Bone turnover and biochemical markers in malignancy.

There are three principal disturbances in bone remodelling that occur in neoplasia affecting the skeleton. The first is an increase in bone turnover that in solid tumors may be confined to sites of metastases or be a generalized phenomenon, most likely related to the secretion of parathyroid hormone-related protein. In the bone remodelling sequence, bone resorption precedes formation, so that increases in turnover result in substantial skeletal deficits more marked at cancellous than cortical bone sites. The second abnormality in bone remodelling is an imbalance between the amount resorbed and that formed at each remodelling site. This is a conspicuous feature of myelomatosis with moderate grades of plasma cell infiltration. The third phenomenon is the process of uncoupling. In osteolytic disease this is associated with the creation of erosion cavities that are never subsequently repaired. Progressive waves of bone resorption result in the destruction of skeletal elements and focal osteolysis. Osteosclerotic metastases formed by uncoupled bone formation represent the deposition of new bone either on quiescent bone surfaces or arising from stromal condensations within the marrow cavity. In solid tumors biopsy evidence suggests that uncoupled bone resorption and formation occur within the same metastases and that the radiographic expression (osteosclerosis, osteolysis) depends on the predominant component. The understanding that abnormalities of skeletal metabolism are mediated by authentic bone cells raises the possibility that skeletal specific markers of bone turnover might be utilized for the diagnosis of metastases, to assess the skeletal prognosis, or to monitor treatment. A variety of skeletal markers have been assessed. The pyridinium crosslinks currently provide the markers of greatest predictive value. Although they have high specificity, their sensitivity is low (< 30%). This indicates that many individuals with skeletal metastases would be missed. In contrast, skeletal markers have proven invaluable in the assessment of the natural history of the disease and response to intervention. They have been particularly useful in assessing the pharmacodynamics of bisphosphonate treatment. However, their day-to-day precision is sufficiently low that they are of limited value in the monitoring of individuals.