood of the joint.

Osteomyelitis is another disease in which bone scan may be invaluable. X-ray changes do not usually appear before 10-14 days after onset and may be delayed further or be difficult to interpret. Bone scan shows abnormalities days or weeks before x-ray films. Although the exact time after onset necessary for most patients to display abnormality is not as clearly defined in osteomyelitis as in fracture, the literature seems to suggest 5-7 days. If the bone scan is normal and osteomyelitis is strongly suspected, a gallium scan may be helpful, or the bone scan may be repeated after several days. Certain conditions resemble osteomyelitis clinically and to some degree on scan. These include cellulitis, arthritis, and focal bone necrosis or infarct. Patients already receiving steroid or antibiotic therapy before onset of osteomyelitis may display changes in normal bone response. These changes can affect the bone scan latent period or the image characteristics.

Metastatic tumor detection is the reason for most bone scan requests. All malignancies capable of metastasis may reach bone. Some of these, including prostate, breast, lung, kidney, urinary bladder, thyroid follicular carcinoma and possibly malignant lymphoma, are more likely to do so than other tumors. Formerly, bone scanning was widely used to establish operability of these neoplasms by excluding bone metastases after a primary tumor was discovered. Most reports on breast carcinoma patients in early (potentially operable) stage disclosed a very small incidence of abnormal bone scans (usually <10%) unless there was some other evidence of bone metastases such as bone pain or elevated alkaline phosphatase levels. For example, in stage I and II breast carcinoma, there is an average reported incidence of 6% bone scan abnormality (literature range, 1%-40%). False positive abnormality due to conditions other than metastasis is also a problem (3%-57% of cases, average about 10%). Therefore, bone (and also liver or brain) scans are not being recommended in most institutions as routine preoperative tests in breast carcinoma. Fewer data are available for lung carcinoma, but there may be a better case for preoperative bone scans in potentially resectable patients based on higher rates of detectable occult metastases and the greater magnitude of the operation, which becomes unnecessary if metastases are found.

Tumor-related bone scanning is indicated in several situations: (1) to investigate bone pain when x-ray films do not show a lesion, (2) to document extent of tumor in nonresectable malignancies known to have a high rate of bone metastasis to follow results of therapy, and (3) in some cases, to help investigate unexplained symptoms that may be due to occult tumor. Until bone scanning became available, skeletal x-ray survey was the mainstay of diagnosis. Numerous comparisons have shown that bone scanning detects 15%-40% more bone metastases (literature range, 7%-57%) than x-ray. This difference is related to osteoblastic reaction induced by the tumor, which may occur even when the lesion is osteolytic on x-ray film. On the other hand, about 5% of metastases are seen on x-ray film but not on scan (literature range, 3%-8%); these are usually pure osteolytic lesions. The implication of these figures is that bone scan is sufficient for routine detection of metastases in the major bone-seeking tumors and that x-ray should be reserved for specific anatomical areas in which the scan is equivocal or the etiology of scan abnormality is in doubt. X-ray is also useful when there is strong suspicion of bone malignancy yet the scan is normal, when the scan is normal in areas of bone pain, and to help differentiate metastasis from focal severe osteoarthritis.

Bone scanning does have disadvantages, chief of which is nonspecificity. The variety of conditions that may produce abnormal bone scans include fractures (even those of long duration), osteomyelitis, active osteoarthritis, joint synovial inflammation, areas of bone necrosis or infarct, myositis ossificans, renal osteodystrophy, Paget’s disease of bone, certain benign bone tumors such as fibrous dysplasia and osteoid osteoma, and various artifacts such as ossification centers in the sternum or costochondral junction calcification. On the other hand, when tumor produces widespread bone marrow invasion, the spine or other bones may sometimes have a homogeneous appearance on scan that may be misinterpreted as normal unless certain other findings are taken into account. As a general rule, the greater the number of focal asymmetric lesions on bone scan, the more metastatic tumor should be suspected. Healing fractures (which actually may have occurred at different times) and Paget’s disease create the most interpretative difficulty. Some institutions routinely scan only the spine, pelvis, and ribs instead of the total body. A question may arise about the probability of metastases elsewhere. One large study encompassing a wide variety of tumors indicates that the incidence of solitary uptake (other skeletal areas negative) in the skull is about 4%; in the extremities as a unit, about 9%; in the humerus, about 1%; in the femur, 5%; in the tibia or fibula, 2%; and elsewhere in the extremities, quite rare.