RADIOGRAPHIC IMAGING OF METASTASES
Radiographs are the fastest, least expensive, and most readily available technique to diagnose metastases. Even though other techniques may be more sensitive, the radiograph gives the best integration of overall bone structure and alignment. It remains the cornerstone for evaluation of the mechanical properties of bone and correlates best with clinical features. It should therefore be the first test ordered in the evaluation of pain. Radiographs greatly assist surgical planning. The ease of measuring and monitoring lesions makes the plain radiograph indispensable.
Disease spreads diffusely within long bones. A frequent problem occurs in patients with proximal femoral metastases in whom radiographic evaluation is confined to the hip and potential lesions distal in the femur are overlooked. It is very important to image the entire bone before internal fixation so that all lesions can be stabilized during the same operative procedure.
Metastatic disease is characterized by the presence of multiple bone lesions. Single metastases occur rarely and must be differentiated from primary bone tumors. Typically, the so-called “solitary metastasis” is merely the first of many lesions to be identified. Thyroid and renal cancers and myeloma (plasmacytoma) are the most likely to present as isolated metastases. Even these favorable cases typically develop widespread disease, suggesting that there is unrecognized dissemination of cancer at the time the first bone metastasis is identified.
There are three typical radiographic patterns of metastatic disease: osteolytic, osteoblastic, and mixed. Because of variations in the bone microenvironment and clonal differentiation of tumors, different patterns may exist throughout the skeleton or within one bone. This is also seen during the course of therapy, where one lesion may be partially treated. Osteoblastic areas seen radiographically correspond to the reaction of the host bone to the metastases. They are not the cancer itself. Fast-growing tumors tend to have a mixed pattern where bone reaction cannot keep up with the tumor rate of growth. The reactive bone often lacks mechanical strength despite its deceptively strong appearance. It forms in a random pattern lacking Haversian structure. Just as in Paget’s disease, disorganized sclerotic bone can be weak and incur fracture.
Periosteal changes may occur for several reasons. Rapidly growing tumor can elevate the periosteum, causing an irregular periosteal reaction. Lung cancer and prostate cancer with cortical involvement commonly show this pattern. Stress fracture through the underlying bone can also be associated with periosteal elevation. Nevertheless, periosteal elevation is usually a hallmark of primary bone neoplasm. Sarcoma should be excluded when there is periostitis.