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Metastases from carcinoma are by far the most common malignant tumors involving the skeleton.Imaging has an important role in the detection, diagnosis, prognostication, treatment planning, andfollow-up monitoring of bone metastases. In patients with proven nonskeletal tumors, imaging isuseful for screening the skeleton to assess metastatic disease and, if it is present, to determine itsextent.[1, 2, 3]

In a patient without a known malignancy, a possible diagnosis of bone metastases may be made byrecognizing radiographic and other imaging findings. If bone metastases are present or suspected,further imaging or imaging-guided techniques may be required to confirm the diagnosis, to establishthe extent of the disease, and to find the primary tumor.

Bone metastases are often multiple at the time of diagnosis. In adults, the lesions generally occur inthe axial skeleton and other sites with residual red marrow, although the lesions may be foundanywhere in the skeletal system. Common sites for metastases are the vertebrae, pelvis, proximalparts of the femur, ribs, proximal part of the humerus, and skull. More than 90% of metastases arefound in this distribution.

Certain carcinomas may have a predilection for particular skeletal sites. For example, metastases tothe bones of the hands and feet are rare, but 50% of hand metastases originate from lung neoplasms(see the image below). Primary tumors arising from the pelvis have a predilection for spread to the

lumbosacral spine.

Bone metastases to the finger. Radiograph shows a destructive expanded osteolyticlesion in the metacarpal of the thumb in a 55-year-old man with lung carcinoma.Occasionally, patients with bone metastases may present with a pathologic fracture; therefore,checking the state of underlying bone for disease is important if such a fracture is suspected (see thefirst image below).[4] In addition, patients may present with complications of bone metastases, such asneurologic impairment due to spinal epidural compression (see the second image below).

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Pathologic fracture. Radiograph shows a displaced fracture through an osteolytic lesion in

the distal femur of a 53-year-old woman with lung carcinoma. Spinal epidural compressionin a 70-year-old man with leg weakness. Lateral lumbar myelogram shows a complete epidural block due to adestructive osteolytic lesion of the L3 vertebral body. Lumbar puncture was performed at the L2-3 level.Pathophysiology

A basic knowledge of the processes by which metastases involve bone helps in understandingradiologic findings.

Bone involvement in metastases occurs by means of 3 main mechanisms: (1) direct extension, (2)retrograde venous flow, and (3) seeding with tumor emboli via the blood circulation. Seeding occursinitially in the red marrow; this process accounts for the predominant distribution of metastatic lesionsin the red marrowcontaining areas in adults. In contrast, bone metastases are usually widespread inchildren. Retrograde venous embolism is probably the major mechanism when spread from intra-abdominal cancer involves the vertebrae. Increased intra-abdominal pressure causes blood to bediverted from the systemic caval system to the valveless vertebral venous plexus of Batson; thisdiversion allows the caudal and cranial f low of blood.

As a metastatic lesion grows in the medullary cavity, the surrounding bone is remodeled by means ofeither osteoclastic or osteoblastic processes. The relative degree of resultant bone resorption ordeposition is highly variable and depends on the type and location of the tumor. The relationship

between the osteoclastic and osteoblastic remodeling processes determines whether a predominantlytic, sclerotic, or mixed pattern is seen on radiographs.[5, 6]

Differential diagnosis and other problems to be considered

When evaluating for bone metastases, also considerbone islands,eosinophilic granuloma of theskeleton,bone lymphoma,osteomalacia and renal osteodystrophy,chronic osteomyelitis,Pagetdisease,pelvic insufficiency fractures,stress fractures,tuberous sclerosis, and secondaryosteoarthritis.

Preferred examination

Technetium-99m (99m Tc) bone scintiscanning (ie, radionuclide bone scanning) is widely regarded asthe most cost-effective and available whole-body screening test for the assessment of bone

metastases. Conventional radiography is the best modality for characterizing lesions that are depictedon bone scintiscans. Combined analysis and reporting of findings on radiographs and99m Tc bone

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scintiscans improve the diagnostic accuracy in detecting bone metastases and assessing theresponse to therapy.[7, 8, 9]

Computed tomography (CT) scanning and magnetic resonance imaging (MRI) are useful in evaluatingsuspicious bone scintiscan findings that appear equivocal on radiographs.[10, 11, 12, 13] MRI can also helpin detecting metastatic lesions before changes in bone metabolism make the lesions detectable on

bone scintiscans.

[14, 15, 16]

CT scanning is useful in guiding needle biopsy, particularly in vertebrallesions. MRI is helpful in determining the extent of local disease in planning surgery or radiationtherapy.

The first screening test used for the detection of bone metastases depends on the relative availabilityof MRI and99m Tc bone scintiscanning. The selection will become less of an issue when more MRIunits are established and when its cost decreases. Factors such as cost and relatively long imagingtimes, as well as considerations of patient throughput, are important. MRI is estimated to cost 2-3times as much as99m Tc bone scintigraphy[17, 18] ; fluorodeoxyglucose (FDG) positron emissiontomography (PET) scanning costs 8 times as much.[19, 20, 21, 22, 23]

Limitations of techniques

Radiographs are relatively insensitive in the detection of early or small metastatic lesions. Although

CT scans are superior to radiographs, CT scanning is also relatively insensitive in showing smallintramedullary lesions, and it has the disadvantage of limited skeletal coverage. Bone scintiscanfindings are sensitive but nonspecific. Whole-body MRI and FDG-PET scanning are accuratetechniques that are currently limited by their high cost.[24, 25, 26, 27, 28]