Biological aspects of bone metastases

In a review of the literature the authors analyze the mechanism of bone metastases in its phases: the separation of tumor cells from the primary mass, the vascular invasion, the transport, the endothelial attachment, the exit of tumor cells from the vessels, the establishing of a metastasis in the bone structure, the active bone resorption and/or the increased bone formation as a response of bone tissue. Also the factors responsible for bone reactions to tumor invasion, both lytic and blastic, have been analyzed. The active bone resorption is made by: osteoclastic activation, the quantitative predominant mechanism, and direct production of osteolytic enzymes by tumor cells. The factors activating osteoclasts are various: PTH related proteins, prostaglandins, osteoclastic activating factors, identified as interleukin 1, transforming growth factors α and β, tumor necrosis factors, linfotoxine. The bone formation is principally due to a osteoblastic activation by some factors: piezoelectric stimuli, TNF-β, PDGF, some interleukins and prostaglandins with diphasic action. A direct ossification of the stroma of some tumors, prostate and breast, in presence of osteoblasts, is also possible. Finally the use of some indicators of bone metabolism for the diagnosis of bone metastases has been reported.