Musculoskeletal Imaging

Metastasis

the dispersion of a disease, cells or organisms from one site of the body to another site not directly connected with it. Although the term metastasis may describe such disparate processes as the shifting of symptoms or local manifestations in mumps or the transportation of bacteria from one part of the body to another, it is used particularly to indicate the spread of malignant tumours to distant sites. The capacity to metastasize is one of the basic characteristics of all malignant tumours; benign neoplasms do not become metastatic.

Mechanisms of transfer include direct extension or seeding of body cavities and surfaces, transport through lymphatics, haematogenous dissemination or intraspinal spread.

In skeletal metastasis, the malignant neoplasm spreads to osseous structures. The skeleton is a frequent site of metastasis, with the most common sites being the spine, bones of the pelvis, ribs, sternum, femoral and humeral shafts and skull. The primary lesions involved most frequently are carcinoma of the breast, prostate and lung. Patients often have bone tenderness, a soft tissue mass, and deformity.

Bone response to metastasis can be classified broadly as bone resorption or bone formation, with the radiographic patterns described as purely osteolytic, purely osteosclerotic, and mixed osteolytic osteosclerotic.

Osteolytic lesions can be further characterized as well circumscribed (geographic bone destruction) or poorly defined (motheaten bone destruction or permeative bone destruction).

Some tumours produce large, osteoblastic skeletal lesions resembling those of Pagets disease or osteosarcoma. Other produce expansile osteolytic lesions. Soft tissue masses of variable size may also occur. Additional features are soft tissue ossification at sites of metastases, with or without adjacent bone involvement, and pathologic fracture.

Various diagnostic techniques, including scintigraphy, CT scanning and MR imaging, can be used in addition to routine radiography in the initial detection and later monitoring of metastatic foci. Radiotherapy itself may produce a number of bone alterations, including osteopenia, coarsening of the trabecular pattern, insufficiency fractures, ischaemic necrosis of bone and secondary neoplasia.

Standard radiographic protocols have been developed (metastatic bone survey) (Table 1), which should be performed in conjunction with bone scintigraphy.

Metastasis, skeletal, Table 1. Conventional radiographic survey for skeletal metastases.

Pelvis:	Anteroposterior projection
Thoracic spine:	Anteroposterior and lateral projections
Lumbar spine:	Anteroposterior and lateral projections
Cervical spine:	Lateral projection
Skull:	Lateral projection
Thorax:	Anteroposterior projection
Femur:	Anteroposterior projection
Humerus:	Anteroposterior projection

The classic finding on scintigraphy is a focus or foci of increased accumulation of the radionuclide ("hot" spot), although occasionally an area of diminished uptake of the radiotracer ("cold" spot) is evident.

On CT scans, further delineation of the nature of a scintigraphically positive bone scan may be possible. CT is an excellent tool in determining the extent of metastatic lesions.

MR imaging has been used most extensively in the evaluation of spinal metastasis. On T1-weighted images, vertebral lesions are o signal intensities are similar but less pronounced in acute (less than 30 days old) benign compression fractures.